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

From: Peter Zijlstra <peterz@infradead.org>
To: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: xen.list@daevel.fr, Rik van Riel <riel@redhat.com>,
	Dongli Zhang <dongli.zhang@oracle.com>,
	dario.faggioli@citrix.com, bevan@bi-co.net,
	Xiaolong Ye <xiaolong.ye@intel.com>,
	linux-kernel@vger.kernel.org, mingo@redhat.com,
	Frederic Weisbecker <fweisbec@gmail.com>,
	joao.m.martins@oracle.com, xen-devel@lists.xen.org,
	Ingo Molnar <mingo@kernel.org>,
	Wanpeng Li <wanpeng.li@hotmail.com>
Subject: Re: [PATCH 1/1] sched/cputime: do not decrease steal time after live migration on xen
Date: Tue, 10 Oct 2017 14:48:16 +0200	[thread overview]
Message-ID: <20171010124816.ujbjwqow4axpzgk2__18462.137951474$1507639771$gmane$org@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20171010124201.GD8263@redhat.com>

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 ?

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