From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Mon, 9 May 2011 17:36:44 +1000 From: Anton Blanchard To: benh@kernel.crashing.org Subject: [PATCH] powerpc: Improve scheduling of system call entry instructions Message-ID: <20110509173644.33053672@kryten> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Cc: linuxppc-dev@lists.ozlabs.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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 --- 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 */