linux-kernel - Re: [PATCH resend 5/8] sched: cgroup cookie API for core scheduling

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Date:   Tue, 30 Mar 2021 14:19:47 -0700
From:   Josh Don <joshdon@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>, torvalds@...ux-foundation.org,
        fweisbec@...il.com, Kees Cook <keescook@...omium.org>,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
        Steven Rostedt <rostedt@...dmis.org>, benbjiang@...cent.com,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        "Hyser,Chris" <chris.hyser@...cle.com>,
        Hao Luo <haoluo@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        dhiatt@...italocean.com, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH resend 5/8] sched: cgroup cookie API for core scheduling

On Tue, Mar 30, 2021 at 2:29 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > On Wed, Mar 24, 2021 at 05:40:17PM -0400, Joel Fernandes (Google) wrote:
> > > +
> > > +   if (!tg->core_tagged && val) {
> > > +           /* Tag is being set. Check ancestors and descendants. */
> > > +           if (cpu_core_get_group_cookie(tg) ||
> > > +               cpu_core_check_descendants(tg, true /* tag */)) {
> > > +                   ret = -EBUSY;
> > > +                   goto out_unlock;
> > > +           }
> >
> > So the desired semantics is to only allow a single tag on any upwards
> > path? Isn't that in conflict with the cgroup requirements?
> >
> > TJ?

I carried this requirement over from the previous iteration, but I
don't see a reason why we can't just dump this and have each task use
the group cookie of its closest tagged ancestor. Joel, is there any
context here I'm missing?

FWIW I also just realized that cpu_core_check_descendants() is busted
as it recurses only on one child.

> > > +   } else if (tg->core_tagged && !val) {
> > > +           /* Tag is being reset. Check descendants. */
> > > +           if (cpu_core_check_descendants(tg, true /* tag */)) {
> >
> > I'm struggling to understand this. If, per the above, you cannot set
> > when either a parent is already set or a child is set, then how can a
> > child be set to refuse clearing?

Yes this is superfluous with the above semantics.