linux-kernel - Re: [patch] CFS scheduler, -v18

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Date:	Tue, 26 Jun 2007 02:00:47 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	S.Çağlar Onur <caglar@...dus.org.tr>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Subject: Re: [patch] CFS scheduler, -v18

On Tue, 26 Jun 2007 10:38:13 +0200 Ingo Molnar <mingo@...e.hu> wrote:

> 
> > - __exit_signal() does apparently-unlocked 64-bit arith.  Is there 
> >   some implicit locking here or do we not care about the occasional 
> >   race-induced inaccuracy?
> 
> do you mean the tsk->se.sum_exec_runtime addition, etc? That runs with 
> interrupts disabled so sum_sched_runtime is protected.
> 
> >   (ditto, lots of places, I expect)
> 
> which places do you mean?

I forget ;) There seemed to be rather a lot of 64-bit addition with no
obvious locking in sight, that's all.

> > ...
> >   (Gee, there's shitloads of 64-bit stuff in there.  Does it all 
> >   _really_ need to be 64-bit on 32-bit?)
>
> yes - CFS is fundamentally designed for 64-bit, with still pretty OK 
> arithmetics performance for 32-bit.

It may have been designed for 64-bit, but was that the correct design?  The
cost on 32-bit appears to be pretty high.  Perhaps a round of uninlining
will help.

> > - overall, CFS takes sched.o from 41157 of .text up to 48781 on x86_64,
> >   which at 18% is rather a large bloat.  Hopefully a lot of this is 
> >   the new debug stuff.
> 
> > - On i386 sched.o went from 33755 up to 43660 which is 29% growth. 
> >   Possibly acceptable, but why did it increase a lot more than the x86_64
> >   version?  All that 64-bit arith, I assume?
> 
> the main reason is the sched debugging stuff:

That would serve to explain the 18% growth on x86_64.  But why did i386
grow by much more: 29%?  I'd be suspecting all the new 64-bit arithmetic.


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