Re: Lower than expected CPU pressure in PSI

Ivan Babrou
2020-01-16T20:25:01Z

This definitely helps! It would be nice to add this as a section here:

* https://www.kernel.org/doc/html/latest/accounting/psi.html

On Wed, Jan 15, 2020 at 8:55 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Fri, Jan 10, 2020 at 11:28:32AM -0800, Ivan Babrou wrote:
> > I applied the patch on top of 5.5.0-rc3 and it's definitely better
> > now, both competing cgroups report 500ms/s delay. Feel free to add
> > Tested-by from me.
>
> Thanks, Ivan!
>
> > I'm still seeing /unified/system.slice at 385ms/s and /unified.slice
> > at 372ms/s, do you have an explanation for that part? Maybe it's
> > totally reasonable, but warrants a patch for documentation.
>
> Yes, this is a combination of CPU pinning and how pressure is
> calculated in SMP systems.
>
> The stall times are defined as lost compute potential - which scales
> with the number of concurrent threads - normalized to wallclock
> time. See the "Multiple CPUs" section in kernel/sched/psi.c.
>
> By restricting the CPUs in system.slice, there is less compute
> available in that group than in the parent, which means that the
> relative loss of potential can be higher.
>
> It's a bit unintuitive because most cgroup metrics are plain numbers
> that add up to bigger numbers as you go up the tree. If we exported
> both the numerator (waste) and denominator (compute potential) here,
> the numbers would act more conventionally, with parent numbers always
> bigger than the child's. But because pressure is normalized to
> wallclock time, you only see the ratio at each level, and that can
> shrink as you go up the tree if lower levels are CPU-constrained.
>
> We could have exported both numbers, but for most usecases that would
> be more confusing than helpful. And in practice it's the ratio that
> really matters: the pressure in the leaf cgroups is high due to the
> CPU restriction; but when you go higher up the tree and look at not
> just the pinned tasks, but also include tasks in other groups that
> have more CPUs available to them, the aggregate productivity at that
> level *is* actually higher.
>
> I hope that helps!