linux-kernel - Re: [PATCH 1/1] cgroup: make per-cgroup pressure stall tracking configurable

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Message-ID: <CAJuCfpGkj9HxbkXnYN58JXJp1j6kVkvQhqscnEfjyB5unKg1NQ@mail.gmail.com>
Date:   Fri, 14 May 2021 11:20:10 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
        lizefan.x@...edance.com, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Benjamin Segall <bsegall@...gle.com>, mgorman@...e.de,
        Minchan Kim <minchan@...nel.org>,
        Jonathan Corbet <corbet@....net>, bristot@...hat.com,
        "Paul E . McKenney" <paulmck@...nel.org>, rdunlap@...radead.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>, macro@...am.me.uk,
        Viresh Kumar <viresh.kumar@...aro.org>,
        mike.kravetz@...cle.com, linux-doc@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        cgroups mailinglist <cgroups@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH 1/1] cgroup: make per-cgroup pressure stall tracking configurable

On Fri, May 14, 2021 at 10:52 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, May 14, 2021 at 08:54:47AM -0700, Suren Baghdasaryan wrote:
>
> > Correct, for this function CONFIG_CGROUPS=n and
> > cgroup_disable=pressure are treated the same. True, from the code it's
> > not very obvious. Do you have some refactoring in mind that would make
> > it more explicit?
>
> Does this make sense?
>
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -744,24 +744,26 @@ static void psi_group_change(struct psi_
>
>  static struct psi_group *iterate_groups(struct task_struct *task, void **iter)
>  {
> +       if (cgroup_psi_enabled()) {
>  #ifdef CONFIG_CGROUPS
> -       struct cgroup *cgroup = NULL;
> +               struct cgroup *cgroup = NULL;
>
> -       if (!*iter)
> -               cgroup = task->cgroups->dfl_cgrp;
> -       else if (*iter == &psi_system)
> -               return NULL;
> -       else
> -               cgroup = cgroup_parent(*iter);
> +               if (!*iter)
> +                       cgroup = task->cgroups->dfl_cgrp;
> +               else if (*iter == &psi_system)
> +                       return NULL;
> +               else
> +                       cgroup = cgroup_parent(*iter);
>
> -       if (cgroup && cgroup_parent(cgroup)) {
> -               *iter = cgroup;
> -               return cgroup_psi(cgroup);
> -       }
> -#else
> -       if (*iter)
> -               return NULL;
> +               if (cgroup && cgroup_parent(cgroup)) {
> +                       *iter = cgroup;
> +                       return cgroup_psi(cgroup);
> +               }
>  #endif
> +       } else {
> +               if (*iter)
> +                       return NULL;
> +       }
>         *iter = &psi_system;
>         return &psi_system;
>  }

Hmm. Looks like the case when cgroup_psi_enabled()==true and
CONFIG_CGROUPS=n would miss the "if (*iter) return NULL;" condition.
Effectively with CONFIG_CGROUPS=n this becomes:

       if (cgroup_psi_enabled()) {           <== assume this is true
#ifdef CONFIG_CGROUPS                <== compiled out
#endif
       } else {
               if (*iter)                                  <== this
statement will never execute
                       return NULL;
       }
       *iter = &psi_system;
        return &psi_system;

>
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>