On Fri, Sep 19 2014, Johannes Weiner wrote:
> In a memcg with even just moderate cache pressure, success rates for
> transparent huge page allocations drop to zero, wasting a lot of
> effort that the allocator puts into assembling these pages.
>
> The reason for this is that the memcg reclaim code was never designed
> for higher-order charges. It reclaims in small batches until there is
> room for at least one page. Huge pages charges only succeed when
> these batches add up over a series of huge faults, which is unlikely
> under any significant load involving order-0 allocations in the group.
>
> Remove that loop on the memcg side in favor of passing the actual
> reclaim goal to direct reclaim, which is already set up and optimized
> to meet higher-order goals efficiently.
>
> This brings memcg's THP policy in line with the system policy: if the
> allocator painstakingly assembles a hugepage, memcg will at least make
> an honest effort to charge it. As a result, transparent hugepage
> allocation rates amid cache activity are drastically improved:
>
> vanilla patched
> pgalloc 4717530.80 ( +0.00%) 4451376.40 ( -5.64%)
> pgfault 491370.60 ( +0.00%) 225477.40 ( -54.11%)
> pgmajfault 2.00 ( +0.00%) 1.80 ( -6.67%)
> thp_fault_alloc 0.00 ( +0.00%) 531.60 (+100.00%)
> thp_fault_fallback 749.00 ( +0.00%) 217.40 ( -70.88%)
>
> [ Note: this may in turn increase memory consumption from internal
> fragmentation, which is an inherent risk of transparent hugepages.
> Some setups may have to adjust the memcg limits accordingly to
> accomodate this - or, if the machine is already packed to capacity,
> disable the transparent huge page feature. ]
We're using an earlier version of this patch, so I approve of the
general direction. But I have some feedback.
The memsw aspect of this change seems somewhat separate. Can it be
split into a different patch?
The memsw aspect of this patch seems to change behavior. Is this
intended? If so, a mention of it in the commit log would assuage the
reader. I'll explain... Assume a machine with swap enabled and
res.limit==memsw.limit, thus memsw_is_minimum is true. My understanding
is that memsw.usage represents sum(ram_usage, swap_usage). So when
memsw_is_minimum=true, then both swap_usage=0 and
memsw.usage==res.usage. In this condition, if res usage is at limit
then there's no point in swapping because memsw.usage is already
maximal. Prior to this patch I think the kernel did the right thing,
but not afterwards.
Before this patch:
if res.usage == res.limit, try_charge() indirectly calls
try_to_free_mem_cgroup_pages(noswap=true)
After this patch:
if res.usage == res.limit, try_charge() calls
try_to_free_mem_cgroup_pages(may_swap=true)
Notice the inverted swap-is-allowed value.
I haven't had time to look at your other outstanding memcg patches.
These comments were made with this patch in isolation.
> Signed-off-by: Johannes Weiner <han...@cmpxchg.org>
> ---
> include/linux/swap.h | 6 ++--
> mm/memcontrol.c | 86
> +++++++++++-----------------------------------------
> mm/vmscan.c | 7 +++--
> 3 files changed, 25 insertions(+), 74 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index ea4f926e6b9b..37a585beef5c 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -327,8 +327,10 @@ extern void lru_cache_add_active_or_unevictable(struct
> page *page,
> extern unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
> gfp_t gfp_mask, nodemask_t *mask);
> extern int __isolate_lru_page(struct page *page, isolate_mode_t mode);
> -extern unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem,
> - gfp_t gfp_mask, bool noswap);
> +extern unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
> + unsigned long nr_pages,
> + gfp_t gfp_mask,
> + bool may_swap);
> extern unsigned long mem_cgroup_shrink_node_zone(struct mem_cgroup *mem,
> gfp_t gfp_mask, bool noswap,
> struct zone *zone,
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 9431024e490c..e2def11f1ec1 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -315,9 +315,6 @@ struct mem_cgroup {
> /* OOM-Killer disable */
> int oom_kill_disable;
>
> - /* set when res.limit == memsw.limit */
> - bool memsw_is_minimum;
> -
> /* protect arrays of thresholds */
> struct mutex thresholds_lock;
>
> @@ -481,14 +478,6 @@ enum res_type {
> #define OOM_CONTROL (0)
>
> /*
> - * Reclaim flags for mem_cgroup_hierarchical_reclaim
> - */
> -#define MEM_CGROUP_RECLAIM_NOSWAP_BIT 0x0
> -#define MEM_CGROUP_RECLAIM_NOSWAP (1 << MEM_CGROUP_RECLAIM_NOSWAP_BIT)
> -#define MEM_CGROUP_RECLAIM_SHRINK_BIT 0x1
> -#define MEM_CGROUP_RECLAIM_SHRINK (1 << MEM_CGROUP_RECLAIM_SHRINK_BIT)
> -
> -/*
> * The memcg_create_mutex will be held whenever a new cgroup is created.
> * As a consequence, any change that needs to protect against new child
> cgroups
> * appearing has to hold it as well.
> @@ -1794,42 +1783,6 @@ static void mem_cgroup_out_of_memory(struct mem_cgroup
> *memcg, gfp_t gfp_mask,
> NULL, "Memory cgroup out of memory");
> }
>
> -static unsigned long mem_cgroup_reclaim(struct mem_cgroup *memcg,
> - gfp_t gfp_mask,
> - unsigned long flags)
> -{
> - unsigned long total = 0;
> - bool noswap = false;
> - int loop;
> -
> - if (flags & MEM_CGROUP_RECLAIM_NOSWAP)
> - noswap = true;
> - if (!(flags & MEM_CGROUP_RECLAIM_SHRINK) && memcg->memsw_is_minimum)
> - noswap = true;
> -
> - for (loop = 0; loop < MEM_CGROUP_MAX_RECLAIM_LOOPS; loop++) {
> - if (loop)
> - drain_all_stock_async(memcg);
> - total += try_to_free_mem_cgroup_pages(memcg, gfp_mask, noswap);
> - /*
> - * Allow limit shrinkers, which are triggered directly
> - * by userspace, to catch signals and stop reclaim
> - * after minimal progress, regardless of the margin.
> - */
> - if (total && (flags & MEM_CGROUP_RECLAIM_SHRINK))
> - break;
> - if (mem_cgroup_margin(memcg))
> - break;
> - /*
> - * If nothing was reclaimed after two attempts, there
> - * may be no reclaimable pages in this hierarchy.
> - */
> - if (loop && !total)
> - break;
> - }
> - return total;
> -}
> -
> /**
> * test_mem_cgroup_node_reclaimable
> * @memcg: the target memcg
> @@ -2532,8 +2485,9 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t
> gfp_mask,
> struct mem_cgroup *mem_over_limit;
> struct res_counter *fail_res;
> unsigned long nr_reclaimed;
> - unsigned long flags = 0;
> unsigned long long size;
> + bool may_swap = true;
> + bool drained = false;
> int ret = 0;
>
> if (mem_cgroup_is_root(memcg))
> @@ -2550,7 +2504,7 @@ retry:
> goto done_restock;
> res_counter_uncharge(&memcg->res, size);
> mem_over_limit = mem_cgroup_from_res_counter(fail_res, memsw);
> - flags |= MEM_CGROUP_RECLAIM_NOSWAP;
> + may_swap = false;
> } else
> mem_over_limit = mem_cgroup_from_res_counter(fail_res, res);
>
> @@ -2576,11 +2530,18 @@ retry:
> if (!(gfp_mask & __GFP_WAIT))
> goto nomem;
>
> - nr_reclaimed = mem_cgroup_reclaim(mem_over_limit, gfp_mask, flags);
> + nr_reclaimed = try_to_free_mem_cgroup_pages(mem_over_limit, nr_pages,
> + gfp_mask, may_swap);
>
> if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
> goto retry;
>
> + if (!drained) {
> + drain_all_stock_async(mem_over_limit);
> + drained = true;
> + goto retry;
> + }
> +
> if (gfp_mask & __GFP_NORETRY)
> goto nomem;
> /*
> @@ -3655,19 +3616,13 @@ static int mem_cgroup_resize_limit(struct mem_cgroup
> *memcg,
> enlarge = 1;
>
> ret = res_counter_set_limit(&memcg->res, val);
> - if (!ret) {
> - if (memswlimit == val)
> - memcg->memsw_is_minimum = true;
> - else
> - memcg->memsw_is_minimum = false;
> - }
> mutex_unlock(&set_limit_mutex);
>
> if (!ret)
> break;
>
> - mem_cgroup_reclaim(memcg, GFP_KERNEL,
> - MEM_CGROUP_RECLAIM_SHRINK);
> + try_to_free_mem_cgroup_pages(memcg, 1, GFP_KERNEL, true);
> +
> curusage = res_counter_read_u64(&memcg->res, RES_USAGE);
> /* Usage is reduced ? */
> if (curusage >= oldusage)
> @@ -3714,20 +3669,13 @@ static int mem_cgroup_resize_memsw_limit(struct
> mem_cgroup *memcg,
> if (memswlimit < val)
> enlarge = 1;
> ret = res_counter_set_limit(&memcg->memsw, val);
> - if (!ret) {
> - if (memlimit == val)
> - memcg->memsw_is_minimum = true;
> - else
> - memcg->memsw_is_minimum = false;
> - }
> mutex_unlock(&set_limit_mutex);
>
> if (!ret)
> break;
>
> - mem_cgroup_reclaim(memcg, GFP_KERNEL,
> - MEM_CGROUP_RECLAIM_NOSWAP |
> - MEM_CGROUP_RECLAIM_SHRINK);
> + try_to_free_mem_cgroup_pages(memcg, 1, GFP_KERNEL, false);
> +
> curusage = res_counter_read_u64(&memcg->memsw, RES_USAGE);
> /* Usage is reduced ? */
> if (curusage >= oldusage)
> @@ -3976,8 +3924,8 @@ static int mem_cgroup_force_empty(struct mem_cgroup
> *memcg)
> if (signal_pending(current))
> return -EINTR;
>
> - progress = try_to_free_mem_cgroup_pages(memcg, GFP_KERNEL,
> - false);
> + progress = try_to_free_mem_cgroup_pages(memcg, 1,
> + GFP_KERNEL, true);
> if (!progress) {
> nr_retries--;
> /* maybe some writeback is necessary */
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b672e2c6becc..97d31ec17d06 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2759,21 +2759,22 @@ unsigned long mem_cgroup_shrink_node_zone(struct
> mem_cgroup *memcg,
> }
>
> unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
> + unsigned long nr_pages,
> gfp_t gfp_mask,
> - bool noswap)
> + bool may_swap)
> {
> struct zonelist *zonelist;
> unsigned long nr_reclaimed;
> int nid;
> struct scan_control sc = {
> - .nr_to_reclaim = SWAP_CLUSTER_MAX,
> + .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
> .gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
> (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK),
> .target_mem_cgroup = memcg,
> .priority = DEF_PRIORITY,
> .may_writepage = !laptop_mode,
> .may_unmap = 1,
> - .may_swap = !noswap,
> + .may_swap = may_swap,
> };
>
> /*
--
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/
