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 15:35:21 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...ux.com>
To: Yosry Ahmed <yosryahmed@...gle.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, 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.