From:   Valentin Schneider <vschneid@redhat.com>
To:     Juri Lelli <juri.lelli@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()
In-Reply-To: <Y9yydSfMvTHhyEqP@localhost.localdomain>
References: <20230202182854.3696665-1-vschneid@redhat.com>
 <Y9yydSfMvTHhyEqP@localhost.localdomain>
Date:   Mon, 06 Feb 2023 12:46:51 +0000
Message-ID: <xhsmhbkm7rmf8.mognet@vschneid.remote.csb>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On 03/02/23 08:06, Juri Lelli wrote:
> 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.
>

Indeed, I thought myself clever by getting rid of the ifdefs...

> Thanks,
> Juri