linux-kernel - Re: [PATCH v1 6/9] memcg: sleep during flushing stats in safe contexts

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Date:   Tue, 28 Mar 2023 15:06:03 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Yosry Ahmed <yosryahmed@...gle.com>
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>,
        Michal Koutný <mkoutny@...e.com>,
        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: [PATCH v1 6/9] memcg: sleep during flushing stats in safe
 contexts

On Tue, Mar 28, 2023 at 11:45:19AM -0700, Yosry Ahmed wrote:
> On Tue, Mar 28, 2023 at 11:35 AM Johannes Weiner <hannes@...xchg.org> wrote:
> > On Tue, Mar 28, 2023 at 06:16:35AM +0000, Yosry Ahmed wrote:
> > >  void mem_cgroup_flush_stats_ratelimited(void)
> > >  {
> > >       if (time_after64(jiffies_64, READ_ONCE(flush_next_time)))
> > > -             mem_cgroup_flush_stats();
> > > +             mem_cgroup_flush_stats_atomic();
> > > +}
> >
> > This should probably be mem_cgroup_flush_stats_atomic_ratelimited().
> >
> > (Whee, kinda long, but that's alright. Very specialized caller...)
> 
> It should, but the following patch makes it non-atomic anyway, so I
> thought I wouldn't clutter the diff by renaming it here and then
> reverting it back in the next patch.
> 
> There is an argument for maintaining a clean history tho in case the
> next patch is reverted separately (which is the reason I put it in a
> separate patch to begin with) -- so perhaps I should rename it here to
> mem_cgroup_flush_stats_atomic_ratelimited () and back to
> mem_cgroup_flush_stats_ratelimited() in the next patch, just for
> consistency?

Sounds good to me. It's pretty minor churn.

> > Btw, can you guys think of a reason against moving the threshold check
> > into the common function? It would then apply to the time-limited
> > flushes as well, but that shouldn't hurt anything. This would make the
> > code even simpler:
> 
> I think the point of having the threshold check outside the common
> function is that the periodic flusher always flushes, regardless of
> the threshold, to keep rstat flushing from critical contexts as cheap
> as possible.

Good point. Yeah, let's keep it separate then.

> > > @@ -2845,7 +2845,7 @@ static void prepare_scan_count(pg_data_t *pgdat, struct scan_control *sc)
> > >        * Flush the memory cgroup stats, so that we read accurate per-memcg
> > >        * lruvec stats for heuristics.
> > >        */
> > > -     mem_cgroup_flush_stats();
> > > +     mem_cgroup_flush_stats_atomic();
> >
> > I'm thinking this one could be non-atomic as well. It's called fairly
> > high up in reclaim without any locks held.
> 
> A later patch does exactly that. I put making the reclaim and refault
> paths non-atomic in separate patches to easily revert them if we see a
> regression. Let me know if this is too defensive and if you'd rather
> have them squashed.

No, good call. I should have just looked ahead first :-)