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From:   Hao Luo <haoluo@google.com>
Date:   Thu, 9 Mar 2023 14:23:15 -0800
Message-ID: <CA+khW7iAeTALH5b6upHXs1RotFBohVeZCcAp5H+r-=L05kqnww@mail.gmail.com>
Subject: Re: [PATCH v3] sched: cpuset: Don't rebuild root domains on suspend-resume
To:     Juri Lelli <juri.lelli@redhat.com>
Cc:     Qais Yousef <qyousef@layalina.io>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Waiman Long <longman@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,
        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 Wed, Mar 8, 2023 at 10:55=E2=80=AFPM Juri Lelli <juri.lelli@redhat.com> =
wrote:
>
> On 08/03/23 10:01, Hao Luo wrote:
> > On Wed, Mar 8, 2023 at 2:20=E2=80=AFAM Juri Lelli <juri.lelli@redhat.co=
m> wrote:
> > >
> > > On 01/03/23 17:03, Qais Yousef wrote:
> > > > On 03/01/23 15:26, Juri Lelli wrote:
> > <...>
> > > > > BTW, do you have a repro script of some sort handy I might play w=
ith?
> > > >
> > > > Sorry no. You'll just need to suspend to ram. I had a simple patch =
to measure
> > > > the time around the call and trace_printk'ed the result.
> > > >
> > > > I was working on a android phone which just suspends to ram if you =
turn the
> > > > screen off and disconnect the usb.
> > >
> > > Looks like I could come up with the following
> > >
> > > https://github.com/jlelli/linux.git deadline/rework-cpusets
> > > https://github.com/jlelli/linux/tree/deadline/rework-cpusets
> > >
> > > which I don't think it's at a point that I feel comfortable to propos=
e
> > > as an RFC (not even sure if it actually makes sense), but it survived=
 my
> > > very light testing.
> > >
> > > Could you please take a look and, if it makes some sense in theory, g=
ive
> > > it a try on your end?
> > >
> >
> > Hi Juri,
> >
> > Thanks for coming up with the RFC. I can test your changes in the
> > server environment. I observed the same issue on my side and I can
> > reproduce.
> >
> > I sync'ed up with Qais offline earlier yesterday, and was preparing a
> > couple of patches that optimize the cpuset.cpus writes. Tracking dl
> > tasks in cpusets is one of them. But I am happy to take your patches
> > and do the testing. Note that I won't be able to test the dl part of
> > the patch, only the latency impact on rebuild_root_domains(), as we
> > don't have dl tasks in our system.
> >
> > The other patch is fixing cpuset_rwsem. I see you switched it back to
> > mutex. I did observe performance issues with cpuset_rwsem. Basically,
> > using percpu_rwsem generates very very long latency tails for writers,
> > but mutex doesn't. After some debugging, I found it was because
> > percpu_rwsem requires every writer to call a synchronize_rcu() for
> > acquiring the lock. So in my patches, I disabled the fastpath of
> > readers for cpuset_rwsem. This has been done before[1]. But mutex also
> > worked.
> >
> > Anyway, I'm happy to test your patches and ack once they are sent out.
>
> Do you strictly need a proper RFC or could you please test the above for
> now? If you could please do the latter, and if tests look ok, I could
> then put together proper changelogs etc. and propose an RFC (it would
> save me some time not to do that if the above doesn't work, apologies
> for not going the proper route from the start). Guess this question
> applies to Qais as well. Hummm, or maybe you are actually saying that
> you are indeed going to test them already, just wanted to make sure
> then. :)

Juri, I ported your patches to a 5.10 kernel, because my workload can
only run on 5.10. But unfortunately the kernel crashed at
cpuset_can_attach(). I'll put a few comments in your github branch.

Hao


Hao