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

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Message-ID: <20160813153601.GA21154@lerouge>
Date:	Sat, 13 Aug 2016 17:36:04 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Wanpeng Li <kernellwp@...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 Fri, Aug 12, 2016 at 11:58:03AM -0400, Rik van Riel wrote:
> On Fri, 12 Aug 2016 15:09:00 +0800
> Wanpeng Li <kernellwp@...il.com> wrote:
> > 2016-08-12 10:44 GMT+08:00 Rik van Riel <riel@...hat.com>:
> 
> > > If you pass ULONG_MAX as the maxtime argument to
> > > steal_account_process_time(), does the steal time
> > > get accounted properly at 75%?  
> > 
> > Yes.
> 
> I talked with Paolo this morning, and it turns out that if a guest
> misses several timer ticks in a row, they will simply get lost.
> 
> That means the functions calling steal_account_process_time may not
> know how much CPU time has passed since the last time it was called,
> but steal_account_process_time will get a good idea on how much time
> the host spent running something else.
> 
> Removing the limit, and documenting why, seems like the right way to
> fix this bug.
> 
> Wanpeng, does the patch below work for you?
> 
> Everybody else, does this patch look acceptable?
> 
> ---8<---
> Subject: time,virt: do not limit steal_account_process_time
> 
> When a guest is interrupted for a longer amount of time, missed clock
> ticks are not redelivered later. Because of that, we should not limit
> the amount of steal time accounted to the amount of time that the
> calling functions think have passed.
> 
> Instead, simply let steal_account_process_time account however much
> steal time the host told us elapsed. This can make up timer ticks
> that were missed when the host scheduled somebody else.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> Reported-by: Wanpeng Li <kernellwp@...il.com>

I much prefer this version. After all, even if that time spent in
host is very large, it's still stolen time and we want to account it.

Thanks.