Date:   Fri, 3 Feb 2023 08:06:29 +0100
From:   Juri Lelli <juri.lelli@redhat.com>
To:     Valentin Schneider <vschneid@redhat.com>
Cc:     linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        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>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Eder Zulian <ezulian@redhat.com>
Subject: Re: [RFC PATCH v1] sched/deadline: Add more reschedule cases to
 prio_changed_dl()
Message-ID: <Y9yydSfMvTHhyEqP@localhost.localdomain>
References: <20230202182854.3696665-1-vschneid@redhat.com>
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Hi,

On 02/02/23 18:28, Valentin Schneider wrote:
> I've been tracking down an issue on a ~5.17ish kernel where:
> 
>   CPUx                           CPUy
> 
>   <DL task p0 owns an rtmutex M>
>   <p0 depletes its runtime, gets throttled>
>   <rq switches to the idle task>
> 				 <DL task p1 blocks on M, boost/replenish p0>
> 				 <No call to resched_curr() happens here>
> 
>   [idle task keeps running here until *something*
>    accidentally sets TIF_NEED_RESCHED]
> 
> On that kernel, it is quite easy to trigger using rt-tests's deadline_test
> [1] with the test running on isolated CPUs (this reduces the chance of
> something unrelated setting TIF_NEED_RESCHED on the idle tasks, making the
> issue even more obvious as the hung task detector chimes in).
> 
> I haven't been able to reproduce this using a mainline kernel, even if I
> revert
> 
>   2972e3050e35 ("tracing: Make trace_marker{,_raw} stream-like")
> 
> which gets rid of the lock involved in the above test, *but* I cannot
> convince myself the issue isn't there from looking at the code.
> 
> Make prio_changed_dl() issue a reschedule if the current task isn't a
> deadline one. While at it, ensure a reschedule is emitted when a
> queued-but-not-current task gets boosted with an earlier deadline that
> current's.

As discussed offline I agree this needs fixing, but .. :)

> [1]: https://git.kernel.org/pub/scm/utils/rt-tests/rt-tests.git
> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
> ---
>  kernel/sched/deadline.c | 45 ++++++++++++++++++++++++++---------------
>  1 file changed, 29 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 0d97d54276cc8..faa382ea084c1 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -2663,17 +2663,28 @@ static void switched_to_dl(struct rq *rq, struct task_struct *p)
>  static void prio_changed_dl(struct rq *rq, struct task_struct *p,
>  			    int oldprio)
>  {
> -	if (task_on_rq_queued(p) || task_current(rq, p)) {
> -#ifdef CONFIG_SMP

Doesn't this break UP? Don't think earlierst_dl etc are defined in UP.

Thanks,
Juri