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

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Date:	Tue, 16 Aug 2016 10:01:44 -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 Tue, 2016-08-16 at 14:54 +0800, Wanpeng Li wrote:
> 2016-08-16 10:11 GMT+08:00 Rik van Riel <riel@...hat.com>:
> > On Tue, 2016-08-16 at 09:31 +0800, Wanpeng Li wrote:
> > > 2016-08-15 23:00 GMT+08:00 Rik van Riel <riel@...hat.com>:
> > > > On Mon, 2016-08-15 at 16:53 +0800, Wanpeng Li wrote:
> > > > > 2016-08-12 23:58 GMT+08:00 Rik van Riel <riel@...hat.com>:
> > > > > [...]
> > > > > > Wanpeng, does the patch below work for you?
> > > > > 
> > > > > It will break steal time for full dynticks guest, and there
> > > > > is a
> > > > > calltrace of thread_group_cputime_adjusted call stack, RIP is
> > > > > cputime_adjust+0xff/0x130.
> > > > 
> > > > How?  This patch is equivalent to passing ULONG_MAX to
> > > > steal_account_process_time, which you tried to no ill
> > > > effect before.
> > > 
> > > https://lkml.org/lkml/2016/6/8/404/ Paolo original suggested to
> > > add
> > > the max cputime limit to the vtime, when the cpu is running in
> > > nohz
> > > full mode and stop the tick, jiffies will be updated depends on
> > > clock
> > > source instead of clock event device in
> > > guest(tick_nohz_update_jiffies() callsite, ktime_get()), so it
> > > will
> > > not be affected by lost clock ticks, my patch keeps the limit for
> > > vtime and remove the limit to non-vtime. However, your patch
> > > removes
> > > the limit for both scenarios and results in the below calltrace
> > > for
> > > vtime.
> > 
> > I understand what it does.
> > 
> > What I would like to understand is WHY enforcing the limit
> > is the right thing when using vtime, and the wrong thing
> > in all other scenarios.
> 
> I observed that function get_vtime_delta() underflow which means that
> delta < other when debugging your bugfix patch, I believe that is why
> Paolo suggested to add the max cputime limit to vtime, he also
> pointed
> out the potentional underflow before
> https://lkml.org/lkml/2016/6/8/404/

Looking at get_vtime_delta() I can see exactly how the underflow
can happen.  The interval returned by account_other_time() is NOT
rounded down to the nearest jiffy, while the base interval it is
subtracted from is.

Furthermore, even if we did not have that rounding issue, a guest
could get preempted in-between determining delta, and calling
account_other_time(), which could also cause the issue.

Could you re-send your patch with a comment in get_vtime_delta(),
as well as the changelog, explaining exactly why account_other_time()
should be limited from get_vtime_delta(), but not from the other
three call sites?

Documentation could save future developers a bunch of debugging
time on this code.

thanks,

Rik
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