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

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Date:   Fri, 14 May 2021 11:50:06 -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 11:20 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> 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;
>

Ah, sorry. I forgot that CONFIG_CGROUPS=n would force
cgroup_psi_enabled()==false (the way function is defined in cgroup.h),
so (CONFIG_CGROUPS=n && cgroup_psi_enabled()==true) is an invalid
configuration. I think adding a comment to your suggestion would make
it more clear.
So your suggestion seems to work. I'll test it and include it in the
next revision. Thanks!


> >
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> >