Re: [PATCH 13/19] mm/migrate: Use xchg instead of spinlock
On Tue, Jun 05, 2018 at 12:24:39AM -0700, Srikar Dronamraju wrote:
> * Peter Zijlstra [2018-06-04 21:28:21]:
>
> > > if (time_after(jiffies, pgdat->numabalancing_migrate_next_window)) {
> > > - spin_lock(>numabalancing_migrate_lock);
> > > - pgdat->numabalancing_migrate_nr_pages = 0;
> > > - pgdat->numabalancing_migrate_next_window = jiffies +
> > > - msecs_to_jiffies(migrate_interval_millisecs);
> > > - spin_unlock(>numabalancing_migrate_lock);
> > > + if (xchg(>numabalancing_migrate_nr_pages, 0))
> > > + pgdat->numabalancing_migrate_next_window = jiffies +
> > > + msecs_to_jiffies(migrate_interval_millisecs);
> >
> > Note that both are in fact wrong. That wants to be something like:
> >
> > pgdat->numabalancing_migrate_next_window += interval;
> >
> > Otherwise you stretch every interval by 'jiffies -
> > numabalancing_migrate_next_window'.
>
> Okay, I get your point.
Note that in practise it probably doesn't matter, but it just upsets my
OCD ;-)
> > Also, that all wants READ_ONCE/WRITE_ONCE, irrespective of the
> > spinlock/xchg.
> unsigned long interval = READ_ONCE(pgdat->numabalancing_migrate_next_window);
>
> if (time_after(jiffies, interval)) {
> interval += msecs_to_jiffies(migrate_interval_millisecs));
> if (xchg(>numabalancing_migrate_nr_pages, 0))
> WRITE_ONCE(pgdat->numabalancing_migrate_next_window, interval);
> }
>
> Something like this?
Almost, you forgot about the case where 'jiffies -
numabalancing_migrate_next_window > interval'.
That wants to be something like:
unsigned long timo = READ_ONCE(stupid_long_name);
if (time_after(jiffies, timo) && xchg(_long_name, 0)) {
do {
timo += msec_to_jiffies(..);
} while (unlikely(time_after(jiffies, timo);
WRITE_ONCE(stupid_long_name, timo);
}
Re: [PATCH 13/19] mm/migrate: Use xchg instead of spinlock
On Tue, Jun 05, 2018 at 12:24:39AM -0700, Srikar Dronamraju wrote:
> * Peter Zijlstra [2018-06-04 21:28:21]:
>
> > > if (time_after(jiffies, pgdat->numabalancing_migrate_next_window)) {
> > > - spin_lock(>numabalancing_migrate_lock);
> > > - pgdat->numabalancing_migrate_nr_pages = 0;
> > > - pgdat->numabalancing_migrate_next_window = jiffies +
> > > - msecs_to_jiffies(migrate_interval_millisecs);
> > > - spin_unlock(>numabalancing_migrate_lock);
> > > + if (xchg(>numabalancing_migrate_nr_pages, 0))
> > > + pgdat->numabalancing_migrate_next_window = jiffies +
> > > + msecs_to_jiffies(migrate_interval_millisecs);
> >
> > Note that both are in fact wrong. That wants to be something like:
> >
> > pgdat->numabalancing_migrate_next_window += interval;
> >
> > Otherwise you stretch every interval by 'jiffies -
> > numabalancing_migrate_next_window'.
>
> Okay, I get your point.
Note that in practise it probably doesn't matter, but it just upsets my
OCD ;-)
> > Also, that all wants READ_ONCE/WRITE_ONCE, irrespective of the
> > spinlock/xchg.
> unsigned long interval = READ_ONCE(pgdat->numabalancing_migrate_next_window);
>
> if (time_after(jiffies, interval)) {
> interval += msecs_to_jiffies(migrate_interval_millisecs));
> if (xchg(>numabalancing_migrate_nr_pages, 0))
> WRITE_ONCE(pgdat->numabalancing_migrate_next_window, interval);
> }
>
> Something like this?
Almost, you forgot about the case where 'jiffies -
numabalancing_migrate_next_window > interval'.
That wants to be something like:
unsigned long timo = READ_ONCE(stupid_long_name);
if (time_after(jiffies, timo) && xchg(_long_name, 0)) {
do {
timo += msec_to_jiffies(..);
} while (unlikely(time_after(jiffies, timo);
WRITE_ONCE(stupid_long_name, timo);
}
Re: [PATCH 13/19] mm/migrate: Use xchg instead of spinlock
* Peter Zijlstra [2018-06-04 21:28:21]:
> > if (time_after(jiffies, pgdat->numabalancing_migrate_next_window)) {
> > - spin_lock(>numabalancing_migrate_lock);
> > - pgdat->numabalancing_migrate_nr_pages = 0;
> > - pgdat->numabalancing_migrate_next_window = jiffies +
> > - msecs_to_jiffies(migrate_interval_millisecs);
> > - spin_unlock(>numabalancing_migrate_lock);
> > + if (xchg(>numabalancing_migrate_nr_pages, 0))
> > + pgdat->numabalancing_migrate_next_window = jiffies +
> > + msecs_to_jiffies(migrate_interval_millisecs);
>
> Note that both are in fact wrong. That wants to be something like:
>
> pgdat->numabalancing_migrate_next_window += interval;
>
> Otherwise you stretch every interval by 'jiffies -
> numabalancing_migrate_next_window'.
Okay, I get your point.
>
> Also, that all wants READ_ONCE/WRITE_ONCE, irrespective of the
> spinlock/xchg.
>
> I suppose the problem here is that PPC has a very nasty test-and-set
> spinlock with fwd progress issues while xchg maps to a fairly simple
> ll/sc that (hopefully) has some hardware fairness.
>
> And pgdata being a rather course data structure (per node?) there could
> be a lot of CPUs stomping on this here thing.
>
> So simpler not really, but better for PPC.
>
unsigned long interval = READ_ONCE(pgdat->numabalancing_migrate_next_window);
if (time_after(jiffies, interval)) {
interval += msecs_to_jiffies(migrate_interval_millisecs));
if (xchg(>numabalancing_migrate_nr_pages, 0))
WRITE_ONCE(pgdat->numabalancing_migrate_next_window, interval);
}
Something like this?
Re: [PATCH 13/19] mm/migrate: Use xchg instead of spinlock
* Peter Zijlstra [2018-06-04 21:28:21]:
> > if (time_after(jiffies, pgdat->numabalancing_migrate_next_window)) {
> > - spin_lock(>numabalancing_migrate_lock);
> > - pgdat->numabalancing_migrate_nr_pages = 0;
> > - pgdat->numabalancing_migrate_next_window = jiffies +
> > - msecs_to_jiffies(migrate_interval_millisecs);
> > - spin_unlock(>numabalancing_migrate_lock);
> > + if (xchg(>numabalancing_migrate_nr_pages, 0))
> > + pgdat->numabalancing_migrate_next_window = jiffies +
> > + msecs_to_jiffies(migrate_interval_millisecs);
>
> Note that both are in fact wrong. That wants to be something like:
>
> pgdat->numabalancing_migrate_next_window += interval;
>
> Otherwise you stretch every interval by 'jiffies -
> numabalancing_migrate_next_window'.
Okay, I get your point.
>
> Also, that all wants READ_ONCE/WRITE_ONCE, irrespective of the
> spinlock/xchg.
>
> I suppose the problem here is that PPC has a very nasty test-and-set
> spinlock with fwd progress issues while xchg maps to a fairly simple
> ll/sc that (hopefully) has some hardware fairness.
>
> And pgdata being a rather course data structure (per node?) there could
> be a lot of CPUs stomping on this here thing.
>
> So simpler not really, but better for PPC.
>
unsigned long interval = READ_ONCE(pgdat->numabalancing_migrate_next_window);
if (time_after(jiffies, interval)) {
interval += msecs_to_jiffies(migrate_interval_millisecs));
if (xchg(>numabalancing_migrate_nr_pages, 0))
WRITE_ONCE(pgdat->numabalancing_migrate_next_window, interval);
}
Something like this?
Re: [PATCH 13/19] mm/migrate: Use xchg instead of spinlock
On Mon, Jun 04, 2018 at 03:30:22PM +0530, Srikar Dronamraju wrote:
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 8c0af0f..1c55956 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1874,11 +1874,9 @@ static bool numamigrate_update_ratelimit(pg_data_t
> *pgdat,
>* all the time is being spent migrating!
>*/
> if (time_after(jiffies, pgdat->numabalancing_migrate_next_window)) {
> - spin_lock(>numabalancing_migrate_lock);
> - pgdat->numabalancing_migrate_nr_pages = 0;
> - pgdat->numabalancing_migrate_next_window = jiffies +
> - msecs_to_jiffies(migrate_interval_millisecs);
> - spin_unlock(>numabalancing_migrate_lock);
> + if (xchg(>numabalancing_migrate_nr_pages, 0))
> + pgdat->numabalancing_migrate_next_window = jiffies +
> + msecs_to_jiffies(migrate_interval_millisecs);
Note that both are in fact wrong. That wants to be something like:
pgdat->numabalancing_migrate_next_window += interval;
Otherwise you stretch every interval by 'jiffies -
numabalancing_migrate_next_window'.
Also, that all wants READ_ONCE/WRITE_ONCE, irrespective of the
spinlock/xchg.
I suppose the problem here is that PPC has a very nasty test-and-set
spinlock with fwd progress issues while xchg maps to a fairly simple
ll/sc that (hopefully) has some hardware fairness.
And pgdata being a rather course data structure (per node?) there could
be a lot of CPUs stomping on this here thing.
So simpler not really, but better for PPC.
> }
> if (pgdat->numabalancing_migrate_nr_pages > ratelimit_pages) {
> trace_mm_numa_migrate_ratelimit(current, pgdat->node_id,
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4526643..464a25c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6208,7 +6208,6 @@ static void __paginginit free_area_init_core(struct
> pglist_data *pgdat)
>
> pgdat_resize_init(pgdat);
> #ifdef CONFIG_NUMA_BALANCING
> - spin_lock_init(>numabalancing_migrate_lock);
> pgdat->numabalancing_migrate_nr_pages = 0;
> pgdat->active_node_migrate = 0;
> pgdat->numabalancing_migrate_next_window = jiffies;
> --
> 1.8.3.1
>
Re: [PATCH 13/19] mm/migrate: Use xchg instead of spinlock
On Mon, Jun 04, 2018 at 03:30:22PM +0530, Srikar Dronamraju wrote:
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 8c0af0f..1c55956 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1874,11 +1874,9 @@ static bool numamigrate_update_ratelimit(pg_data_t
> *pgdat,
>* all the time is being spent migrating!
>*/
> if (time_after(jiffies, pgdat->numabalancing_migrate_next_window)) {
> - spin_lock(>numabalancing_migrate_lock);
> - pgdat->numabalancing_migrate_nr_pages = 0;
> - pgdat->numabalancing_migrate_next_window = jiffies +
> - msecs_to_jiffies(migrate_interval_millisecs);
> - spin_unlock(>numabalancing_migrate_lock);
> + if (xchg(>numabalancing_migrate_nr_pages, 0))
> + pgdat->numabalancing_migrate_next_window = jiffies +
> + msecs_to_jiffies(migrate_interval_millisecs);
Note that both are in fact wrong. That wants to be something like:
pgdat->numabalancing_migrate_next_window += interval;
Otherwise you stretch every interval by 'jiffies -
numabalancing_migrate_next_window'.
Also, that all wants READ_ONCE/WRITE_ONCE, irrespective of the
spinlock/xchg.
I suppose the problem here is that PPC has a very nasty test-and-set
spinlock with fwd progress issues while xchg maps to a fairly simple
ll/sc that (hopefully) has some hardware fairness.
And pgdata being a rather course data structure (per node?) there could
be a lot of CPUs stomping on this here thing.
So simpler not really, but better for PPC.
> }
> if (pgdat->numabalancing_migrate_nr_pages > ratelimit_pages) {
> trace_mm_numa_migrate_ratelimit(current, pgdat->node_id,
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4526643..464a25c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6208,7 +6208,6 @@ static void __paginginit free_area_init_core(struct
> pglist_data *pgdat)
>
> pgdat_resize_init(pgdat);
> #ifdef CONFIG_NUMA_BALANCING
> - spin_lock_init(>numabalancing_migrate_lock);
> pgdat->numabalancing_migrate_nr_pages = 0;
> pgdat->active_node_migrate = 0;
> pgdat->numabalancing_migrate_next_window = jiffies;
> --
> 1.8.3.1
>
Re: [PATCH 13/19] mm/migrate: Use xchg instead of spinlock
On Mon, 2018-06-04 at 15:30 +0530, Srikar Dronamraju wrote:
>
> +++ b/mm/migrate.c
> @@ -1874,11 +1874,9 @@ static bool
> numamigrate_update_ratelimit(pg_data_t *pgdat,
>* all the time is being spent migrating!
>*/
> if (time_after(jiffies, pgdat-
> >numabalancing_migrate_next_window)) {
> - spin_lock(>numabalancing_migrate_lock);
> - pgdat->numabalancing_migrate_nr_pages = 0;
> - pgdat->numabalancing_migrate_next_window = jiffies +
> - msecs_to_jiffies(migrate_interval_millisecs)
> ;
> - spin_unlock(>numabalancing_migrate_lock);
> + if (xchg(>numabalancing_migrate_nr_pages, 0))
> + pgdat->numabalancing_migrate_next_window =
> jiffies +
> + msecs_to_jiffies(migrate_interval_mi
> llisecs);
> }
I am not convinced this is simpler, but no real
objection either way :)
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Re: [PATCH 13/19] mm/migrate: Use xchg instead of spinlock
On Mon, 2018-06-04 at 15:30 +0530, Srikar Dronamraju wrote:
>
> +++ b/mm/migrate.c
> @@ -1874,11 +1874,9 @@ static bool
> numamigrate_update_ratelimit(pg_data_t *pgdat,
>* all the time is being spent migrating!
>*/
> if (time_after(jiffies, pgdat-
> >numabalancing_migrate_next_window)) {
> - spin_lock(>numabalancing_migrate_lock);
> - pgdat->numabalancing_migrate_nr_pages = 0;
> - pgdat->numabalancing_migrate_next_window = jiffies +
> - msecs_to_jiffies(migrate_interval_millisecs)
> ;
> - spin_unlock(>numabalancing_migrate_lock);
> + if (xchg(>numabalancing_migrate_nr_pages, 0))
> + pgdat->numabalancing_migrate_next_window =
> jiffies +
> + msecs_to_jiffies(migrate_interval_mi
> llisecs);
> }
I am not convinced this is simpler, but no real
objection either way :)
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