linux-kernel - Lower than expected CPU pressure in PSI

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Message-ID: <CABWYdi25Y=zrfdnitT3sSgC3UqcFHfz6-N2YP7h2TJai=JH_zg@mail.gmail.com>
Date:   Wed, 8 Jan 2020 11:47:10 -0800
From:   Ivan Babrou <ivan@...udflare.com>
To:     linux-kernel <linux-kernel@...r.kernel.org>
Cc:     kernel-team <kernel-team@...udflare.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>
Subject: Lower than expected CPU pressure in PSI

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

I'm running Linux 5.4.8 with hybrid cgroup hierarchy under systemd.

P.S.: ./scripts/get_maintainer.pl kernel/sched/psi.c does not say
Johannes Weiner is a maintainer