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Tue, 28 Mar 2023 15:19:05 -0700 (PDT) MIME-Version: 1.0 References: <20230328061638.203420-1-yosryahmed@google.com> <20230328061638.203420-5-yosryahmed@google.com> In-Reply-To: From: Yosry Ahmed Date: Tue, 28 Mar 2023 15:18:28 -0700 Message-ID: Subject: Re: [PATCH v1 4/9] cgroup: rstat: add WARN_ON_ONCE() if flushing outside task context To: Johannes Weiner Cc: Tejun Heo , Josef Bacik , Jens Axboe , Zefan Li , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , =?UTF-8?Q?Michal_Koutn=C3=BD?= , Vasily Averin , cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, bpf@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: DD4731A0019 X-Stat-Signature: 9m4a9qe3bd9ahx6ypphugk94sp5hu8hy X-HE-Tag: 1680041946-645734 X-HE-Meta: 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 fWDUZ1nx E8+ZLKgGOvryjufr+CnG0ac7sGR/WEqk3scTMQ1IUU+aDp8QRhDZWAdLKfq0Q0XxND3fzwreGyKgRdSCPJW/99s8hi1GnrQ1w2nJg3zkhKjXbmovXfXyH63QdPcgsel0dKmUcilkNaljk1DfG7ZTHJLaOOoClp+fS6/Ehu/25hH9Lq9XlSC+74wNiFTGTzJ1MS2b9/ljTTKTjobQHgmBdsrRaeuoxU3Ui8CjB3vz+xG4T+slDznwi0K2wwahNzORdQNCVu8UICKVd+sFSZpFe9cOq+ybvXZYKMYIPKEBIPf1uYoLdfzTYPzdlhsT+6h0SiDRBNE8oFFHjtZujEVao5+gY+opI3JJq33a2NHak4PrKN/W+J6vIQf/BsWUB8CasggFjz0oaziVEhn11Nq5APFDPHhXEjg/hvRrfPegFHDLAdnAk9cQt5nHnUA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Mar 28, 2023 at 11:59=E2=80=AFAM Yosry Ahmed wrote: > > On Tue, Mar 28, 2023 at 10:49=E2=80=AFAM Johannes Weiner wrote: > > > > On Tue, Mar 28, 2023 at 06:16:33AM +0000, Yosry Ahmed wrote: > > > rstat flushing is too expensive to perform in irq context. > > > The previous patch removed the only context that may invoke an rstat > > > flush from irq context, add a WARN_ON_ONCE() to detect future > > > violations, or those that we are not aware of. > > > > > > Signed-off-by: Yosry Ahmed > > > --- > > > kernel/cgroup/rstat.c | 2 ++ > > > 1 file changed, 2 insertions(+) > > > > > > diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c > > > index d3252b0416b6..c2571939139f 100644 > > > --- a/kernel/cgroup/rstat.c > > > +++ b/kernel/cgroup/rstat.c > > > @@ -176,6 +176,8 @@ static void cgroup_rstat_flush_locked(struct cgro= up *cgrp, bool may_sleep) > > > { > > > int cpu; > > > > > > + /* rstat flushing is too expensive for irq context */ > > > + WARN_ON_ONCE(!in_task()); > > > lockdep_assert_held(&cgroup_rstat_lock); > > > > This seems a bit arbitrary. Why is an irq caller forbidden, but an > > irq-disabled, non-preemptible section caller is allowed? The latency > > impact on the system would be the same, right? > > Thanks for taking a look. > > So in the first patch series the initial purpose was to make sure > cgroup_rstat_lock was never acquired in an irq context, so that we can > stop disabling irqs while holding it. Tejun disagreed with this > approach though. > > We currently have one caller that calls flushing with irqs disabled > (mem_cgroup_usage()) -- so we cannot forbid such callers (yet), but I > thought we can at least forbid callers from irq context now (or catch > those that we are not aware of), and then maybe forbid irqs_disabled() > contexts as well we can get rid of that callsite. > > WDYT? I added more context in the commit log in the v2 respin [1]. Let me know if you want me to change something else, rephrase the comment, or drop the patch entirely. [1]https://lore.kernel.org/linux-mm/20230328221644.803272-5-yosryahmed@goog= le.com/