linux-kernel - Re: [PATCH v2 3/3] mm: Fix missing mem cgroup soft limit tree updates

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Date:   Mon, 22 Feb 2021 09:41:40 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ying Huang <ying.huang@...el.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] mm: Fix missing mem cgroup soft limit tree updates

On Fri 19-02-21 11:28:47, Tim Chen wrote:
> 
> 
> On 2/19/21 1:16 AM, Michal Hocko wrote:
> 
> >>
> >> Something like this?
> >>
> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >> index 8bddee75f5cb..b50cae3b2a1a 100644
> >> --- a/mm/memcontrol.c
> >> +++ b/mm/memcontrol.c
> >> @@ -3472,6 +3472,14 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
> >>                 if (!mz)
> >>                         break;
> >>
> >> +               /*
> >> +                * Soft limit tree is updated based on memcg events sampling.
> >> +                * We could have missed some updates on page uncharge and
> >> +                * the cgroup is below soft limit.  Skip useless soft reclaim.
> >> +                */
> >> +               if (!soft_limit_excess(mz->memcg))
> >> +                       continue;
> >> +
> >>                 nr_scanned = 0;
> >>                 reclaimed = mem_cgroup_soft_reclaim(mz->memcg, pgdat,
> > 
> > Yes I meant something like this but then I have looked more closely and
> > this shouldn't be needed afterall. __mem_cgroup_largest_soft_limit_node
> > already does all the work
> >         if (!soft_limit_excess(mz->memcg) ||
> >             !css_tryget(&mz->memcg->css))
> >                 goto retry;
> > so this shouldn't really happen.
> > 
> 
> Ah, that's true.  The added check for soft_limit_excess is not needed.
> 
> Do you think it is still a good idea to add patch 3 to
> restrict the uncharge update in page batch of the same node and cgroup?

I would rather drop it. The less the soft limit reclaim code is spread
around the better.
-- 
Michal Hocko
SUSE Labs