linux-kernel - Re: [PATCH 8/8] Convert PDA into the percpu section

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Date:	Wed, 07 Mar 2007 11:33:02 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Andi Kleen <ak@...e.de>
Cc:	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Zachary Amsden <zach@...are.com>,
	Jeremy Fitzhardinge <jeremy@...source.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 8/8] Convert PDA into the percpu section

On Tue, 2007-03-06 at 20:34 +0100, Andi Kleen wrote:
> Sigh -- i had hoped this had settled down because it was a
> merging nightmare last time. Ok. 

Indeed, that's why I waited until everything else was fully merged and
accepted.

> > +#define percpu_to_op(op,var,val)				\
> > +	do {							\
> > +		typedef typeof(var) T__;			\
> > +		if (0) { T__ tmp__; tmp__ = (val); }		\
> > +		switch (sizeof(var)) {				\
> > +		case 1:						\
> > +			asm(op "b %1,"__percpu_seg"%0"		\
> > +			    : "+m" (var)			\
> > +			    :"ri" ((T__)val));			\
> 
> Perhaps I'm blind but I can't see where the %fs reference is there.
> How does this work? 

Here:

+/* fs segment starts at (positive) offset == __per_cpu_offset[cpu] */
+#define __percpu_seg "%%fs:"
+#else  /* !SMP */
+#include <asm-generic/percpu.h>
+#define __percpu_seg ""
+#endif /* SMP */

That's how we get SMP & non-SMP unification in that code.

> Do you have text size comparisons before/after and possible lmbench? 

No, but I'll run them this evening.  Last time the size reduction was
slight, and there was no measurable performance improvement in
microbenchmarks.

Thanks,
Rusty.

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