linux-kernel - Re: [PATCH 3/4] sched/cpuacct: fix user/system in shown cpuacct.usage*

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Date:   Wed, 17 Mar 2021 18:22:57 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Andrey Ryabinin <arbn@...dex-team.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     Boris Burkov <boris@....io>,
        Bharata B Rao <bharata@...ux.vnet.ibm.com>,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Andrey Ryabinin <arbn@...dex-team.com>, stable@...r.kernel.org
Subject: Re: [PATCH 3/4] sched/cpuacct: fix user/system in shown cpuacct.usage*

Andrey Ryabinin <arbn@...dex-team.com> writes:

> cpuacct has 2 different ways of accounting and showing user
> and system times.
>
> The first one uses cpuacct_account_field() to account times
> and cpuacct.stat file to expose them. And this one seems to work ok.
>
> The second one is uses cpuacct_charge() function for accounting and
> set of cpuacct.usage* files to show times. Despite some attempts to
> fix it in the past it still doesn't work. E.g. while running KVM
> guest the cpuacct_charge() accounts most of the guest time as
> system time. This doesn't match with user&system times shown in
> cpuacct.stat or proc/<pid>/stat.

I couldn't reproduce this running a cpu bound load in a kvm guest on a
nohz_full cpu on 5.11.  The time is almost entirely in cpuacct.usage and
_user, while _sys stays low.

Could you say more about how you're seeing this?  Don't really doubt
there's a problem, just wondering what you're doing.

> diff --git a/kernel/sched/cpuacct.c b/kernel/sched/cpuacct.c
> index 941c28cf9738..7eff79faab0d 100644
> --- a/kernel/sched/cpuacct.c
> +++ b/kernel/sched/cpuacct.c
> @@ -29,7 +29,7 @@ struct cpuacct_usage {
>  struct cpuacct {
>  	struct cgroup_subsys_state	css;
>  	/* cpuusage holds pointer to a u64-type object on every CPU */
> -	struct cpuacct_usage __percpu	*cpuusage;

Definition of struct cpuacct_usage can go away now.

> @@ -99,7 +99,8 @@ static void cpuacct_css_free(struct cgroup_subsys_state *css)
>  static u64 cpuacct_cpuusage_read(struct cpuacct *ca, int cpu,
>  				 enum cpuacct_stat_index index)
>  {
> -	struct cpuacct_usage *cpuusage = per_cpu_ptr(ca->cpuusage, cpu);
> +	u64 *cpuusage = per_cpu_ptr(ca->cpuusage, cpu);
> +	u64 *cpustat = per_cpu_ptr(ca->cpustat, cpu)->cpustat;
>  	u64 data;

There's a BUG_ON below this that could probably be WARN_ON_ONCE while
you're here

> @@ -278,8 +274,8 @@ static int cpuacct_stats_show(struct seq_file *sf, void *v)
>  	for_each_possible_cpu(cpu) {
>  		u64 *cpustat = per_cpu_ptr(ca->cpustat, cpu)->cpustat;
>  
> -		val[CPUACCT_STAT_USER]   += cpustat[CPUTIME_USER];
> -		val[CPUACCT_STAT_USER]   += cpustat[CPUTIME_NICE];
> +		val[CPUACCT_STAT_USER] += cpustat[CPUTIME_USER];
> +		val[CPUACCT_STAT_USER] += cpustat[CPUTIME_NICE];

unnecessary whitespace change?