2017-03-30 14:47 GMT+08:00 Wanpeng Li <kernel...@gmail.com>: > Cc Peterz, Thomas, > 2017-03-30 12:27 GMT+08:00 Mike Galbraith <efa...@gmx.de>: >> On Wed, 2017-03-29 at 16:08 -0400, Rik van Riel wrote: >> >>> 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? >> >> You really want skew_tick=1, especially on big boxen. >> >>> 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? >> >> (I or low rq->clock bits as crude NOHZ collision avoidance) >> >>> 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. >> >> That improves jitter, especially on big boxen. I have an 8 socket box >> that thinks it's an extra large PC, there, collision avoidance matters >> hugely. I couldn't reproduce bean counting woes, no idea if collision >> avoidance will help that. > > So I implement two methods, one is from Rik's random offset proposal
If we should just add random offset to the cpu in the nohz_full mode? > through skew tick, the other one is from Frederic's proposal and it is > the same as my original idea through use nanosecond granularity to > check deltas but only perform an actual cputime update when that delta >>= TICK_NSEC. Both methods can solve the bug which Luiz reported. This can just solves two cpu hogs running on the cpu in nohz_full mode. However, Luiz's testcase w/ ./acct-bug 1 995 shows idle 100%. Regards, Wanpeng Li > Peterz, Thomas, any ideas? > > --------------------------->8------------------------------------------------------------- > > skew tick: > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > index 7fe53be..9981437 100644 > --- a/kernel/time/tick-sched.c > +++ b/kernel/time/tick-sched.c > @@ -1198,7 +1198,11 @@ void tick_setup_sched_timer(void) > hrtimer_set_expires(&ts->sched_timer, tick_init_jiffy_update()); > > /* Offset the tick to avert jiffies_lock contention. */ > +#ifdef CONFIG_NO_HZ_FULL > + if (sched_skew_tick || tick_nohz_full_running) { > +#else > if (sched_skew_tick) { > +#endif > u64 offset = ktime_to_ns(tick_period) >> 1; > do_div(offset, num_possible_cpus()); > offset *= smp_processor_id(); > > -------------------------------------->8----------------------------------------------------- > > use nanosecond granularity to check deltas but only perform an actual > cputime update when that delta >= TICK_NSEC. > > diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c > index f3778e2b..f1ee393 100644 > --- a/kernel/sched/cputime.c > +++ b/kernel/sched/cputime.c > @@ -676,18 +676,21 @@ void thread_group_cputime_adjusted(struct > task_struct *p, u64 *ut, u64 *st) > #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN > static u64 vtime_delta(struct task_struct *tsk) > { > - unsigned long now = READ_ONCE(jiffies); > + u64 now = local_clock(); > + u64 delta; > + > + delta = now - tsk->vtime_snap; > > - if (time_before(now, (unsigned long)tsk->vtime_snap)) > + if (delta < TICK_NSEC) > return 0; > > - return jiffies_to_nsecs(now - tsk->vtime_snap); > + return jiffies_to_nsecs(delta / TICK_NSEC); > } > > static u64 get_vtime_delta(struct task_struct *tsk) > { > - unsigned long now = READ_ONCE(jiffies); > - u64 delta, other; > + u64 delta = vtime_delta(tsk); > + u64 other; > > /* > * Unlike tick based timing, vtime based timing never has lost > @@ -696,10 +699,9 @@ static u64 get_vtime_delta(struct task_struct *tsk) > * elapsed time. Limit account_other_time to prevent rounding > * errors from causing elapsed vtime to go negative. > */ > - delta = jiffies_to_nsecs(now - tsk->vtime_snap); > other = account_other_time(delta); > WARN_ON_ONCE(tsk->vtime_snap_whence == VTIME_INACTIVE); > - tsk->vtime_snap = now; > + tsk->vtime_snap += delta; > > return delta - other; > } > @@ -776,7 +778,7 @@ void arch_vtime_task_switch(struct task_struct *prev) > > write_seqcount_begin(¤t->vtime_seqcount); > current->vtime_snap_whence = VTIME_SYS; > - current->vtime_snap = jiffies; > + current->vtime_snap = sched_clock_cpu(smp_processor_id()); > write_seqcount_end(¤t->vtime_seqcount); > } > > @@ -787,7 +789,7 @@ void vtime_init_idle(struct task_struct *t, int cpu) > local_irq_save(flags); > write_seqcount_begin(&t->vtime_seqcount); > t->vtime_snap_whence = VTIME_SYS; > - t->vtime_snap = jiffies; > + t->vtime_snap = sched_clock_cpu(cpu); > write_seqcount_end(&t->vtime_seqcount); > local_irq_restore(flags); > } > > Regards, > Wanpeng Li