powerpc: Improve scheduling of system call entry instructions
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>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index 0ec3b42..a85f487 100644
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S
@@ -211,11 +211,11 @@
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 */