linux-kernel - Re: [git] CFS-devel, group scheduler, fixes

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Message-ID: <20070924130851.201d18ed@twins>
Date:	Mon, 24 Sep 2007 13:08:51 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Mike Galbraith <efault@....de>
Cc:	Tong Li <tong.n.li@...el.com>, Ingo Molnar <mingo@...e.hu>,
	dimm <dmitry.adamushko@...il.com>, linux-kernel@...r.kernel.org,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Subject: Re: [git] CFS-devel, group scheduler, fixes

On Mon, 24 Sep 2007 12:42:15 +0200 Mike Galbraith <efault@....de> wrote:

> On Mon, 2007-09-24 at 12:24 +0200, Peter Zijlstra wrote:
> 
> > how about something like:
> > 
> >  s64 delta = (s64)(vruntime - min_vruntime);
> >  if (delta > 0)
> >   min_vruntime += delta;
> > 
> > That would rid us of most of the funny conditionals there.
> 
> That still left me with negative min_vruntimes.  The pinned hogs didn't
> lock my box up, but I quickly got the below, so hastily killed it.
> 
> se.wait_max              :             7.846949
> se.wait_max              :           301.951601
> se.wait_max              :             7.071359
>

Odd, the idea (which I think is clear) is that min_vruntime can wrap
around the u64 spectrum. And by using min_vruntime as offset to base
the key around, we get a signed but limited range key-space. (because
we update min_vruntime to be the leftmost task (in a monotonic fashion))

So I'm having trouble with these patches, that is, both your wrap
around condition of:

  if (likely(new_rq->cfs.min_vruntime))

as well as the last patchlet:

  if (((s64)vruntime > (s64)min_vruntime) ||

in that neither of these changes make sense in what its trying to do.

Its perfectly valid for min_vruntime to exist in 1ULL << 63.

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