Messages in this thread | | | Date | Wed, 6 Nov 2019 09:37:49 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] timers/nohz: Update nohz load even if tick already stopped |
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On Tue, Nov 05, 2019 at 01:43:51PM +0100, Peter Zijlstra wrote: > On Tue, Nov 05, 2019 at 01:30:58AM -0600, Scott Wood wrote: > > The warning is due to kernel/sched/idle.c not updating curr->se.exec_start. > > Ah, indeed so. > > > While debugging I noticed an issue with a particular load pattern. The CPU > > goes non-nohz for a brief time at an interval very close to twice > > tick_period. When the tick is started, the timer expiration is more than > > tick_period in the past, so hrtimer_forward() tries to catch up by adding > > 2*tick_period to the expiration. Then the tick is stopped before that new > > expiration, and when the tick is woken up the expiry is again advanced by > > 2*tick_period with the timer never actually running. sched_tick_remote() > > does fire every second, but there are streaks of several seconds where it > > keeps catching the CPU in a non-nohz state, so neither the normal nor remote > > ticks are calling calc_load_nohz_remote(). > > > > Is there a reason to not just remove the hrtimer_forward() from > > tick_nohz_restart(), letting the timer fire if it's in the past, which will > > take care of doing hrtimer_forward()? > > I'll have to look into that. I always get confused by all that nohz code > :/ > > > As for the warning in sched_tick_remote(), it seems like a test for time > > since the last tick on this cpu (remote or otherwise) would be better than > > relying on curr->se.exec_start, in order to detect things like this. > > I don't think we have a timestamp that is shared between the remote and > local tick. Also, there is a reason this warning uses the task time > accounting, there used to be (as in, I can't find it in a hurry) code > that could not deal with >u32 (~4s) clock updates. > > The below should have idle keep the timestamp up-to-date. Keeping > accurate idle->se.sum_exec_runtime doesn't seem too interesting, the > idle code already keeps track of total idle times. > > ---
The obvious alternative is something like:
if (rq->curr != rq->idle) { s64 delta = rq_clock_task(rq) - curr->se.exec_start; WARN_ON_ONCE(delta > 3ULL * NSEC_PER_SEC); }
Which would avoid polluting the idle path with that extra assignment.
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