Messages in this thread | | | Date | Tue, 3 Jul 2018 17:07:11 +0200 | From | Juri Lelli <> | Subject | Re: [BUG] Caused by: sched/deadline: Move CPU frequency selection triggering points |
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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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