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

[Shakeel Butt <shakeel.butt@linux.dev>]

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