linux-kernel - Re: [RFC PATCH 6/7] workingset: memcg: sleep when flushing stats in workingset

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Date:   Thu, 23 Mar 2023 09:02:25 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Tejun Heo <tj@...nel.org>, Josef Bacik <josef@...icpanda.com>,
        Jens Axboe <axboe@...nel.dk>,
        Zefan Li <lizefan.x@...edance.com>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vasily Averin <vasily.averin@...ux.dev>,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        bpf@...r.kernel.org
Subject: Re: [RFC PATCH 6/7] workingset: memcg: sleep when flushing stats in workingset_refault()

On Thu, Mar 23, 2023 at 9:00 AM Johannes Weiner <hannes@...xchg.org> wrote:
>
> On Thu, Mar 23, 2023 at 04:00:36AM +0000, Yosry Ahmed wrote:
> > In workingset_refault(), we call mem_cgroup_flush_stats_delayed() to
> > flush stats within an RCU read section and with sleeping disallowed.
> > Move the call to mem_cgroup_flush_stats_delayed() above the RCU read
> > section and allow sleeping to avoid unnecessarily performing a lot of
> > work without sleeping.
> >
> > Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> > ---
> >
> > A lot of code paths call into workingset_refault(), so I am not
> > generally sure at all whether it's okay to sleep in all contexts or not.
> > Feedback here would be very helpful.
> >
> > ---
> >  mm/workingset.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/workingset.c b/mm/workingset.c
> > index 042eabbb43f6..410bc6684ea7 100644
> > --- a/mm/workingset.c
> > +++ b/mm/workingset.c
> > @@ -406,6 +406,8 @@ void workingset_refault(struct folio *folio, void *shadow)
> >       unpack_shadow(shadow, &memcgid, &pgdat, &eviction, &workingset);
> >       eviction <<= bucket_order;
> >
> > +     /* Flush stats (and potentially sleep) before holding RCU read lock */
> > +     mem_cgroup_flush_stats_delayed(true);
>
> Btw, it might be a good time to rename this while you're in the
> area. delayed suggests this is using a delayed_work, but this is
> actually sometimes flushing directly from the callsite.
>
> What it's doing is ratelimited calls. A better name would be:
>
>         mem_cgroup_flush_stats_ratelimited()

Agreed. Will do in the next version.