Messages in this thread | | | Date | Mon, 10 Apr 2017 20:01:46 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH] tick/nohz: Fix wrong user and system time accouting against vtime sampling |
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On Mon, Apr 10, 2017 at 05:45:56PM +0200, Thomas Gleixner wrote: > On Wed, 5 Apr 2017, Wanpeng Li wrote: > > + /* > > + * Offset the tick to avert jiffies_lock contention, and all ticks > > + * alignment in order that the vtime sampling does not end up "in > > + * phase" with the jiffies incrementing. > > + */ > > + if (sched_skew_tick || tick_nohz_full_enabled()) { > > u64 offset = ktime_to_ns(tick_period) >> 1; > > do_div(offset, num_possible_cpus()); > > offset *= smp_processor_id(); > > That's not a fix, that's just papering over the problem. > > offset = 1ms / 2 = 500us = 500000ns; > offset /= 144 = 3472ns > > So CPU0 and CPU1 ticks are ~3 microseconds apart. That merily reduces the > probability of the issue, but does not prevent it.
I worried about it but didn't realize it could be that tight.
So the alternative is the solution involving sched_clock() as the source for cputime. Wanpeng Li could you please resubmit your patch that does that?
Thanks.
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