linux-kernel - Re: Discrepancy between comments for sched_find_first

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Date:	Fri, 2 Apr 2010 22:16:48 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Matt Turner <mattst88@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-alpha@...r.kernel.org
Subject: Re: Discrepancy between comments for sched_find_first_bit


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Sun, 2010-03-28 at 23:37 -0400, Matt Turner wrote:
> > include/asm-generic/bitops/sched.h says
> > /*
> >  * Every architecture must define this function. It's the fastest
> >  * way of searching a 100-bit bitmap.  It's guaranteed that at least
> >  * one of the 100 bits is cleared.
> >  */
> > 
> > arch/alpha/include/asm/bitops.h says
> > /*
> >  * Every architecture must define this function. It's the fastest
> >  * way of searching a 140-bit bitmap where the first 100 bits are
> >  * unlikely to be set. It's guaranteed that at least one of the 140
> >  * bits is set.
> >  */
> > 
> > Is the guarantee that one of the first 100-bits set (and that the last
> > 40 are useless?), or 140-bits? If it's just the first 100 bits we care
> > about, then the alpha version needs to be fixed.
> > 
> > I'm just checking this out, because gcc produces horrendous code for
> > sched_find_first_bit on alpha. I rewrote it in assembly and it's
> > better than 4 times faster.
> > 
> > Also, is it even worth optimizing that function? It looks like it's
> > only used in kernel/sched_rt.c.
> 
> (might help if you CC the scheduler people on scheduler functions :-)
> 
> Right, so it used to be 140 bits with the old O(1) scheduler, currently
> (as you noted) sched_rt is the only user left and will therefore only
> care about the first 100 bits.
> 
> As it stands I think it might still make sense to optimize this as for
> rt loads it still on a hot path.
> 
> As to the 100 vs 140 length, would it really make much of difference to
> shorten the implementation to 100? If not I'd leave it at 140.
> 
> Ingo, any comments?

I guess getting below the 128 bits boundary would shave an instruction and a 
branch off or so?

	Ingo
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