linux-kernel - Re: [PATCH -tip 22/32] sched: Split the cookie and setup per-task cookie on fork

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Message-ID: <20201202075447.GC3021@hirez.programming.kicks-ass.net>
Date:   Wed, 2 Dec 2020 08:54:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Don <joshdon@...gle.com>
Cc:     Joel Fernandes <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>, mingo@...nel.org,
        torvalds@...ux-foundation.org, fweisbec@...il.com,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <kerrnel@...gle.com>, 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,
        Paul Turner <pjt@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Patrick Bellasi <derkling@...gle.com>, benbjiang@...cent.com,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jesse Barnes <jsbarnes@...gle.com>, chris.hyser@...cle.com,
        Ben Segall <bsegall@...gle.com>, Hao Luo <haoluo@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>,
        Oleg Rombakh <olegrom@...gle.com>
Subject: Re: [PATCH -tip 22/32] sched: Split the cookie and setup per-task
 cookie on fork

On Tue, Dec 01, 2020 at 10:36:18PM -0800, Josh Don wrote:
> On Tue, Dec 1, 2020 at 11:35 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > So I don't think that later patch is right... That is, it works, but
> > afaict it's massive overkill.
> >
> >         COOKIE_CMP_RETURN(task_cookie);
> >         COOKIE_CMP_RETURN(group_cookie);
> >         COOKIE_CMP_RETURN(color);
> >
> > So if task_cookie matches, we consider group_cookie, if that matches we
> > consider color.
> >
> > Now, afaict that's semantically exactly the same as just using the
> > narrowest cookie. That is, use the task cookie if there is, and then,
> > walking the cgroup hierarchy (up) pick the first cgroup cookie.
> >
> > (I don't understand the color thing, but lets have that discussion in
> > that subthread)
> >
> > Which means you only need a single active cookie field.
> >
> > IOW, you're just making things complicated and expensive.
> >
> 
> For the per-task interface, I believe we still want to prevent two
> tasks that share a task cookie from sharing an overall cookie if they
> are in two separately tagged groups (Joel please correct me if I'm
> mistaken there). That's why in Joel's older patch, the overall cookie
> was a combination of the task and group cookies.  My concern about
> that was the potential cookie collision.

Then disallow sharing a task cookie when the tasks are in different
cgroups or disallow cgroup movement when they share a cookie.