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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. > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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