linux-kernel - Re: [PATCH 3/3] mm: memcg: use non-unified stats flushing for userspace reads

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Date:   Mon, 28 Aug 2023 10:00:21 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>,
        Ivan Babrou <ivan@...udflare.com>, Tejun Heo <tj@...nel.org>,
        linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] mm: memcg: use non-unified stats flushing for
 userspace reads

On Mon, Aug 28, 2023 at 9:15 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
[...]
> >
> > Well, I really have to say that I do not like the notion that reading
> > stats is unpredictable. This just makes it really hard to use. If
> > the precision is to be sarificed then this should be preferable over
> > potentially high global lock contention. We already have that model in
> > place of /proc/vmstat (configurable timeout for flusher and a way to
> > flush explicitly). I appreciate you would like to have a better
> > precision but as you have explored the locking is really hard to get rid
> > of here.
>
> Reading the stats *is* unpredictable today. In terms of
> accuracy/staleness and cost. Avoiding the flush entirely on the read
> path will surely make the cost very stable and cheap, but will make
> accuracy even less predictable.
>

On average you would get the stats at most 2 second old, so I would
not say it is less predictable.

> >
> > So from my POV I would prefer to avoid flushing from the stats reading
> > path and implement force flushing by writing to stat file. If the 2s
> > flushing interval is considered to coarse I would be OK to allow setting
> > it from userspace. This way this would be more in line with /proc/vmstat
> > which seems to be working quite well.
> >
> > If this is not accaptable or deemed a wrong approach long term then it
> > would be good to reonsider the current cgroup_rstat_lock at least.
> > Either by turning it into mutex or by dropping the yielding code which
> > can severly affect the worst case latency AFAIU.
>
> Honestly I think it's better if we do it the other way around. We make
> flushing on the stats reading path non-unified and deterministic. That
> model also exists and is used for cpu.stat. If we find a problem with
> the locking being held from userspace, we can then remove flushing
> from the read path and add interface(s) to configure the periodic
> flusher and do a force flush.
>

Here I agree with you. Let's go with the approach which is easy to
undo for now. Though I prefer the new explicit interface for flushing,
that step would be very hard to undo. Let's reevaluate if the proposed
approach shows negative impact on production traffic and I think
Cloudflare folks can give us the results soon.