Re: [PATCH] time,virt: resync steal time when guest & host lose sync

[Rik van Riel]

On Thu, 2016-08-11 at 18:11 +0800, Wanpeng Li wrote:
> 2016-08-11 0:52 GMT+08:00 Rik van Riel <riel@xxxxxxxxxx>:
> > On Wed, 10 Aug 2016 07:39:08 +0800
> > Wanpeng Li <kernellwp@xxxxxxxxx> 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%?

If that is the case, I have a hypothesis:
1) The guest is running so much slower when sharing
 Âa CPU 4 ways, that it is accounting only ~90% of
 Âwall clock time as CPU time, due to missing the
 Âother 10% or so of clock ticks.
2) account_process_tick() only ever processes one tick
 Âat a time - if it gets called only 90x a second for
 Âa 100Hz guest, but all the steal time recorded by
 Âthe host is fully accounted (ULONG_MAX limit), then
 Âthat could make up for lost/skipped timer ticks.
3) not accounting "extra" steal time (beyond the amount
 Âof time accounted by account_process_tick) would reduce
 Âthe total amount of time that gets accounted if there
 Âare 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? ÂI looked through the
code in kernel/time/ briefly, but did not spot it...

-- 

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