Re: [PATCH -mm] i386: cpu_relax() smp.c
From: Nick Piggin
Date: Tue Jun 13 2006 - 22:09:09 EST
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@xxxxxxxx>
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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