linux-kernel - Re: [PATCH v1 3/3] cgroup/rstat: introduce ratelimited rstat flushing

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Date: Fri, 19 Apr 2024 12:25:32 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Jesper Dangaard Brouer <hawk@...nel.org>
Cc: tj@...nel.org, hannes@...xchg.org, lizefan.x@...edance.com, 
	cgroups@...r.kernel.org, longman@...hat.com, netdev@...r.kernel.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, shakeel.butt@...ux.dev, 
	kernel-team@...udflare.com, Arnaldo Carvalho de Melo <acme@...nel.org>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, mhocko@...nel.org, Wei Xu <weixugc@...gle.com>
Subject: Re: [PATCH v1 3/3] cgroup/rstat: introduce ratelimited rstat flushing

On Fri, Apr 19, 2024 at 3:17 AM Jesper Dangaard Brouer <hawk@...nelorg> wrote:
>
>
> On 18/04/2024 23.00, Yosry Ahmed wrote:
> > On Thu, Apr 18, 2024 at 4:00 AM Jesper Dangaard Brouer<hawk@...nel.org>  wrote:
> >> On 18/04/2024 04.21, Yosry Ahmed wrote:
> >>> On Tue, Apr 16, 2024 at 10:51 AM Jesper Dangaard Brouer<hawk@...nel.org>  wrote:
> >>>> This patch aims to reduce userspace-triggered pressure on the global
> >>>> cgroup_rstat_lock by introducing a mechanism to limit how often reading
> >>>> stat files causes cgroup rstat flushing.
> >>>>
> [...]
>
> > Taking a step back, I think this series is trying to address two
> > issues in one go: interrupt handling latency and lock contention.
>
> Yes, patch 2 and 3 are essentially independent and address two different
> aspects.
>
> > While both are related because reducing flushing reduces irq
> > disablement, I think it would be better if we can fix that issue
> > separately with a more fundamental solution (e.g. using a mutex or
> > dropping the lock at each CPU boundary).
> >
> > After that, we can more clearly evaluate the lock contention problem
> > with data purely about flushing latency, without taking into
> > consideration the irq handling problem.
> >
> > Does this make sense to you?
>
> Yes, make sense.
>
> So, you are suggesting we start with the mutex change? (patch 2)
> (which still needs some adjustments/tuning)

Yes. Let's focus on (what I assume to be) the more important problem,
IRQ serving latency. Once this is fixed, let's consider the tradeoffs
for improving lock contention separately.

Thanks!

>
> This make sense to me, as there are likely many solutions to reducing
> the pressure on the lock.
>
> With the tracepoint patch in-place, I/we can measure the pressure on the
> lock, and I plan to do this across our CF fleet.  Then we can slowly
> work on improving lock contention and evaluate this on our fleets.
>
> --Jesper
> p.s.
> Setting expectations:
>   - Going on vacation today, so will resume work after 29th April.