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

[Qais Yousef <qyousef@layalina.io>]

On 03/09/23 14:23, Hao Luo wrote:
> 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.

Yeah I am working on 5.10 too (this will need to be backported to 5.10 and 5.15
ultimately) and had the same crash because task is NULL.

Fixed it this way which I think what you intended to do Juri? It moves the
check for dl_task(task) inside cgroup_taskset_for_each() loop.

	diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
	index 83a8943467fb..06d6bb68d86b 100644
	--- a/kernel/cgroup/cpuset.c
	+++ b/kernel/cgroup/cpuset.c
	@@ -2495,11 +2495,11 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
			ret = security_task_setscheduler(task);
			if (ret)
				goto out_unlock;
	-       }
	 
	-       if (dl_task(task)) {
	-               cs->deadline_tasks++;
	-               cpuset_attach_old_cs->deadline_tasks--;
	+               if (dl_task(task)) {
	+                       cs->deadline_tasks++;
	+                       cpuset_attach_old_cs->deadline_tasks--;
	+               }
		}
	 
		/*

Like Hao I don't have any deadline tasks in the system. With the fix above
I don't notice the delay on suspend resume using your patches.

If you want any debug; please feel free to add them into your branch so I can
run with that and give you the log.


Thanks!

--
Qais Yousef