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

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Date:	Thu, 11 Aug 2016 22:44:52 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Radim Krcmar <rkrcmar@...hat.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH] time,virt: resync steal time when guest & host lose sync

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

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...

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