Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 29 Mar 2023 12:22:24 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Tejun Heo <tj@kernel.org>
cc:     Yosry Ahmed <yosryahmed@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        Josef Bacik <josef@toxicpanda.com>,
        Jens Axboe <axboe@kernel.dk>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Muchun Song <muchun.song@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vasily Averin <vasily.averin@linux.dev>,
        cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        bpf@vger.kernel.org
Subject: Re: [RFC PATCH 1/7] cgroup: rstat: only disable interrupts for the
 percpu lock
In-Reply-To: <ZCSJDpPPOVvBYfOy@slm.duckdns.org>
Message-ID: <f9b6410-ee17-635f-a35d-559fa0191dc3@google.com>
References: <20230323040037.2389095-1-yosryahmed@google.com> <20230323040037.2389095-2-yosryahmed@google.com> <ZBz/V5a7/6PZeM7S@slm.duckdns.org> <CAJD7tkYNZeEytm_Px9_73Y-AYJfHAxaoTmmnO71HW5hd1B5tPg@mail.gmail.com> <ZB5UalkjGngcBDEJ@slm.duckdns.org>
 <CAJD7tkYhyMkD8SFf8b8L1W9QUrLOdw-HJ2NUbENjw5dgFnH3Aw@mail.gmail.com> <CALvZod6rF0D21hcV7xnqD+oRkn=x5NLi5GOkPpyaPa859uDH+Q@mail.gmail.com> <CAJD7tkY_ESpMYMw72bsATpp6tPphv8qS6VbfEUjpKZW6vUqQSQ@mail.gmail.com> <CALvZod41ecuCKmuFBNtAjoKJjQgWYzoe4_B8zRK37HYk-rYDkA@mail.gmail.com>
 <CAJD7tkZrp=4zWvjE9_010TAG1T_crCbf9P64UzJABspgcrGPKg@mail.gmail.com> <ZCSJDpPPOVvBYfOy@slm.duckdns.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Precedence: bulk

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