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Subject: Re: [PATCH 1/1] sched/cputime: do not decrease steal time after
 live migration on xen
From: Rik van Riel <riel@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>,
        Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>, Dongli Zhang <dongli.zhang@oracle.com>,
        Wanpeng Li <wanpeng.li@hotmail.com>,
        Xiaolong Ye <xiaolong.ye@intel.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, linux-kernel@vger.kernel.org,
        xen-devel@lists.xen.org, mingo@redhat.com, dario.faggioli@citrix.com,
        bevan@bi-co.net, xen.list@daevel.fr, joao.m.martins@oracle.com
Date: Tue, 10 Oct 2017 10:01:02 -0400
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On Tue, 2017-10-10 at 14:48 +0200, Peter Zijlstra wrote:
> On Tue, Oct 10, 2017 at 02:42:01PM +0200, Stanislaw Gruszka wrote:
> > > > +		u64 steal, steal_time;
> > > > +		s64 steal_delta;
> > > > +
> > > > +		steal_time =
> > > > paravirt_steal_clock(smp_processor_id());
> > > > +		steal = steal_delta = steal_time - this_rq()-
> > > > >prev_steal_time;
> > > > +
> > > > +		if (unlikely(steal_delta < 0)) {
> > > > +			this_rq()->prev_steal_time =
> > > > steal_time;
> > 
> > I don't think setting prev_steal_time to smaller value is right
> > thing to do. 
> > 
> > Beside, I don't think we need to check for overflow condition for
> > cputime variables (it will happen after 279 years :-). So instead
> > of introducing signed steal_delta variable I would just add
> > below check, which should be sufficient to fix the problem:
> > 
> > 	if (unlikely(steal <= this_rq()->prev_steal_time))
> > 		return 0;
> 
> How about you just fix up paravirt_steal_time() on migration and not
> muck with the users ?

Not just migration, either. CPU hotplug is another time to fix up
the steal time.