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From: Ivan Babrou <ivan@cloudflare.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
	kernel-team <kernel-team@cloudflare.com>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>
Subject: Re: Lower than expected CPU pressure in PSI
Date: Thu, 16 Jan 2020 12:24:47 -0800
Message-ID: <CABWYdi3BrQHaM_Np81W8f=EFU09cqqJARbywEvhq_XWoDqrBPw@mail.gmail.com> (raw)
In-Reply-To: <20200115165543.GA47772@cmpxchg.org>

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!

  reply index

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-08 19:47 Ivan Babrou
2020-01-09 16:16 ` Johannes Weiner
2020-01-10 19:28   ` Ivan Babrou
2020-01-15 16:55     ` Johannes Weiner
2020-01-16 20:24       ` Ivan Babrou [this message]
2020-01-09 16:23 ` Johannes Weiner

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