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From:   Josh Don <joshdon@google.com>
Date:   Thu, 4 Feb 2021 19:45:39 -0800
Message-ID: <CABk29NuS=7U4kgvGPf1=tVJxG3tiPqjr_H4ZhAFHaKSS4uqn2Q@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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On Thu, Feb 4, 2021 at 5:54 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, Jan 22, 2021 at 08:17:01PM -0500, Joel Fernandes (Google) wrote:
> > From: Peter Zijlstra <peterz@infradead.org>
>
> I'm thinking this is too much credit, I didn't write much of this.
>
> > Marks all tasks in a cgroup as matching for core-scheduling.
> >
> > A task will need to be moved into the core scheduler queue when the cgroup
> > it belongs to is tagged to run with core scheduling.  Similarly the task
> > will need to be moved out of the core scheduler queue when the cgroup
> > is untagged.
> >
> > Also after we forked a task, its core scheduler queue's presence will
> > need to be updated according to its new cgroup's status.
> >
> > Use stop machine mechanism to update all tasks in a cgroup to prevent a
> > new task from sneaking into the cgroup, and missed out from the update
> > while we iterates through all the tasks in the cgroup.  A more complicated
> > scheme could probably avoid the stop machine.  Such scheme will also
> > need to resovle inconsistency between a task's cgroup core scheduling
> > tag and residency in core scheduler queue.
> >
> > We are opting for the simple stop machine mechanism for now that avoids
> > such complications.
> >
> > Core scheduler has extra overhead.  Enable it only for core with
> > more than one SMT hardware threads.
>
> Very little actual words on the desired and implemented semantics of the
> interface, while the patch contains much non-obvious complication.

Ack to both, will fix in the next posting.