linux-kernel - Re: [PATCH V2 2/2] rt: Increase/decrease the nr of migratory tasks when enabling/disabling migration

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Date:   Fri, 30 Jun 2017 09:30:55 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     linux-rt-users <linux-rt-users@...r.kernel.org>,
        "Luis Claudio R . Goncalves" <lgoncalv@...hat.com>,
        Clark Williams <williams@...hat.com>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 2/2] rt: Increase/decrease the nr of migratory tasks
 when  enabling/disabling migration


* Daniel Bristot de Oliveira <bristot@...hat.com> wrote:

> There is a problem in the migrate_disable()/enable() implementation
> regarding the number of migratory tasks in the rt/dl RQs. The problem
> is the following:
> 
> When a task is attached to the rt runqueue, it is checked if it either
> can run in more than one CPU, or if it is with migration disable. If
> either check is true, the rt_rq->rt_nr_migratory counter is not
> increased. The counter increases otherwise.
> 
> When the task is detached, the same check is done. If either check is
> true, the rt_rq->rt_nr_migratory counter is not decreased. The counter
> decreases otherwise. The same check is done in the dl scheduler.
> 
> One important thing is that, migrate disable/enable does not touch this
> counter for tasks attached to the rt rq. So suppose the following chain
> of events.
> 
> Assumptions:
> Task A is the only runnable task in A      Task B runs on the CPU B
> Task A runs on CFS (non-rt)                Task B has RT priority
> Thus, rt_nr_migratory is 0                 B is running
> Task A can run on all CPUS.
> 
> Timeline:
>         CPU A/TASK A                        CPU B/TASK B
> A takes the rt mutex X                           .
> A disables migration                             .
>            .                          B tries to take the rt mutex X
>            .                          As it is held by A {
>            .                            A inherits the rt priority of B
>            .                            A is dequeued from CFS RQ of CPU A
>            .                            A is enqueued in the RT RQ of CPU A
>            .                            As migration is disabled
>            .                              rt_nr_migratory in A is not increased
>            .
> A enables migration
> A releases the rt mutex X {
>   A returns to its original priority
>   A ask to be dequeued from RT RQ {
>     As migration is now enabled and it can run on all CPUS {
>        rt_nr_migratory should be decreased
>        As rt_nr_migratory is 0, rt_nr_migratory under flows
>     }
> }
> 
> This variable is important because it notifies if there are more than one
> runnable & migratory task in the runqueue. If there are more than one
> tasks, the rt_rq is set as overloaded, and then tries to migrate some
> tasks. This rule is important to keep the scheduler working conserving,
> that is, in a system with M CPUs, the M highest priority tasks should be
> running.
> 
> As rt_nr_migratory is unsigned, it will become > 0, notifying that the
> RQ is overloaded, activating pushing mechanism without need.
> 
> This patch fixes this problem by decreasing/increasing the
> rt/dl_nr_migratory in the migrate disable/enable operations.
> 
> Reported-by: Pei Zhang <pezhang@...hat.com>
> Reported-by: Luiz Capitulino <lcapitulino@...hat.com>
> Signed-off-by: Daniel Bristot de Oliveira <bristot@...hat.com>
> Cc: Luis Claudio R. Goncalves <lgoncalv@...hat.com>
> Cc: Clark Williams <williams@...hat.com>
> Cc: Luiz Capitulino <lcapitulino@...hat.com>
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: LKML <linux-kernel@...r.kernel.org>
> Cc: linux-rt-users <linux-rt-users@...r.kernel.org>
> ---
>  kernel/sched/core.c | 49 ++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 44 insertions(+), 5 deletions(-)

This second patch does not apply to the latest scheduler tree (tip:master) cleanly 
- which tree is it against?

Thanks,

	Ingo