linux-kernel - Re: [PATCH] sched: prevent getting too much vruntime

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Date:	Wed, 11 Nov 2015 12:50:43 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Byungchul Park <byungchul.park@....com>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org, efault@....de,
	tglx@...utronix.de, yuyang.du@...el.com, pjt@...gle.com
Subject: Re: [PATCH] sched: prevent getting too much vruntime

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@....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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