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From:   Josh Don <joshdon@google.com>
Date:   Fri, 5 Feb 2021 17:15:49 -0800
Message-ID: <CABk29NuSB0ynM3w=o8CwMPAdQPm-z4LTTOsQrdP5Nu4y+v8dsA@mail.gmail.com>
Subject: Re: [PATCH v10 2/5] sched: CGroup tagging interface for core scheduling
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
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        Aubrey Li <aubrey.intel@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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On Fri, Feb 5, 2021 at 3:53 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, Jan 22, 2021 at 08:17:01PM -0500, Joel Fernandes (Google) wrote:
>
> > +/* All active sched_core_cookies */
> > +static struct rb_root sched_core_cookies = RB_ROOT;
> > +static DEFINE_RAW_SPINLOCK(sched_core_cookies_lock);
>
> > +/*
> > + * Returns the following:
> > + * a < b  => -1
> > + * a == b => 0
> > + * a > b  => 1
> > + */
> > +static int sched_core_cookie_cmp(const struct sched_core_cookie *a,
> > +                              const struct sched_core_cookie *b)
> > +{
> > +#define COOKIE_CMP_RETURN(field) do {                \
> > +     if (a->field < b->field)                \
> > +             return -1;                      \
> > +     else if (a->field > b->field)           \
> > +             return 1;                       \
> > +} while (0)                                  \
> > +
> > +     COOKIE_CMP_RETURN(task_cookie);
> > +     COOKIE_CMP_RETURN(group_cookie);
> > +
> > +     /* all cookie fields match */
> > +     return 0;
> > +
> > +#undef COOKIE_CMP_RETURN
> > +}
>
> AFAICT all this madness exists because cgroup + task interaction, yet
> none of that code is actually dependent on cgroups being on.
>
> So this seems to implement semantics that will make two tasks that share
> a cookie, but are then placed in different cgroups not actually share.
>
> Is that desired? Can we justify these semantics and the resulting code
> complexity.

Yes that is the desired result. IMO it is less optimal from an
interface perspective if we were to instead have group or task cookie
override the other. Joel gave some additional justification here:
https://lkml.org/lkml/2020/12/6/389.