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SubjectRe: [BUG] Caused by: sched/deadline: Move CPU frequency selection triggering points
Hi Steve,

On 03/07/18 10:54, Steven Rostedt wrote:
> When looking to test SCHED_DEADLINE, I triggered a lockup. The lockup
> appears to be caused by WARN_ON() done inside the scheduling path, and
> I'm guessing it tried to grab the rq lock and caused a deadlock (all I
> would get would be the "--- cut here ---" line, and then nothing after
> that. But a bit of playing with the printks() I figured out it was
> happening at:
>
> assert_clock_updated() {
> SCHED_WARN_ON(rq->clock_update_flags < RQCF_ACT_SKIP);
> }
>
> I bisected it down to commit e0367b126 ("sched/deadline: Move CPU
> frequency selection triggering points"). Reverting it indeed makes the
> deadlock go away.
>
> I commented out the WARN_ON that was being triggered, and that let the
> system continue to run. I did the following change:
>
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -914,7 +914,12 @@ static inline void assert_clock_updated(struct rq *rq)
> * The only reason for not seeing a clock update since the
> * last rq_pin_lock() is if we're currently skipping updates.
> */
> - SCHED_WARN_ON(rq->clock_update_flags < RQCF_ACT_SKIP);
> + if (rq->clock_update_flags < RQCF_ACT_SKIP)
> + trace_printk("WARN_ON: [%d] rq->clock_update_flags (%d) < %d)\n",
> + rq->cpu, rq->clock_update_flags, RQCF_ACT_SKIP);
> + else
> + trace_printk("GOOD: [%d] rq->clock_update_flags (%d) >= %d)\n",
> + rq->cpu, rq->clock_update_flags, RQCF_ACT_SKIP);
> }
>
> To see what was happening. I also, added trace_printk()s to all the
> updates to clock_update_flags, and ran my test again.
>
> Here's what I got:
>
> deadline_test-1393 [002] 162.127132: bprint: push_dl_task.part.40: WARN_ON: [1] rq->clock_update_flags (0) < 2)
> deadline_test-1393 [002] 162.127133: bprint: update_rq_clock: [1] clock was 0 now 4
> deadline_test-1393 [002] 162.127134: bprint: rq_clock: GOOD: [1] rq->clock_update_flags (4) >= 2)
>
> It appears that we hit this in this path:
>
> push_dl_task {
> add_running_bw() {
> __add_running_bw() {
> cpufreq_update_util() {
> data->func(data, rq_clock(rq), flags);
> rq_clock() {
> assert_clock_updated()
>
> And here the clock isn't updated and we get the splat.
>
> Reverting the stated patch works because it added the call to
> cpufreq_update_util() that does the rq_clock() at an inappropriate
> time, which causes the splat.
>
> I'm not sure what the right answer to this is. Reverting obviously
> works, but I'm also guessing a proper placement of update_rq_clock()
> may also work. I just don't know where that placement is, as I don't
> understand the rq_clock() updates enough.
>
> Help?

This got into tip quite recently

ecda2b66e263 ("sched/deadline: Fix missing clock update")

could you please double check that you have that in your stack?

Thanks,

- Juri

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