Re: [PATCH -mm] i386: cpu_relax() smp.c

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andreas Mohr wrote:

>Hi,
>
>On Mon, Jun 12, 2006 at 08:37:43PM +0200, Andreas Mohr wrote:
>
>>Hi all,
>>
>>while reviewing 2.6.17-rc6-mm1, I found some places that might
>>want to make use of cpu_relax() in order to not block secondary
>>pipelines while busy-polling (probably especially useful on SMT CPUs):
>>
>
>OK, no replies arguing against anything, thus patch follow-up. ;)
>(no. 1 of 3)
>

The other two look fine. This one should remove the mb(). cpu_relax
IIRC already includes a barrier(), and we are not concerned about
consistency here, only coherency, which the hardware takes care of
for us.

The flush_cpumask is guaranteed to be cleared *after* all other
variables (eg. flush_mm) have been used... that happens in the IPI
handler of course.

Aside, if we *were* worried about consistency here, smp_mb would
have been the more correct barrier to use.

>
>Signed-off-by: Andreas Mohr <andi@lisas.de>
>
>
>diff -urN linux-2.6.17-rc6-mm2.orig/arch/i386/kernel/smp.c linux-2.6.17-rc6-mm2.my/arch/i386/kernel/smp.c
>--- linux-2.6.17-rc6-mm2.orig/arch/i386/kernel/smp.c	2006-06-08 10:38:04.000000000 +0200
>+++ linux-2.6.17-rc6-mm2.my/arch/i386/kernel/smp.c	2006-06-13 19:33:22.000000000 +0200
>@@ -388,9 +388,11 @@
> 	 */
> 	send_IPI_mask(cpumask, INVALIDATE_TLB_VECTOR);
> 
>-	while (!cpus_empty(flush_cpumask))
>+	while (!cpus_empty(flush_cpumask)) {
>+		cpu_relax();
> 		/* nothing. lockup detection does not belong here */
> 		mb();
>+	}
> 
> 	flush_mm = NULL;
> 	flush_va = 0;
>

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