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

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Message-ID: <t5vnayr43kpi2nn7adjgbct4ijfganbowoubfcxynpewiixvei@7kprlv6ek7vd>
Date: Mon, 22 Jul 2024 13:02:00 -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 V7 1/2] cgroup/rstat: Avoid thundering herd problem by
 kswapd across NUMA nodes

On Fri, Jul 19, 2024 at 09:52:17PM GMT, Yosry Ahmed wrote:
> 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?

The last statement gives me the impression that you are trying to fix
something that is not broken. The current ratelimiting users are ok, the
issue is with the sync flushers. Or maybe you are suggesting that the new
ratelimiting will be used for all sync flushers and current ratelimiting
users and the new ratelimiting will make a good tradeoff between the
accuracy and potential flush stall?