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

[Hao Luo]

On Wed, Mar 8, 2023 at 2:20 AM Juri Lelli <juri.lelli@xxxxxxxxxx> 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.

[1] https://lore.kernel.org/all/20160811165413.GA22807@xxxxxxxxxx/T/#m13ef9a728c89cccc9c02af3c96924096c76162a5

Hao