2017-03-30 4:08 GMT+08:00 Rik van Riel <r...@redhat.com>: > On Wed, 2017-03-29 at 13:16 -0400, Luiz Capitulino wrote: >> On Tue, 28 Mar 2017 13:24:06 -0400 >> Luiz Capitulino <lcapitul...@redhat.com> wrote: >> >> > 1. In my tracing I'm seeing that sometimes (always?) the >> > time interval between two timer interrupts is less than 1ms >> >> I think that's the root cause. >> >> In this trace, we see the following: >> >> 1. On CPU15, we transition from user-space to kernel-space because >> of a timer interrupt (it's the tick) >> >> 2. vtimer_delta() returns 0, because jiffies didn't change since the >> last accounting >> >> 3. While CPU15 is executing in kernel-space, jiffies is updated >> by CPU0 >> >> 4. When going back to user-space, vtime_delta() returns non-zero >> and the whole time is accounted for system time (observe how >> the cputime parameter in account_system_time() is less than 1ms) > > In other words, the tick on cpu0 is aligned > with the tick on the nohz_full cpus, and > jiffies is advanced while the nohz_full cpus > with an active tick happen to be in kernel > mode? > > Frederic, can you think of any reason why > the tick on nohz_full CPUs would end up aligned > with the tick on cpu0, instead of running at some > random offset? > > A random offset, or better yet a somewhat randomized > tick length to make sure that simultaneous ticks are > fairly rare and the vtime sampling does not end up > "in phase" with the jiffies incrementing, could make > the accounting work right again. > > Of course, that assumes the above hypothesis is correct :)
There is such a feature skew_tick currently, refer to commit 5307c9556bc (tick: add tick skew boot option), w/ skew_tick=1 boot parameter, the bug disappear, however, the commit also mentioned that it will hurt power consumption. I will try Frederic's proposal which is similar to my original idea "how bad would it be to revert to sched_clock() instead of jiffies in vtime_delta()? We could use nanosecond granularity to check deltas but only perform an actual cputime update when that delta >= TICK_NSEC." Regards, Wanpeng Li