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

[Peter Zijlstra]

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 ?