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Subject: Re: [PATCH] sched/deadline: Fix stale throttling on de-/boosted tasks
To:     peterz@infradead.org, Juri Lelli <juri.lelli@redhat.com>
Cc:     Lucas Stach <l.stach@pengutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel@vger.kernel.org, kernel@pengutronix.de,
        patchwork-lst@pengutronix.de
References: <20200831110719.2126930-1-l.stach@pengutronix.de>
 <20200902060024.GK16601@localhost.localdomain>
 <20200902094339.GZ1362448@hirez.programming.kicks-ass.net>
From:   Daniel Bristot de Oliveira <bristot@redhat.com>
Message-ID: <bbcd9183-5a48-b3b1-516c-041db8e2f5aa@redhat.com>
Date:   Wed, 16 Sep 2020 09:40:03 +0200
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On 9/2/20 11:43 AM, peterz@infradead.org wrote:
> On Wed, Sep 02, 2020 at 08:00:24AM +0200, Juri Lelli wrote:
>> On 31/08/20 13:07, Lucas Stach wrote:
>>> When a boosted task gets throttled, what normally happens is that it's
>>> immediately enqueued again with ENQUEUE_REPLENISH, which replenishes the
>>> runtime and clears the dl_throttled flag. There is a special case however:
>>> if the throttling happened on sched-out and the task has been deboosted in
>>> the meantime, the replenish is skipped as the task will return to its
>>> normal scheduling class. This leaves the task with the dl_throttled flag
>>> set.
>>>
>>> Now if the task gets boosted up to the deadline scheduling class again
>>> while it is sleeping, it's still in the throttled state. The normal wakeup
>>> however will enqueue the task with ENQUEUE_REPLENISH not set, so we don't
>>> actually place it on the rq. Thus we end up with a task that is runnable,
>>> but not actually on the rq and neither a immediate replenishment happens,
>>> nor is the replenishment timer set up, so the task is stuck in
>>> forever-throttled limbo.
>>>
>>> Clear the dl_throttled flag before dropping back to the normal scheduling
>>> class to fix this issue.
>>>
>>> Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
> 
>> Acked-by: Juri Lelli <juri.lelli@redhat.com>

I faced a similar issue, but involving DL tasks (not !DL):
  https://lore.kernel.org/lkml/5076e003450835ec74e6fa5917d02c4fa41687e6.1600170294.git.bristot@redhat.com/

While debugging that problem, I reviewed and tested this patch,
and we need it. So:

Reviewed-by: Daniel Bristot de Oliveira <bristot@redhat.com>

Thanks!
-- Daniel


> Thanks!
>