linux-kernel - Re: [PATCH V2] cgroup/rstat: Avoid thundering herd problem by kswapd across NUMA nodes

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Date: Tue, 25 Jun 2024 15:59:09 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: "Christoph Lameter (Ampere)" <cl@...ux.com>
Cc: Shakeel Butt <shakeel.butt@...ux.dev>, Jesper Dangaard Brouer <hawk@...nel.org>, tj@...nel.org, 
	cgroups@...r.kernel.org, hannes@...xchg.org, lizefan.x@...edance.com, 
	longman@...hat.com, kernel-team@...udflare.com, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] cgroup/rstat: Avoid thundering herd problem by kswapd
 across NUMA nodes

On Tue, Jun 25, 2024 at 3:35 PM Christoph Lameter (Ampere) <cl@...ux.com> wrote:
>
> On Tue, 25 Jun 2024, Yosry Ahmed wrote:
>
> >> In my reply above, I am not arguing to go back to the older
> >> stats_flush_ongoing situation. Rather I am discussing what should be the
> >> best eventual solution. From the vmstats infra, we can learn that
> >> frequent async flushes along with no sync flush, users are fine with the
> >> 'non-determinism'. Of course cgroup stats are different from vmstats
> >> i.e. are hierarchical but I think we can try out this approach and see
> >> if this works or not.
> >
> > If we do not do sync flushing, then the same problem that happened
> > with stats_flush_ongoing could occur again, right? Userspace could
> > read the stats after an event, and get a snapshot of the system before
> > that event.
> >
> > Perhaps this is fine for vmstats if it has always been like that (I
> > have no idea), or if no users make assumptions about this. But for
> > cgroup stats, we have use cases that rely on this behavior.
>
> vmstat updates are triggered initially as needed by the shepherd task and
> there is no requirement that this is triggered simultaenously. We
> could actually randomize the intervals in vmstat_update() a bit if this
> will help.

The problem is that for cgroup stats, the behavior has been that a
userspace read will trigger a flush (i.e. propagating updates). We
have use cases that depend on this. If we switch to the vmstat model
where updates are triggered independently from user reads, it
constitutes a behavioral change.