linux-kernel - Re: [PATCH] sched/deadline: Fix stale throttling on de-/boosted tasks

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Message-ID: <20200902094339.GZ1362448@hirez.programming.kicks-ass.net>
Date:   Wed, 2 Sep 2020 11:43:39 +0200
From:   peterz@...radead.org
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     Lucas Stach <l.stach@...gutronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel@...r.kernel.org, kernel@...gutronix.de,
        patchwork-lst@...gutronix.de
Subject: Re: [PATCH] sched/deadline: Fix stale throttling on de-/boosted tasks

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@...gutronix.de>

> Acked-by: Juri Lelli <juri.lelli@...hat.com>

Thanks!