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

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Date:	Wed, 11 Nov 2015 10:26:32 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	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 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.

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