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

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Message-ID: <33295077-e969-427a-badb-3e29698f5cfb@kernel.org>
Date: Fri, 19 Apr 2024 12:16:52 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
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 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)

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.