On Wed, Nov 11, 2015 at 12:50:43PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 11, 2015 at 06:48:49PM +0900, Byungchul Park wrote:
> > On Wed, Nov 11, 2015 at 10:26:32AM +0100, Peter Zijlstra wrote:
> > > On Wed, Nov 11, 2015 at 05:50:27PM +0900, byungchul.p...@lge.com wrote:
> > >
> > > I've not actually read anything; my brain isn't working right today.
> > >
> > > > +static inline void vruntime_unnormalize(struct cfs_rq *cfs_rq, struct
> > > > sched_entity *se)
> > > > +{
> > > > + se->vruntime += cfs_rq->min_vruntime;
> > > > + if (unlikely((s64)se->vruntime < 0))
> > > > + se->vruntime = 0;
> > > > +}
> > >
> > > But this is broken. This simply _cannot_ be right.
> > >
> > > vruntime very much needs to wrap in u64 space. While regular time in ns
> > > takes some 584 year to wrap, vruntime is scaled. The fastest vruntime is
> > > 2/1024 or 512 times faster than normal time. Making it take just over a
> > > year to wrap around. This will happen.
> >
> > Then, do you mean it's no problem even if we compare between a vruntime
> > not wrapped yet and another vruntime already wrapped? I really wonder it.
>
> It should be; we were really careful with this back when we wrote all
> that. All vruntime comparisons should be of the form (s64)(a-b). Which
> gets you the correct order assuming things haven't drifted more than
> 2^63 apart.
I checked it. It looks no problem as you said.
Thank you very much.
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