Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages
On 08.05.2015 [15:47:26 -0700], Andrew Morton wrote: > On Wed, 06 May 2015 11:28:12 +0200 Vlastimil Babka wrote: > > > On 05/06/2015 12:09 AM, Nishanth Aravamudan wrote: > > > On 03.04.2015 [10:45:56 -0700], Nishanth Aravamudan wrote: > > >>> What I find somewhat worrying though is that we could potentially > > >>> break the pfmemalloc_watermark_ok() test in situations where > > >>> zone_reclaimable_pages(zone) == 0 is a transient situation (and not > > >>> a permanently allocated hugepage). In that case, the throttling is > > >>> supposed to help system recover, and we might be breaking that > > >>> ability with this patch, no? > > >> > > >> Well, if it's transient, we'll skip it this time through, and once there > > >> are reclaimable pages, we should notice it again. > > >> > > >> I'm not familiar enough with this logic, so I'll read through the code > > >> again soon to see if your concern is valid, as best I can. > > > > > > In reviewing the code, I think that transiently unreclaimable zones will > > > lead to some higher direct reclaim rates and possible contention, but > > > shouldn't cause any major harm. The likelihood of that situation, as > > > well, in a non-reserved memory setup like the one I described, seems > > > exceedingly low. > > > > OK, I guess when a reasonably configured system has nothing to reclaim, > > it's already busted and throttling won't change much. > > > > Consider the patch Acked-by: Vlastimil Babka > > OK, thanks, I'll move this patch into the queue for 4.2-rc1. Thank you! > Or is it important enough to merge into 4.1? I think 4.2 is sufficient, but I wonder now if I should have included a stable tag? The issue has been around for a while and there's a relatively easily workaround (use the per-node sysfs files to manually round-robin around the exhausted node) in older kernels, so I had decided against it before. Thanks, Nish -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages
On Wed, 06 May 2015 11:28:12 +0200 Vlastimil Babka wrote:
> On 05/06/2015 12:09 AM, Nishanth Aravamudan wrote:
> > On 03.04.2015 [10:45:56 -0700], Nishanth Aravamudan wrote:
> >>> What I find somewhat worrying though is that we could potentially
> >>> break the pfmemalloc_watermark_ok() test in situations where
> >>> zone_reclaimable_pages(zone) == 0 is a transient situation (and not
> >>> a permanently allocated hugepage). In that case, the throttling is
> >>> supposed to help system recover, and we might be breaking that
> >>> ability with this patch, no?
> >>
> >> Well, if it's transient, we'll skip it this time through, and once there
> >> are reclaimable pages, we should notice it again.
> >>
> >> I'm not familiar enough with this logic, so I'll read through the code
> >> again soon to see if your concern is valid, as best I can.
> >
> > In reviewing the code, I think that transiently unreclaimable zones will
> > lead to some higher direct reclaim rates and possible contention, but
> > shouldn't cause any major harm. The likelihood of that situation, as
> > well, in a non-reserved memory setup like the one I described, seems
> > exceedingly low.
>
> OK, I guess when a reasonably configured system has nothing to reclaim,
> it's already busted and throttling won't change much.
>
> Consider the patch Acked-by: Vlastimil Babka
OK, thanks, I'll move this patch into the queue for 4.2-rc1.
Or is it important enough to merge into 4.1?
From: Nishanth Aravamudan
Subject: mm: vmscan: do not throttle based on pfmemalloc reserves if node has
no reclaimable pages
Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc
reserves if node has no ZONE_NORMAL") from Mel.
We have a system with the following topology:
# numactl -H
available: 3 nodes (0,2-3)
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
23 24 25 26 27 28 29 30 31
node 0 size: 28273 MB
node 0 free: 27323 MB
node 2 cpus:
node 2 size: 16384 MB
node 2 free: 0 MB
node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
node 3 size: 30533 MB
node 3 free: 13273 MB
node distances:
node 0 2 3
0: 10 20 20
2: 20 10 20
3: 20 20 10
Node 2 has no free memory, because:
# cat /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages
1
This leads to the following zoneinfo:
Node 2, zone DMA
pages free 0
min 1840
low 2300
high 2760
scanned 0
spanned 262144
present 262144
managed 262144
...
all_unreclaimable: 1
If one then attempts to allocate some normal 16M hugepages via
echo 37 > /proc/sys/vm/nr_hugepages
The echo never returns and kswapd2 consumes CPU cycles.
This is because throttle_direct_reclaim ends up calling
wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...).
pfmemalloc_watermark_ok() in turn checks all zones on the node if there
are any reserves, and if so, then indicates the watermarks are ok, by
seeing if there are sufficient free pages.
675becce15 added a condition already for memoryless nodes. In this case,
though, the node has memory, it is just all consumed (and not
reclaimable). Effectively, though, the result is the same on this call to
pfmemalloc_watermark_ok() and thus seems like a reasonable additional
condition.
With this change, the afore-mentioned 16M hugepage allocation attempt
succeeds and correctly round-robins between Nodes 1 and 3.
Signed-off-by: Nishanth Aravamudan
Reviewed-by: Michal Hocko
Acked-by: Vlastimil Babka
Cc: Dave Hansen
Cc: Mel Gorman
Cc: Anton Blanchard
Cc: Johannes Weiner
Cc: Michal Hocko
Cc: Rik van Riel
Cc: Dan Streetman
Signed-off-by: Andrew Morton
---
mm/vmscan.c |3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff -puN
mm/vmscan.c~mm-vmscan-do-not-throttle-based-on-pfmemalloc-reserves-if-node-has-no-reclaimable-pages
mm/vmscan.c
---
a/mm/vmscan.c~mm-vmscan-do-not-throttle-based-on-pfmemalloc-reserves-if-node-has-no-reclaimable-pages
+++ a/mm/vmscan.c
@@ -2646,7 +2646,8 @@ static bool pfmemalloc_watermark_ok(pg_d
for (i = 0; i <= ZONE_NORMAL; i++) {
zone = &pgdat->node_zones[i];
- if (!populated_zone(zone))
+ if (!populated_zone(zone) ||
+ zone_reclaimable_pages(zone) == 0)
continue;
pfmemalloc_reserve += min_wmark_pages(zone);
_
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Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages
On 05/06/2015 12:09 AM, Nishanth Aravamudan wrote: On 03.04.2015 [10:45:56 -0700], Nishanth Aravamudan wrote: What I find somewhat worrying though is that we could potentially break the pfmemalloc_watermark_ok() test in situations where zone_reclaimable_pages(zone) == 0 is a transient situation (and not a permanently allocated hugepage). In that case, the throttling is supposed to help system recover, and we might be breaking that ability with this patch, no? Well, if it's transient, we'll skip it this time through, and once there are reclaimable pages, we should notice it again. I'm not familiar enough with this logic, so I'll read through the code again soon to see if your concern is valid, as best I can. In reviewing the code, I think that transiently unreclaimable zones will lead to some higher direct reclaim rates and possible contention, but shouldn't cause any major harm. The likelihood of that situation, as well, in a non-reserved memory setup like the one I described, seems exceedingly low. OK, I guess when a reasonably configured system has nothing to reclaim, it's already busted and throttling won't change much. Consider the patch Acked-by: Vlastimil Babka Thanks, Nish -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages
On 03.04.2015 [10:45:56 -0700], Nishanth Aravamudan wrote:
> On 03.04.2015 [09:57:35 +0200], Vlastimil Babka wrote:
> > On 03/31/2015 11:48 AM, Michal Hocko wrote:
> > >On Fri 27-03-15 15:23:50, Nishanth Aravamudan wrote:
> > >>On 27.03.2015 [13:17:59 -0700], Dave Hansen wrote:
> > >>>On 03/27/2015 12:28 PM, Nishanth Aravamudan wrote:
> > @@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t
> > *pgdat)
> >
> > for (i = 0; i <= ZONE_NORMAL; i++) {
> > zone = &pgdat->node_zones[i];
> > - if (!populated_zone(zone))
> > + if (!populated_zone(zone) || !zone_reclaimable(zone))
> > continue;
> >
> > pfmemalloc_reserve += min_wmark_pages(zone);
> > >>>
> > >>>Do you really want zone_reclaimable()? Or do you want something more
> > >>>direct like "zone_reclaimable_pages(zone) == 0"?
> > >>
> > >>Yeah, I guess in my testing this worked out to be the same, since
> > >>zone_reclaimable_pages(zone) is 0 and so zone_reclaimable(zone) will
> > >>always be false. Thanks!
> > >>
> > >>Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc
> > >>reserves if node has no ZONE_NORMAL") from Mel.
> > >>
> > >>We have a system with the following topology:
> > >>
> > >># numactl -H
> > >>available: 3 nodes (0,2-3)
> > >>node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
> > >>23 24 25 26 27 28 29 30 31
> > >>node 0 size: 28273 MB
> > >>node 0 free: 27323 MB
> > >>node 2 cpus:
> > >>node 2 size: 16384 MB
> > >>node 2 free: 0 MB
> > >>node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
> > >>node 3 size: 30533 MB
> > >>node 3 free: 13273 MB
> > >>node distances:
> > >>node 0 2 3
> > >> 0: 10 20 20
> > >> 2: 20 10 20
> > >> 3: 20 20 10
> > >>
> > >>Node 2 has no free memory, because:
> > >># cat
> > >>/sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages
> > >>1
> > >>
> > >>This leads to the following zoneinfo:
> > >>
> > >>Node 2, zone DMA
> > >> pages free 0
> > >> min 1840
> > >> low 2300
> > >> high 2760
> > >> scanned 0
> > >> spanned 262144
> > >> present 262144
> > >> managed 262144
> > >>...
> > >> all_unreclaimable: 1
> > >
> > >Blee, this is a weird configuration.
> > >
> > >>If one then attempts to allocate some normal 16M hugepages via
> > >>
> > >>echo 37 > /proc/sys/vm/nr_hugepages
> > >>
> > >>The echo never returns and kswapd2 consumes CPU cycles.
> > >>
> > >>This is because throttle_direct_reclaim ends up calling
> > >>wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...).
> > >>pfmemalloc_watermark_ok() in turn checks all zones on the node if there
> > >>are any reserves, and if so, then indicates the watermarks are ok, by
> > >>seeing if there are sufficient free pages.
> > >>
> > >>675becce15 added a condition already for memoryless nodes. In this case,
> > >>though, the node has memory, it is just all consumed (and not
> > >>reclaimable). Effectively, though, the result is the same on this call
> > >>to pfmemalloc_watermark_ok() and thus seems like a reasonable additional
> > >>condition.
> > >>
> > >>With this change, the afore-mentioned 16M hugepage allocation attempt
> > >>succeeds and correctly round-robins between Nodes 1 and 3.
> > >
> > >I am just wondering whether this is the right/complete fix. Don't we
> > >need a similar treatment at more places?
> > >I would expect kswapd would be looping endlessly because the zone
> > >wouldn't be balanced obviously. But I would be wrong... because
> > >pgdat_balanced is doing this:
> > > /*
> > >* A special case here:
> > >*
> > >* balance_pgdat() skips over all_unreclaimable after
> > >* DEF_PRIORITY. Effectively, it considers them balanced so
> > >* they must be considered balanced here as well!
> > >*/
> > > if (!zone_reclaimable(zone)) {
> > > balanced_pages += zone->managed_pages;
> > > continue;
> > > }
> > >
> > >and zone_reclaimable is false for you as you didn't have any
> > >zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it
> > >would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those
> > >zones right?) and so the kswapd would be woken up easily. So it looks
> > >like a mess.
> >
> > Yeah, looks like a much cleaner/complete solution would be to remove
> > such zones from zonelists. But that means covering all situations
> > when these hugepages are allocated/removed and the approach then
> > looks similar to memory hotplug.
> > Also I'm not sure if the ability to actually allocate the reserved
> > hugepage would be impossible due to not being reachable by a
> > zonelist...
> >
> > >There are possibly other places which rely on populated_zone or
> > >fo
Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages
On 03.04.2015 [20:24:45 +0200], Michal Hocko wrote:
> On Fri 03-04-15 10:43:57, Nishanth Aravamudan wrote:
> > On 31.03.2015 [11:48:29 +0200], Michal Hocko wrote:
> [...]
> > > I would expect kswapd would be looping endlessly because the zone
> > > wouldn't be balanced obviously. But I would be wrong... because
> > > pgdat_balanced is doing this:
> > > /*
> > >* A special case here:
> > >*
> > >* balance_pgdat() skips over all_unreclaimable after
> > >* DEF_PRIORITY. Effectively, it considers them balanced so
> > >* they must be considered balanced here as well!
> > >*/
> > > if (!zone_reclaimable(zone)) {
> > > balanced_pages += zone->managed_pages;
> > > continue;
> > > }
> > >
> > > and zone_reclaimable is false for you as you didn't have any
> > > zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it
> > > would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those
> > > zones right?) and so the kswapd would be woken up easily. So it looks
> > > like a mess.
> >
> > My understanding, and I could easily be wrong, is that kswapd2 (node 2
> > is the exhausted one) spins endlessly, because the reclaim logic sees
> > that we are reclaiming from somewhere but the allocation request for
> > node 2 (which is __GFP_THISNODE for hugepages, not GFP_THISNODE) will
> > never complete, so we just continue to reclaim.
>
> __GFP_THISNODE would be waking up kswapd2 again and again, that is true.
Right, one idea I had for this was ensuring that we perform reclaim with
somehow some knowledge of __GFP_THISNODE -- that is it needs to be
somewhat targetted in order to actually help satisfy the current
allocation. But it got pretty hairy fast and I didn't want to break the
world :)
> I am just wondering whether we will have any __GFP_THISNODE allocations
> for a node without CPUs (numa_node_id() shouldn't return such a node
> AFAICS). Maybe if somebody is bound to Node2 explicitly but I would
> consider this as a misconfiguration.
Right, I'd need to check what happens if in our setup you taskset to
node2 and tried to force memory to be local -- I think you'd either be
killed immediately, or the kernel will just disagree with your binding
since it's invalid (e.g., that will happen if you try to bind to a
memoryless node, I think).
Keep in mind that although in my config node2 had no CPUs, that's not a
hard & fast requirement. I do believe in a previous iteration of this
bug, the exhausted node had no free memory but did have cpus assigned to
it.
-Nish
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Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages
On Fri 03-04-15 10:43:57, Nishanth Aravamudan wrote:
> On 31.03.2015 [11:48:29 +0200], Michal Hocko wrote:
[...]
> > I would expect kswapd would be looping endlessly because the zone
> > wouldn't be balanced obviously. But I would be wrong... because
> > pgdat_balanced is doing this:
> > /*
> > * A special case here:
> > *
> > * balance_pgdat() skips over all_unreclaimable after
> > * DEF_PRIORITY. Effectively, it considers them balanced so
> > * they must be considered balanced here as well!
> > */
> > if (!zone_reclaimable(zone)) {
> > balanced_pages += zone->managed_pages;
> > continue;
> > }
> >
> > and zone_reclaimable is false for you as you didn't have any
> > zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it
> > would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those
> > zones right?) and so the kswapd would be woken up easily. So it looks
> > like a mess.
>
> My understanding, and I could easily be wrong, is that kswapd2 (node 2
> is the exhausted one) spins endlessly, because the reclaim logic sees
> that we are reclaiming from somewhere but the allocation request for
> node 2 (which is __GFP_THISNODE for hugepages, not GFP_THISNODE) will
> never complete, so we just continue to reclaim.
__GFP_THISNODE would be waking up kswapd2 again and again, that is true.
I am just wondering whether we will have any __GFP_THISNODE allocations
for a node without CPUs (numa_node_id() shouldn't return such a node
AFAICS). Maybe if somebody is bound to Node2 explicitly but I would
consider this as a misconfiguration.
--
Michal Hocko
SUSE Labs
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Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages
On 03.04.2015 [09:57:35 +0200], Vlastimil Babka wrote:
> On 03/31/2015 11:48 AM, Michal Hocko wrote:
> >On Fri 27-03-15 15:23:50, Nishanth Aravamudan wrote:
> >>On 27.03.2015 [13:17:59 -0700], Dave Hansen wrote:
> >>>On 03/27/2015 12:28 PM, Nishanth Aravamudan wrote:
> @@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t
> *pgdat)
>
> for (i = 0; i <= ZONE_NORMAL; i++) {
> zone = &pgdat->node_zones[i];
> - if (!populated_zone(zone))
> + if (!populated_zone(zone) || !zone_reclaimable(zone))
> continue;
>
> pfmemalloc_reserve += min_wmark_pages(zone);
> >>>
> >>>Do you really want zone_reclaimable()? Or do you want something more
> >>>direct like "zone_reclaimable_pages(zone) == 0"?
> >>
> >>Yeah, I guess in my testing this worked out to be the same, since
> >>zone_reclaimable_pages(zone) is 0 and so zone_reclaimable(zone) will
> >>always be false. Thanks!
> >>
> >>Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc
> >>reserves if node has no ZONE_NORMAL") from Mel.
> >>
> >>We have a system with the following topology:
> >>
> >># numactl -H
> >>available: 3 nodes (0,2-3)
> >>node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
> >>23 24 25 26 27 28 29 30 31
> >>node 0 size: 28273 MB
> >>node 0 free: 27323 MB
> >>node 2 cpus:
> >>node 2 size: 16384 MB
> >>node 2 free: 0 MB
> >>node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
> >>node 3 size: 30533 MB
> >>node 3 free: 13273 MB
> >>node distances:
> >>node 0 2 3
> >> 0: 10 20 20
> >> 2: 20 10 20
> >> 3: 20 20 10
> >>
> >>Node 2 has no free memory, because:
> >># cat
> >>/sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages
> >>1
> >>
> >>This leads to the following zoneinfo:
> >>
> >>Node 2, zone DMA
> >> pages free 0
> >> min 1840
> >> low 2300
> >> high 2760
> >> scanned 0
> >> spanned 262144
> >> present 262144
> >> managed 262144
> >>...
> >> all_unreclaimable: 1
> >
> >Blee, this is a weird configuration.
> >
> >>If one then attempts to allocate some normal 16M hugepages via
> >>
> >>echo 37 > /proc/sys/vm/nr_hugepages
> >>
> >>The echo never returns and kswapd2 consumes CPU cycles.
> >>
> >>This is because throttle_direct_reclaim ends up calling
> >>wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...).
> >>pfmemalloc_watermark_ok() in turn checks all zones on the node if there
> >>are any reserves, and if so, then indicates the watermarks are ok, by
> >>seeing if there are sufficient free pages.
> >>
> >>675becce15 added a condition already for memoryless nodes. In this case,
> >>though, the node has memory, it is just all consumed (and not
> >>reclaimable). Effectively, though, the result is the same on this call
> >>to pfmemalloc_watermark_ok() and thus seems like a reasonable additional
> >>condition.
> >>
> >>With this change, the afore-mentioned 16M hugepage allocation attempt
> >>succeeds and correctly round-robins between Nodes 1 and 3.
> >
> >I am just wondering whether this is the right/complete fix. Don't we
> >need a similar treatment at more places?
> >I would expect kswapd would be looping endlessly because the zone
> >wouldn't be balanced obviously. But I would be wrong... because
> >pgdat_balanced is doing this:
> > /*
> > * A special case here:
> > *
> > * balance_pgdat() skips over all_unreclaimable after
> > * DEF_PRIORITY. Effectively, it considers them balanced so
> > * they must be considered balanced here as well!
> > */
> > if (!zone_reclaimable(zone)) {
> > balanced_pages += zone->managed_pages;
> > continue;
> > }
> >
> >and zone_reclaimable is false for you as you didn't have any
> >zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it
> >would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those
> >zones right?) and so the kswapd would be woken up easily. So it looks
> >like a mess.
>
> Yeah, looks like a much cleaner/complete solution would be to remove
> such zones from zonelists. But that means covering all situations
> when these hugepages are allocated/removed and the approach then
> looks similar to memory hotplug.
> Also I'm not sure if the ability to actually allocate the reserved
> hugepage would be impossible due to not being reachable by a
> zonelist...
>
> >There are possibly other places which rely on populated_zone or
> >for_each_populated_zone without checking reclaimability. Are those
> >working as expected?
>
> Yeah. At least the wakeup_kswapd case should be fixed IMHO. No point
> in waking it up just to let it immediately go to sleep again.
>
> >That being said. I am not objecting to this pa
Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages
On 31.03.2015 [11:48:29 +0200], Michal Hocko wrote:
> On Fri 27-03-15 15:23:50, Nishanth Aravamudan wrote:
> > On 27.03.2015 [13:17:59 -0700], Dave Hansen wrote:
> > > On 03/27/2015 12:28 PM, Nishanth Aravamudan wrote:
> > > > @@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t
> > > > *pgdat)
> > > >
> > > > for (i = 0; i <= ZONE_NORMAL; i++) {
> > > > zone = &pgdat->node_zones[i];
> > > > - if (!populated_zone(zone))
> > > > + if (!populated_zone(zone) || !zone_reclaimable(zone))
> > > > continue;
> > > >
> > > > pfmemalloc_reserve += min_wmark_pages(zone);
> > >
> > > Do you really want zone_reclaimable()? Or do you want something more
> > > direct like "zone_reclaimable_pages(zone) == 0"?
> >
> > Yeah, I guess in my testing this worked out to be the same, since
> > zone_reclaimable_pages(zone) is 0 and so zone_reclaimable(zone) will
> > always be false. Thanks!
> >
> > Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc
> > reserves if node has no ZONE_NORMAL") from Mel.
> >
> > We have a system with the following topology:
> >
> > # numactl -H
> > available: 3 nodes (0,2-3)
> > node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
> > 23 24 25 26 27 28 29 30 31
> > node 0 size: 28273 MB
> > node 0 free: 27323 MB
> > node 2 cpus:
> > node 2 size: 16384 MB
> > node 2 free: 0 MB
> > node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
> > node 3 size: 30533 MB
> > node 3 free: 13273 MB
> > node distances:
> > node 0 2 3
> > 0: 10 20 20
> > 2: 20 10 20
> > 3: 20 20 10
> >
> > Node 2 has no free memory, because:
> > # cat
> > /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages
> > 1
> >
> > This leads to the following zoneinfo:
> >
> > Node 2, zone DMA
> > pages free 0
> > min 1840
> > low 2300
> > high 2760
> > scanned 0
> > spanned 262144
> > present 262144
> > managed 262144
> > ...
> > all_unreclaimable: 1
>
> Blee, this is a weird configuration.
Yep, super gross. It's relatively rare in the field, thankfully. But 16G
pages definitely make it pretty likely to hit (as in, I've seen it once
before :)
> > If one then attempts to allocate some normal 16M hugepages via
> >
> > echo 37 > /proc/sys/vm/nr_hugepages
> >
> > The echo never returns and kswapd2 consumes CPU cycles.
> >
> > This is because throttle_direct_reclaim ends up calling
> > wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...).
> > pfmemalloc_watermark_ok() in turn checks all zones on the node if there
> > are any reserves, and if so, then indicates the watermarks are ok, by
> > seeing if there are sufficient free pages.
> >
> > 675becce15 added a condition already for memoryless nodes. In this case,
> > though, the node has memory, it is just all consumed (and not
> > reclaimable). Effectively, though, the result is the same on this call
> > to pfmemalloc_watermark_ok() and thus seems like a reasonable additional
> > condition.
> >
> > With this change, the afore-mentioned 16M hugepage allocation attempt
> > succeeds and correctly round-robins between Nodes 1 and 3.
>
> I am just wondering whether this is the right/complete fix. Don't we
> need a similar treatment at more places?
Almost certainly needs an audit. Exhausted nodes are tough to reproduce
easily (fully exhausted, that is), for me.
> I would expect kswapd would be looping endlessly because the zone
> wouldn't be balanced obviously. But I would be wrong... because
> pgdat_balanced is doing this:
> /*
>* A special case here:
>*
>* balance_pgdat() skips over all_unreclaimable after
>* DEF_PRIORITY. Effectively, it considers them balanced so
>* they must be considered balanced here as well!
>*/
> if (!zone_reclaimable(zone)) {
> balanced_pages += zone->managed_pages;
> continue;
> }
>
> and zone_reclaimable is false for you as you didn't have any
> zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it
> would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those
> zones right?) and so the kswapd would be woken up easily. So it looks
> like a mess.
My understanding, and I could easily be wrong, is that kswapd2 (node 2
is the exhausted one) spins endlessly, because the reclaim logic sees
that we are reclaiming from somewhere but the allocation request for
node 2 (which is __GFP_THISNODE for hugepages, not GFP_THISNODE) will
never complete, so we just continue to reclaim.
> There are possibly other places which rely on populated_zone or
> for_each_populated_zone without checking reclaimability. Are those
> working as expected?
Not yet verified, admittedly.
> That
Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages
On 03/31/2015 11:48 AM, Michal Hocko wrote:
On Fri 27-03-15 15:23:50, Nishanth Aravamudan wrote:
On 27.03.2015 [13:17:59 -0700], Dave Hansen wrote:
On 03/27/2015 12:28 PM, Nishanth Aravamudan wrote:
@@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
for (i = 0; i <= ZONE_NORMAL; i++) {
zone = &pgdat->node_zones[i];
- if (!populated_zone(zone))
+ if (!populated_zone(zone) || !zone_reclaimable(zone))
continue;
pfmemalloc_reserve += min_wmark_pages(zone);
Do you really want zone_reclaimable()? Or do you want something more
direct like "zone_reclaimable_pages(zone) == 0"?
Yeah, I guess in my testing this worked out to be the same, since
zone_reclaimable_pages(zone) is 0 and so zone_reclaimable(zone) will
always be false. Thanks!
Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc
reserves if node has no ZONE_NORMAL") from Mel.
We have a system with the following topology:
# numactl -H
available: 3 nodes (0,2-3)
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
23 24 25 26 27 28 29 30 31
node 0 size: 28273 MB
node 0 free: 27323 MB
node 2 cpus:
node 2 size: 16384 MB
node 2 free: 0 MB
node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
node 3 size: 30533 MB
node 3 free: 13273 MB
node distances:
node 0 2 3
0: 10 20 20
2: 20 10 20
3: 20 20 10
Node 2 has no free memory, because:
# cat /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages
1
This leads to the following zoneinfo:
Node 2, zone DMA
pages free 0
min 1840
low 2300
high 2760
scanned 0
spanned 262144
present 262144
managed 262144
...
all_unreclaimable: 1
Blee, this is a weird configuration.
If one then attempts to allocate some normal 16M hugepages via
echo 37 > /proc/sys/vm/nr_hugepages
The echo never returns and kswapd2 consumes CPU cycles.
This is because throttle_direct_reclaim ends up calling
wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...).
pfmemalloc_watermark_ok() in turn checks all zones on the node if there
are any reserves, and if so, then indicates the watermarks are ok, by
seeing if there are sufficient free pages.
675becce15 added a condition already for memoryless nodes. In this case,
though, the node has memory, it is just all consumed (and not
reclaimable). Effectively, though, the result is the same on this call
to pfmemalloc_watermark_ok() and thus seems like a reasonable additional
condition.
With this change, the afore-mentioned 16M hugepage allocation attempt
succeeds and correctly round-robins between Nodes 1 and 3.
I am just wondering whether this is the right/complete fix. Don't we
need a similar treatment at more places?
I would expect kswapd would be looping endlessly because the zone
wouldn't be balanced obviously. But I would be wrong... because
pgdat_balanced is doing this:
/*
* A special case here:
*
* balance_pgdat() skips over all_unreclaimable after
* DEF_PRIORITY. Effectively, it considers them balanced so
* they must be considered balanced here as well!
*/
if (!zone_reclaimable(zone)) {
balanced_pages += zone->managed_pages;
continue;
}
and zone_reclaimable is false for you as you didn't have any
zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it
would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those
zones right?) and so the kswapd would be woken up easily. So it looks
like a mess.
Yeah, looks like a much cleaner/complete solution would be to remove
such zones from zonelists. But that means covering all situations when
these hugepages are allocated/removed and the approach then looks
similar to memory hotplug.
Also I'm not sure if the ability to actually allocate the reserved
hugepage would be impossible due to not being reachable by a zonelist...
There are possibly other places which rely on populated_zone or
for_each_populated_zone without checking reclaimability. Are those
working as expected?
Yeah. At least the wakeup_kswapd case should be fixed IMHO. No point in
waking it up just to let it immediately go to sleep again.
That being said. I am not objecting to this patch. I am just trying to
wrap my head around possible issues from such a weird configuration and
all the consequences.
Signed-off-by: Nishanth Aravamudan
The patch as is doesn't seem to be harmful.
Reviewed-by: Michal Hocko
---
v1 -> v2:
Check against zone_reclaimable_pages, rather zone_reclaimable, based
upon feedback from Dave Hansen.
Dunno, but shouldn't we use the same thing here and in pgdat_balanced?
zone_reclaimable_pages seems to be used only
Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages
On Fri 27-03-15 15:23:50, Nishanth Aravamudan wrote:
> On 27.03.2015 [13:17:59 -0700], Dave Hansen wrote:
> > On 03/27/2015 12:28 PM, Nishanth Aravamudan wrote:
> > > @@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t
> > > *pgdat)
> > >
> > > for (i = 0; i <= ZONE_NORMAL; i++) {
> > > zone = &pgdat->node_zones[i];
> > > - if (!populated_zone(zone))
> > > + if (!populated_zone(zone) || !zone_reclaimable(zone))
> > > continue;
> > >
> > > pfmemalloc_reserve += min_wmark_pages(zone);
> >
> > Do you really want zone_reclaimable()? Or do you want something more
> > direct like "zone_reclaimable_pages(zone) == 0"?
>
> Yeah, I guess in my testing this worked out to be the same, since
> zone_reclaimable_pages(zone) is 0 and so zone_reclaimable(zone) will
> always be false. Thanks!
>
> Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc
> reserves if node has no ZONE_NORMAL") from Mel.
>
> We have a system with the following topology:
>
> # numactl -H
> available: 3 nodes (0,2-3)
> node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
> 23 24 25 26 27 28 29 30 31
> node 0 size: 28273 MB
> node 0 free: 27323 MB
> node 2 cpus:
> node 2 size: 16384 MB
> node 2 free: 0 MB
> node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
> node 3 size: 30533 MB
> node 3 free: 13273 MB
> node distances:
> node 0 2 3
> 0: 10 20 20
> 2: 20 10 20
> 3: 20 20 10
>
> Node 2 has no free memory, because:
> # cat
> /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages
> 1
>
> This leads to the following zoneinfo:
>
> Node 2, zone DMA
> pages free 0
> min 1840
> low 2300
> high 2760
> scanned 0
> spanned 262144
> present 262144
> managed 262144
> ...
> all_unreclaimable: 1
Blee, this is a weird configuration.
> If one then attempts to allocate some normal 16M hugepages via
>
> echo 37 > /proc/sys/vm/nr_hugepages
>
> The echo never returns and kswapd2 consumes CPU cycles.
>
> This is because throttle_direct_reclaim ends up calling
> wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...).
> pfmemalloc_watermark_ok() in turn checks all zones on the node if there
> are any reserves, and if so, then indicates the watermarks are ok, by
> seeing if there are sufficient free pages.
>
> 675becce15 added a condition already for memoryless nodes. In this case,
> though, the node has memory, it is just all consumed (and not
> reclaimable). Effectively, though, the result is the same on this call
> to pfmemalloc_watermark_ok() and thus seems like a reasonable additional
> condition.
>
> With this change, the afore-mentioned 16M hugepage allocation attempt
> succeeds and correctly round-robins between Nodes 1 and 3.
I am just wondering whether this is the right/complete fix. Don't we
need a similar treatment at more places?
I would expect kswapd would be looping endlessly because the zone
wouldn't be balanced obviously. But I would be wrong... because
pgdat_balanced is doing this:
/*
* A special case here:
*
* balance_pgdat() skips over all_unreclaimable after
* DEF_PRIORITY. Effectively, it considers them balanced so
* they must be considered balanced here as well!
*/
if (!zone_reclaimable(zone)) {
balanced_pages += zone->managed_pages;
continue;
}
and zone_reclaimable is false for you as you didn't have any
zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it
would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those
zones right?) and so the kswapd would be woken up easily. So it looks
like a mess.
There are possibly other places which rely on populated_zone or
for_each_populated_zone without checking reclaimability. Are those
working as expected?
That being said. I am not objecting to this patch. I am just trying to
wrap my head around possible issues from such a weird configuration and
all the consequences.
> Signed-off-by: Nishanth Aravamudan
The patch as is doesn't seem to be harmful.
Reviewed-by: Michal Hocko
> ---
> v1 -> v2:
> Check against zone_reclaimable_pages, rather zone_reclaimable, based
> upon feedback from Dave Hansen.
Dunno, but shouldn't we use the same thing here and in pgdat_balanced?
zone_reclaimable_pages seems to be used only from zone_reclaimable().
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 5e8eadd71bac..c627fa4c991f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2646,7 +2646,8 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
>
> for (i = 0; i <= ZONE_NORMAL; i++) {
> zone = &pgdat->node_zones[i];
> - if (!populated_zone(zone))
> +
[PATCH v2] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages
On 27.03.2015 [13:17:59 -0700], Dave Hansen wrote:
> On 03/27/2015 12:28 PM, Nishanth Aravamudan wrote:
> > @@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
> >
> > for (i = 0; i <= ZONE_NORMAL; i++) {
> > zone = &pgdat->node_zones[i];
> > - if (!populated_zone(zone))
> > + if (!populated_zone(zone) || !zone_reclaimable(zone))
> > continue;
> >
> > pfmemalloc_reserve += min_wmark_pages(zone);
>
> Do you really want zone_reclaimable()? Or do you want something more
> direct like "zone_reclaimable_pages(zone) == 0"?
Yeah, I guess in my testing this worked out to be the same, since
zone_reclaimable_pages(zone) is 0 and so zone_reclaimable(zone) will
always be false. Thanks!
Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc
reserves if node has no ZONE_NORMAL") from Mel.
We have a system with the following topology:
# numactl -H
available: 3 nodes (0,2-3)
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
23 24 25 26 27 28 29 30 31
node 0 size: 28273 MB
node 0 free: 27323 MB
node 2 cpus:
node 2 size: 16384 MB
node 2 free: 0 MB
node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
node 3 size: 30533 MB
node 3 free: 13273 MB
node distances:
node 0 2 3
0: 10 20 20
2: 20 10 20
3: 20 20 10
Node 2 has no free memory, because:
# cat /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages
1
This leads to the following zoneinfo:
Node 2, zone DMA
pages free 0
min 1840
low 2300
high 2760
scanned 0
spanned 262144
present 262144
managed 262144
...
all_unreclaimable: 1
If one then attempts to allocate some normal 16M hugepages via
echo 37 > /proc/sys/vm/nr_hugepages
The echo never returns and kswapd2 consumes CPU cycles.
This is because throttle_direct_reclaim ends up calling
wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...).
pfmemalloc_watermark_ok() in turn checks all zones on the node if there
are any reserves, and if so, then indicates the watermarks are ok, by
seeing if there are sufficient free pages.
675becce15 added a condition already for memoryless nodes. In this case,
though, the node has memory, it is just all consumed (and not
reclaimable). Effectively, though, the result is the same on this call
to pfmemalloc_watermark_ok() and thus seems like a reasonable additional
condition.
With this change, the afore-mentioned 16M hugepage allocation attempt
succeeds and correctly round-robins between Nodes 1 and 3.
Signed-off-by: Nishanth Aravamudan
---
v1 -> v2:
Check against zone_reclaimable_pages, rather zone_reclaimable, based
upon feedback from Dave Hansen.
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5e8eadd71bac..c627fa4c991f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2646,7 +2646,8 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
for (i = 0; i <= ZONE_NORMAL; i++) {
zone = &pgdat->node_zones[i];
- if (!populated_zone(zone))
+ if (!populated_zone(zone) ||
+ zone_reclaimable_pages(zone) == 0)
continue;
pfmemalloc_reserve += min_wmark_pages(zone);
--
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