linux-kernel - Re: [RFC PATCH 12/13] sched: cgroup tagging interface for core scheduling

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Date:   Fri, 26 Jun 2020 11:06:35 -0400
From:   Vineeth Remanan Pillai <vpillai@...italocean.com>
To:     Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Turner <pjt@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <kerrnel@...gle.com>, Phil Auld <pauld@...hat.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [RFC PATCH 12/13] sched: cgroup tagging interface for core scheduling

On Wed, Mar 4, 2020 at 12:00 PM vpillai <vpillai@...italocean.com> wrote:
>
>
> 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.
>
This came up during a private discussion with Joel and thanks to
him for bringing this up! Details below..

> @@ -7910,7 +7986,12 @@ static void cpu_cgroup_fork(struct task_struct *task)
>         rq = task_rq_lock(task, &rf);
>
>         update_rq_clock(rq);
> +       if (sched_core_enqueued(task))
> +               sched_core_dequeue(rq, task);
A newly created task will not be enqueued and hence do we need this
here?

>         sched_change_group(task, TASK_SET_GROUP);
> +       if (sched_core_enabled(rq) && task_on_rq_queued(task) &&
> +           task->core_cookie)
> +               sched_core_enqueue(rq, task);
>
Do we need this here? Soon after this, wake_up_new_task() is called
which will ultimately call enqueue_task() and adds the task to the
coresched rbtree. So we will be trying to enqueue twice. Also, this
code will not really enqueue,  because task_on_rq_queued() would
return false at this point(activate_task is not yet called for this
new task).

I am not sure if I missed any other code path reaching here that
does not proceed with wake_up_new_task().Please let me know, if I
missed anything here.

Thanks,
Vineeth