On Tue, Apr 11, 2017 at 07:03:17PM +0800, Wanpeng Li wrote: > 2017-03-30 21:38 GMT+08:00 Frederic Weisbecker <[email protected]>: > > On Thu, Mar 30, 2017 at 02:47:11PM +0800, Wanpeng Li wrote: > > [...] > > > > >> > >> -------------------------------------->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(); > > > > I fear we need a global clock, because the reader (task_cputime()) needs > > to compute the delta and therefore use the same clock from any CPU. > > > > Or we can use the local_clock() but the reader must access the same. > > > > So there would be vtime_delta_writer() which uses local_clock and stores > > the current CPU to tsk->vtime_cpu (under the vtime_seqcount). And then > > vtime_delta_reader() which calls sched_clock_cpu(tsk->vtime_cpu) which > > is protected by vtime_seqcount as well. > > > > Although those sched_clock_cpu() things seem to only matter when the > > sched_clock() is unstable. And that stability is a condition for nohz_full > > to work anyway. So probably sched_clock() alone would be enough. > > I observed ~60% user time and ~40% sys time when replace local_clock() > above by sched_clock()(two cpu hogs on the cpu in nohz_full mode). In > addition, Luiz's testcast ./acct-bug 1 995 will show 100% idle time. > If keep local_clock() in vtime_delta(), cpu hogs testcase will > success. However, Luiz's testcase still show 100% idle time.
Assuming a stable TSC, there should be no difference between local_clock() and sched_clock().
