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From:   Hao Luo <haoluo@google.com>
Date:   Tue, 7 Mar 2023 13:06:57 -0800
Message-ID: <CA+khW7iWAn6bbXdkJX1Lt4dWUsN6o4KqVQ8OFTs0B+VTtVjBkw@mail.gmail.com>
Subject: Re: [PATCH v3] sched: cpuset: Don't rebuild root domains on suspend-resume
To:     Waiman Long <longman@redhat.com>
Cc:     Qais Yousef <qyousef@layalina.io>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>, tj@kernel.org,
        linux-kernel@vger.kernel.org, luca.abeni@santannapisa.it,
        claudio@evidence.eu.com, tommaso.cucinotta@santannapisa.it,
        bristot@redhat.com, mathieu.poirier@linaro.org,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        cgroups@vger.kernel.org,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Wei Wang <wvw@google.com>, Rick Yiu <rickyiu@google.com>,
        Quentin Perret <qperret@google.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Sudeep Holla <sudeep.holla@arm.com>,
        Zefan Li <lizefan.x@bytedance.com>, linux-s390@vger.kernel.org,
        x86@kernel.org
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On Tue, Mar 7, 2023 at 12:09=E2=80=AFPM Waiman Long <longman@redhat.com> wr=
ote:
>
> On 3/7/23 14:56, Hao Luo wrote:
> > On Mon, Feb 6, 2023 at 2:15=E2=80=AFPM Qais Yousef <qyousef@layalina.io=
> wrote:
> >> Commit f9a25f776d78 ("cpusets: Rebuild root domain deadline accounting=
 information")
> >> enabled rebuilding root domain on cpuset and hotplug operations to
> >> correct deadline accounting.
> >>
> >> Rebuilding root domain is a slow operation and we see 10+ of ms delays
> >> on suspend-resume because of that (worst case captures 20ms which
> >> happens often).
> >>
> >> Since nothing is expected to change on suspend-resume operation; skip
> >> rebuilding the root domains to regain the some of the time lost.
> >>
> >> Achieve this by refactoring the code to pass whether dl accoutning nee=
ds
> >> an update to rebuild_sched_domains(). And while at it, rename
> >> rebuild_root_domains() to update_dl_rd_accounting() which I believe is
> >> a more representative name since we are not really rebuilding the root
> >> domains, but rather updating dl accounting at the root domain.
> >>
> >> Some users of rebuild_sched_domains() will skip dl accounting update
> >> now:
> >>
> >>          * Update sched domains when relaxing the domain level in cpus=
et
> >>            which only impacts searching level in load balance
> >>          * update sched domains when cpufreq governor changes and we n=
eed
> >>            to create the perf domains
> >>
> >> Users in arch/x86 and arch/s390 are left with the old behavior.
> >>
> >> Debugged-by: Rick Yiu <rickyiu@google.com>
> >> Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> >> ---
> > Hi Qais,
> >
> > Thank you for reporting this. We observed the same issue in our
> > production environment. Rebuild_root_domains() is also called under
> > cpuset_write_resmask, which handles writing to cpuset.cpus. Under
> > production workloads, on a 4.15 kernel, we observed the median latency
> > of writing cpuset.cpus at 3ms, p99 at 7ms. Now the median becomes
> > 60ms, p99 at >100ms. Writing cpuset.cpus is a fairly frequent and
> > critical path in production, but blindly traversing every task in the
> > system is not scalable. And its cost is really unnecessary for users
> > who don't use deadline tasks at all.
>
> The rebuild_root_domains() function shouldn't be called when updating
> cpuset.cpus unless it is a partition root. Is it?
>

I think it's because we were using the legacy hierarchy. I'm not
familiar with cpuset partition though.

Hao