Messages in this thread | | | Date | Wed, 11 Nov 2015 12:50:43 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched: prevent getting too much vruntime |
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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.park@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.
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