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

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Message-ID: <20151112051038.GF7136@byungchulpark-X58A-UD3R>
Date:	Thu, 12 Nov 2015 14:10:39 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Peter Zijlstra <peterz@...radead.org>
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 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.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.

I checked it. It looks no problem as you said.

Thank you very much.

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