linux-kernel - Re: [RFC PATCH 1/7] cgroup: rstat: only disable interrupts for the percpu lock

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Date:   Wed, 29 Mar 2023 12:22:24 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Tejun Heo <tj@...nel.org>
cc:     Yosry Ahmed <yosryahmed@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Josef Bacik <josef@...icpanda.com>,
        Jens Axboe <axboe@...nel.dk>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vasily Averin <vasily.averin@...ux.dev>,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        bpf@...r.kernel.org
Subject: Re: [RFC PATCH 1/7] cgroup: rstat: only disable interrupts for the
 percpu lock

Hi Tejun,

On Wed, 29 Mar 2023, Tejun Heo wrote:
> On Mon, Mar 27, 2023 at 04:23:13PM -0700, Yosry Ahmed wrote:
> > Tejun, if having the lock be non-irq is a non-starter for you, I can
> 
> This is an actual hazard. We see in prod these unprotected locks causing
> very big spikes in tail latencies and they can be tricky to root cause too
> and given the way rstat lock is used it's highly likely to be involved in
> those scenarios with the proposed change, so it's gonna be a nack from my
> end.

Butting in here, I'm fascinated.  This is certainly not my area, I know
nothing about rstat, but this is the first time I ever heard someone
arguing for more disabling of interrupts rather than less.

An interrupt coming in while holding a contended resource can certainly
add to latencies, that I accept of course.  But until now, I thought it
was agreed best practice to disable irqs only regretfully, when strictly
necessary.

If that has changed, I for one want to know about it.  How should we
now judge which spinlocks should disable interrupts and which should not?
Page table locks are currently my main interest - should those be changed?

Thanks,
Hugh