Re: Lower than expected CPU pressure in PSI

Peter Zijlstra
2020-02-07T13:08:45Z

On Thu, Jan 09, 2020 at 11:16:32AM -0500, Johannes Weiner wrote:
> On Wed, Jan 08, 2020 at 11:47:10AM -0800, Ivan Babrou wrote:
> > We added reporting for PSI in cgroups and results are somewhat surprising.
> >
> > My test setup consists of 3 services:
> >
> > * stress-cpu1-no-contention.service : taskset -c 1 stress --cpu 1
> > * stress-cpu2-first-half.service : taskset -c 2 stress --cpu 1
> > * stress-cpu2-second-half.service : taskset -c 2 stress --cpu 1
> >
> > First service runs unconstrained, the other two compete for CPU.
> >
> > As expected, I can see 500ms/s sched delay for the latter two and
> > aggregated 1000ms/s delay for /system.slice, no surprises here.
> >
> > However, CPU pressure reported by PSI says that none of my services
> > have any pressure on them. I can see around 434ms/s pressure on
> > /unified/system.slice and 425ms/s pressure on /unified cgroup, which
> > is surprising for three reasons:
> >
> > * Pressure is absent for my services (I expect it to match scheed delay)
> > * Pressure on /unified/system.slice is lower than both 500ms/s and 1000ms/s
> > * Pressure on root cgroup is lower than on system.slice
>
> CPU pressure is currently implemented based only on the number of
> *runnable* tasks, not on who gets to actively use the CPU. This works
> for contention within cgroups or at the global scope, but it doesn't
> correctly reflect competition between cgroups. It also doesn't show
> the effects of e.g. cpu cycle limiting through cpu.max where there
> might *be* only one runnable task, but it's not getting the CPU.
>
> I've been working on fixing this, but hadn't gotten around to sending
> the patch upstream. Attaching it below. Would you mind testing it?
>
> Peter, what would you think of the below?

I'm not loving it; but I see what it does and I can't quickly see an
alternative.

My main gripe is doing even more of those cgroup traversals.

One thing pick_next_task_fair() does is try and limit the cgroup
traversal to the sub-tree that contains both prev and next. Not sure
that is immediately applicable here, but it might be worth looking into.