Messages in this thread |  | | Date | Thu, 12 Dec 2013 19:24:14 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 2/2] sched: update runqueue clock before migrations away |
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On Tue, Dec 10, 2013 at 03:55:43PM +0000, Chris Redpath wrote: > >That's guestimating the last_runnable_update based on decay_count, and > >per the previous the decay count can get slightly out of sync. > > The guesstimation works fine, the issue is only that we can't tell at > this point how much time that entity was asleep when the CPU it ran on > has no tick and since it is too expensive to synchronize the clocks, > there isn't (currently) a way to find out without updating the rq we > came from. > > I can't see anything handy lying around in struct rq, but is there a > copy of the jiffies held anywhere per-cpu presuming it would stop being > updated when the tick stops? If not, I could store one somewhere as part > of turning the tick off and then we could use the difference between > that and the current jiffies count to estimate the amount of time in > limbo. That would almost certainly be accurate enough for me - a few ms > won't hurt but when we lose seconds it does.
Would pre_schedule_idle() -> rq_last_tick_reset() -> rq->last_sched_tick be useful?
I suppose we could easily lift that to NO_HZ_COMMON.
Which raises another point; I dislike this idle pre/post business. Why can't the post_schedule_idle() calls be done from pick_next_task_idle() and pre_schedule_idle() done from put_prev_task_idle() ?
That would avoid the post_schedule() rq->lock dance.
Vince?
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