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Date: Mon, 23 Jan 2012 11:14:23 -0800
From: Ying Han <yinghan@...gle.com>
To: Michal Hocko <mhocko@...e.cz>
Cc: Hillf Danton <dhillf@...il.com>, linux-mm@...ck.org,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Hugh Dickins <hughd@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>,
Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] mm: memcg: fix over reclaiming mem cgroup
On Mon, Jan 23, 2012 at 5:02 AM, Michal Hocko <mhocko@...e.cz> wrote:
> On Sat 21-01-12 22:49:23, Hillf Danton wrote:
>> In soft limit reclaim, overreclaim occurs when pages are reclaimed from mem
>> group that is under its soft limit, or when more pages are reclaimd than the
>> exceeding amount, then performance of reclaimee goes down accordingly.
>
> First of all soft reclaim is more a help for the global memory pressure
> balancing rather than any guarantee about how much we reclaim for the
> group.
> We need to do more changes in order to make it a guarantee.
> For example you implementation will cause severe problems when all
> cgroups are soft unlimited (default conf.) or when nobody is above the
> limit but the total consumption triggers the global reclaim. Therefore
> nobody is in excess and you would skip all groups and only bang on the
> root memcg.
>
> Ying Han has a patch which basically skips all cgroups which are under
> its limit until we reach a certain reclaim priority but even for this we
> need some additional changes - e.g. reverse the current default setting
> of the soft limit.
>
> Anyway, I like the nr_to_reclaim reduction idea because we have to do
> this in some way because the global reclaim starts with ULONG
> nr_to_scan.
Agree with Michal where there are quite a lot changes we need to get
in for soft limit before any further optimization.
Hillf, please refer to the patch from Johannes
https://lkml.org/lkml/2012/1/13/99 which got quite a lot recent
discussions. I am expecting to get that in before further soft limit
changes.
Thanks
--Ying
>
>> A helper function is added to compute the number of pages that exceed the soft
>> limit of given mem cgroup, then the excess pages are used when every reclaimee
>> is reclaimed to avoid overreclaim.
>>
>> Signed-off-by: Hillf Danton <dhillf@...il.com>
>> ---
>>
>> --- a/mm/memcontrol.c Tue Jan 17 20:41:36 2012
>> +++ b/mm/memcontrol.c Sat Jan 21 21:18:46 2012
>> @@ -1662,6 +1662,21 @@ static int mem_cgroup_soft_reclaim(struc
>> return total;
>> }
>>
>> +unsigned long mem_cgroup_excess_pages(struct mem_cgroup *memcg)
>> +{
>> + unsigned long pages;
>> +
>> + if (mem_cgroup_disabled())
>> + return 0;
>> + if (!memcg)
>> + return 0;
>> + if (mem_cgroup_is_root(memcg))
>> + return 0;
>> +
>> + pages = res_counter_soft_limit_excess(&memcg->res) >> PAGE_SHIFT;
>> + return pages;
>> +}
>> +
>> /*
>> * Check OOM-Killer is already running under our hierarchy.
>> * If someone is running, return false.
>> --- a/mm/vmscan.c Sat Jan 14 14:02:20 2012
>> +++ b/mm/vmscan.c Sat Jan 21 21:30:06 2012
>> @@ -2150,8 +2150,34 @@ static void shrink_zone(int priority, st
>> .mem_cgroup = memcg,
>> .zone = zone,
>> };
>> + unsigned long old;
>> + bool clobbered = false;
>> +
>> + if (memcg != NULL) {
>> + unsigned long excess;
>> +
>> + excess = mem_cgroup_excess_pages(memcg);
>> + /*
>> + * No bother reclaiming pages from mem cgroup that
>> + * is under soft limit
>> + */
>> + if (!excess)
>> + goto next;
>> + /*
>> + * And reclaim no more pages than excess
>> + */
>> + if (excess < sc->nr_to_reclaim) {
>> + old = sc->nr_to_reclaim;
>> + sc->nr_to_reclaim = excess;
>> + clobbered = true;
>> + }
>> + }
>>
>> shrink_mem_cgroup_zone(priority, &mz, sc);
>> +
>> + if (clobbered)
>> + sc->nr_to_reclaim = old;
>> +next:
>> /*
>> * Limit reclaim has historically picked one memcg and
>> * scanned it with decreasing priority levels until
>> --- a/include/linux/memcontrol.h Thu Jan 19 22:03:14 2012
>> +++ b/include/linux/memcontrol.h Sat Jan 21 21:35:50 2012
>> @@ -161,6 +161,7 @@ unsigned long mem_cgroup_soft_limit_recl
>> gfp_t gfp_mask,
>> unsigned long *total_scanned);
>> u64 mem_cgroup_get_limit(struct mem_cgroup *memcg);
>> +unsigned long mem_cgroup_excess_pages(struct mem_cgroup *memcg);
>>
>> void mem_cgroup_count_vm_event(struct mm_struct *mm, enum vm_event_item idx);
>> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> @@ -376,6 +377,11 @@ unsigned long mem_cgroup_soft_limit_recl
>>
>> static inline
>> u64 mem_cgroup_get_limit(struct mem_cgroup *memcg)
>> +{
>> + return 0;
>> +}
>> +
>> +static inline unsigned long mem_cgroup_excess_pages(struct mem_cgroup *memcg)
>> {
>> return 0;
>> }
>> --
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>
> --
> Michal Hocko
> SUSE Labs
> SUSE LINUX s.r.o.
> Lihovarska 1060/12
> 190 00 Praha 9
> Czech Republic
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