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

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Message-ID: <1471022601.32433.2.camel@redhat.com>
Date:	Fri, 12 Aug 2016 13:23:21 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Wanpeng Li <kernellwp@...il.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	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 Fri, 2016-08-12 at 18:33 +0200, Paolo Bonzini wrote:
> 
> On 10/08/2016 18:52, Rik van Riel wrote:
> > Paolo, what is your opinion on this issue?
> > 
> > I can think of all kinds of ways in which guest and host might lose
> > sync with steal time, from uninitialized values at boot, to guest
> > pause, followed by save to disk, and reload, to live migration,
> > to...
> 
> Guest and host _cannot_ lose sync because there is only one copy of
> the
> values.  When the host wants to update the steal time value it just
> reads the old value and writes the new value.  There cannot be a
> guest
> pause, save to disk, live migration or whatever between these two
> steps
> (and uninitialized values at boot are not how percpu values work).

There is one copy of paravirt_steal_clock(smp_processor_id()),
but what keeps it in sync with this_rq()->prev_steal_time?

Is it something simple like them both being zeroed out when
the structures are first allocated at boot time?

> Your hypothesis of lost ticks makes the most sense to me, and then
> changing the argument to ULONG_MAX is the right thing to do.

I sent out a patch that just removes the parameter instead,
and documents why steal_account_process_time can encounter
more elapsed time than the calling functions expected.

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