2016-08-12 10:44 GMT+08:00 Rik van Riel <r...@redhat.com>: > On Thu, 2016-08-11 at 18:11 +0800, Wanpeng Li wrote: >> 2016-08-11 0:52 GMT+08:00 Rik van Riel <r...@redhat.com>: >> > On Wed, 10 Aug 2016 07:39:08 +0800 >> > Wanpeng Li <kernel...@gmail.com> wrote: >> > >> > > The regression is caused by your commit "sched,time: Count >> > > actually >> > > elapsed irq & softirq time". >> > >> > Wanpeng, does this patch fix your issue? >> >> I test this against kvm guest (nohz_full, four vCPUs running on one >> pCPU, four cpuhog processes running on four vCPUs). >> before this fix patch: >> vCPU0's st is 100%, other vCPUs' st are ~75%. >> after this fix patch: >> all vCPUs' st are ~85%. >> However, w/o commit "sched,time: Count actually elapsed irq & softirq >> time", all vCPUs' st are ~75%. > > If you pass ULONG_MAX as the maxtime argument to > steal_account_process_time(), does the steal time > get accounted properly at 75%?
Yes. > > If that is the case, I have a hypothesis: > 1) The guest is running so much slower when sharing > a CPU 4 ways, that it is accounting only ~90% of > wall clock time as CPU time, due to missing the > other 10% or so of clock ticks. > 2) account_process_tick() only ever processes one tick > at a time - if it gets called only 90x a second for > a 100Hz guest, but all the steal time recorded by > the host is fully accounted (ULONG_MAX limit), then > that could make up for lost/skipped timer ticks. > 3) not accounting "extra" steal time (beyond the amount > of time accounted by account_process_tick) would reduce > the total amount of time that gets accounted if there > are missed ticks, taking time away from user/system/etc > > Does the above make sense? > > Am I overlooking some mechanism through which lost/skipped > ticks are made up for in the kernel? I looked through the > code in kernel/time/ briefly, but did not spot it... > > -- > > All Rights Reversed.
