linux-kernel - Re: [PATCH 1/1] sched/cputime: do not decrease steal time after live migration on xen

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Date:   Tue, 10 Oct 2017 10:01:02 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Stanislaw Gruszka <sgruszka@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Dongli Zhang <dongli.zhang@...cle.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Xiaolong Ye <xiaolong.ye@...el.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xen.org,
        mingo@...hat.com, dario.faggioli@...rix.com, bevan@...co.net,
        xen.list@...vel.fr, joao.m.martins@...cle.com
Subject: Re: [PATCH 1/1] sched/cputime: do not decrease steal time after
 live migration on xen

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.