linux-kernel - Re: [PATCH v2] sched/cpuacct: Fix charge cpuacct.usage

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Date:   Tue, 2 Mar 2021 18:43:23 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        mingo@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/cpuacct: Fix charge cpuacct.usage_sys
 incorrently.

Hello.

(Sorry for necroposting, found this upstream reference only now.)

On Mon, Apr 20, 2020 at 03:04:53PM +0800, Muchun Song <songmuchun@...edance.com> wrote:
>  /* Time spent by the tasks of the CPU accounting group executing in ... */
> @@ -339,7 +340,7 @@ void cpuacct_charge(struct task_struct *tsk, u64 cputime)
>  {
>  	struct cpuacct *ca;
>  	int index = CPUACCT_STAT_SYSTEM;
> -	struct pt_regs *regs = task_pt_regs(tsk);
> +	struct pt_regs *regs = get_irq_regs() ? : task_pt_regs(tsk);
I've read the discussion in [1] but I don't think this approach is
correct either (and I don't know what is better :-/).

I only have a qualitative proof:

host:~ # uname -r
5.10.16-1-default

host:~ # systemd-run -p CPUAccounting=yes sh -c 'time sh -c "i=0 ; while [ \"\$i\" -lt 10000 ] ; do i=\$((\$i+1)) ; cat /proc/slabinfo >/dev/null ; done" ; sleep inf'
Running as unit: run-r101b9f53efcb4d2a9bfb65feb6f120ca.service

host:~ # cat /sys/fs/cgroup/cpuacct/system.slice/run-r101b9f53efcb4d2a9bfb65feb6f120ca.service/cpuacct.usage{,_user,_sys}
16138535165
14332580468
1805954697

(See that sys/user ~ 0.1)

host:~ # journalctl -u run-r101b9f53efcb4d2a9bfb65feb6f120ca.service
-- Logs begin at Tue 2021-03-02 18:06:41 CET, end at Tue 2021-03-02 18:27:45 CET. --
Mar 02 18:27:29 host systemd[1]: Started /usr/bin/sh -c time sh -c "i=0 ; while [ \"\$i\" -lt 10000 ] ; do i=\$((\$i+1)) ; cat /proc/slabinfo >/dev/null ; done" ; sleep inf.
Mar 02 18:27:45 host sh[19117]: real        0m15.543s
Mar 02 18:27:45 host sh[19117]: user        0m10.752s
Mar 02 18:27:45 host sh[19117]: sys        0m5.379s

(See that sys/user ~ 0.5)

host:~ # cat /sys/fs/cgroup/cpuacct/system.slice/run-r101b9f53efcb4d2a9bfb65feb6f120ca.service/cpuacct.stat
user 415
system 1209

(See that sys/user ~ 3.0 :-o)

The expectation is that significant amount of the loop is spent in
kernel (dumping slabinfo). I can't tell which of the ratios fits the
reality best but the cpuacct.usage_sys still seems too low.


Michal

[1] https://lore.kernel.org/lkml/20200416141833.50663-1-songmuchun@bytedance.com/

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