linux-kernel - Re: [PATCH V7 1/2] cgroup/rstat: Avoid thundering herd problem by kswapd across NUMA nodes

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Message-ID: <CAJD7tkYWnT8bB8UjPPWa1eFvRY3G7RbiM_8cKrj+jhHz_6N_YA@mail.gmail.com>
Date: Fri, 19 Jul 2024 21:52:17 -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 V7 1/2] cgroup/rstat: Avoid thundering herd problem by
 kswapd across NUMA nodes

On Fri, Jul 19, 2024 at 3:48 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
>
> On Fri, Jul 19, 2024 at 09:54:41AM GMT, Jesper Dangaard Brouer wrote:
> >
> >
> > On 19/07/2024 02.40, Shakeel Butt wrote:
> > > Hi Jesper,
> > >
> > > On Wed, Jul 17, 2024 at 06:36:28PM GMT, Jesper Dangaard Brouer wrote:
> > > >
> > > [...]
> > > >
> > > >
> > > > Looking at the production numbers for the time the lock is held for level 0:
> > > >
> > > > @locked_time_level[0]:
> > > > [4M, 8M)     623 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@               |
> > > > [8M, 16M)    860 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> > > > [16M, 32M)   295 |@@@@@@@@@@@@@@@@@                                   |
> > > > [32M, 64M)   275 |@@@@@@@@@@@@@@@@                                    |
> > > >
> > >
> > > Is it possible to get the above histogram for other levels as well?
> >
> > Data from other levels available in [1]:
> >  [1]
> > https://lore.kernel.org/all/8c123882-a5c5-409a-938b-cb5aec9b9ab5@kernel.org/
> >
> > IMHO the data shows we will get most out of skipping level-0 root-cgroup
> > flushes.
> >
>
> Thanks a lot of the data. Are all or most of these locked_time_level[0]
> from kswapds? This just motivates me to strongly push the ratelimited
> flush patch of mine (which would be orthogonal to your patch series).

Jesper and I were discussing a better ratelimiting approach, whether
it's measuring the time since the last flush, or only skipping if we
have a lot of flushes in a specific time frame (using __ratelimit()).
I believe this would be better than the current memcg ratelimiting
approach, and we can remove the latter.

WDYT?

>
> Shakeel