From: Anton Blanchard <anton@samba.org>
To: benh@kernel.crashing.org
Cc: linuxppc-dev@lists.ozlabs.org
Subject: [PATCH] powerpc: Improve scheduling of system call entry instructions
Date: Mon, 9 May 2011 17:36:44 +1000 [thread overview]
Message-ID: <20110509173644.33053672@kryten> (raw)
After looking at our system call path, Mary Brown suggested that we
should put all mfspr SRR* instructions before any mtspr SRR*.
To test this I used a very simple null syscall (actually getppid)
testcase at http://ozlabs.org/~anton/junkcode/null_syscall.c
I tested with the following changes against the pseries_defconfig:
CONFIG_VIRT_CPU_ACCOUNTING=n
CONFIG_AUDIT=n
to remove the overhead of virtual CPU accounting and syscall
auditing.
POWER6:
baseline: mean = 757.2 cycles sd = 2.108
modified: mean = 759.1 cycles sd = 2.020
POWER7:
baseline: mean = 411.4 cycles sd = 0.138
modified: mean = 404.1 cycles sd = 0.109
So we have 1.77% improvement on POWER7 which looks significant. The
POWER6 suggest a 0.25% slowdown, but the results are within 1
standard deviation and may be in the noise.
Signed-off-by: Anton Blanchard <anton@samba.org>
---
Index: linux-powerpc/arch/powerpc/kernel/exceptions-64s.S
===================================================================
--- linux-powerpc.orig/arch/powerpc/kernel/exceptions-64s.S 2011-05-07 10:56:04.000000000 +1000
+++ linux-powerpc/arch/powerpc/kernel/exceptions-64s.S 2011-05-09 11:20:23.872220052 +1000
@@ -211,11 +211,11 @@ END_FTR_SECTION_IFSET(CPU_FTR_REAL_LE)
mr r9,r13
GET_PACA(r13)
mfspr r11,SPRN_SRR0
- ld r12,PACAKBASE(r13)
- ld r10,PACAKMSR(r13)
- LOAD_HANDLER(r12, system_call_entry)
- mtspr SPRN_SRR0,r12
mfspr r12,SPRN_SRR1
+ ld r10,PACAKBASE(r13)
+ LOAD_HANDLER(r10, system_call_entry)
+ mtspr SPRN_SRR0,r10
+ ld r10,PACAKMSR(r13)
mtspr SPRN_SRR1,r10
rfid
b . /* prevent speculative execution */
reply other threads:[~2011-05-09 7:36 UTC|newest]
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