Re: [PATCH] sched: prevent getting too much vruntime

[Byungchul Park]

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@xxxxxxx 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.

> 
> 
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