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

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Date: Mon, 24 Jun 2024 10:40:48 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
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 Mon, Jun 24, 2024 at 10:32 AM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
>
> On Mon, Jun 24, 2024 at 05:46:05AM GMT, Yosry Ahmed wrote:
> > On Mon, Jun 24, 2024 at 4:55 AM Jesper Dangaard Brouer <hawk@...nel.org> wrote:
> >
> [...]
> > I am assuming this supersedes your other patch titled "[PATCH RFC]
> > cgroup/rstat: avoid thundering herd problem on root cgrp", so I will
> > only respond here.
> >
> > I have two comments:
> > - There is no reason why this should be limited to the root cgroup. We
> > can keep track of the cgroup being flushed, and use
> > cgroup_is_descendant() to find out if the cgroup we want to flush is a
> > descendant of it. We can use a pointer and cmpxchg primitives instead
> > of the atomic here IIUC.
> >
> > - More importantly, I am not a fan of skipping the flush if there is
> > an ongoing one. For all we know, the ongoing flush could have just
> > started and the stats have not been flushed yet. This is another
> > example of non deterministic behavior that could be difficult to
> > debug.
>
> Even with the flush, there will almost always per-cpu updates which will
> be missed. This can not be fixed unless we block the stats updaters as
> well (which is not going to happen). So, we are already ok with this
> level of non-determinism. Why skipping flushing would be worse? One may
> argue 'time window is smaller' but this still does not cap the amount of
> updates. So, unless there is concrete data that this skipping flushing
> is detrimental to the users of stats, I don't see an issue in the
> presense of periodic flusher.

As you mentioned, the updates that happen during the flush are
unavoidable anyway, and the window is small. On the other hand, we
should be able to maintain the current behavior that at least all the
stat updates that happened *before* the call to cgroup_rstat_flush()
are flushed after the call.

The main concern here is that the stats read *after* an event occurs
should reflect the system state at that time. For example, a proactive
reclaimer reading the stats after writing to memory.reclaim should
observe the system state after the reclaim operation happened.

Please see [1] for more details about why this is important, which was
the rationale for removing stats_flush_ongoing in the first place.

[1]https://lore.kernel.org/lkml/20231129032154.3710765-6-yosryahmed@google.com/

>
> >
> > I tried a similar approach before where we sleep and wait for the
> > ongoing flush to complete instead, without contending on the lock,
> > using completions [1]. Although that patch has a lot of complexity,
>
> We can definitely add complexity but only if there are no simple good
> enough mitigations.

I agree that my patch was complicated. I am hoping we can have a
simpler version here that just waits for ongoing flushers if the
cgroup is a descendant of the cgroup already being flushed.