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 09:21:45 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: 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 09:00:03AM GMT, Yosry Ahmed wrote:
[...]
> 
> My point is not about accuracy, although I think it's a reasonable
> argument on its own (a lot of things could change in a short amount of
> time, which is why I prefer magnitude-based ratelimiting).
> 
> My point is about logical ordering. If a userspace program reads the
> stats *after* an event occurs, it expects to get a snapshot of the
> system state after that event. Two examples are:
> 
> - A proactive reclaimer reading the stats after a reclaim attempt to
> check if it needs to reclaim more memory or fallback.
> - A userspace OOM killer reading the stats after a usage spike to
> decide which workload to kill.
> 
> I listed such examples with more detail in [1], when I removed
> stats_flush_ongoing from the memcg code.
> 
> [1]https://lore.kernel.org/lkml/20231129032154.3710765-6-yosryahmed@google.com/

You are kind of arbitrarily adding restrictions and rules here. Why not
follow the rules of a well established and battle tested stats infra
used by everyone i.e. vmstats? There is no sync flush and there are
frequent async flushes. I think that is what Jesper wants as well.

Shakeel