Re: [PATCH v3] sched: cpuset: Don't rebuild root domains on suspend-resume

Hao Luo
2023-03-09T22:23:34Z

On Wed, Mar 8, 2023 at 10:55 PM Juri Lelli <juri.lelli@redhat.com> wrote:
>
> On 08/03/23 10:01, Hao Luo wrote:
> > On Wed, Mar 8, 2023 at 2:20 AM Juri Lelli <juri.lelli@redhat.com> 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 with?
> > > >
> > > > 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 propose
> > > 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, give
> > > 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