From: Ingo Molnar Date: Sat, 8 Nov 2008 15:19:55 +0000 (+0100) Subject: sched: improve sched_clock() performance X-Git-Tag: v2.6.28-rc4~10^2~1 X-Git-Url: http://pilppa.com/gitweb/?a=commitdiff_plain;h=0d12cdd5f883f508d33b85c1bae98fa28987c8c7;p=linux-2.6-omap-h63xx.git sched: improve sched_clock() performance in scheduler-intense workloads native_read_tsc() overhead accounts for 20% of the system overhead: 659567 system_call 41222.9375 686796 schedule 435.7843 718382 __switch_to 665.1685 823875 switch_mm 4526.7857 1883122 native_read_tsc 55385.9412 9761990 total 2.8468 this is large part due to the rdtsc_barrier() that is done before and after reading the TSC. But sched_clock() is not a precise clock in the GTOD sense, using such barriers is completely pointless. So remove the barriers and only use them in vget_cycles(). This improves lat_ctx performance by about 5%. Signed-off-by: Ingo Molnar --- diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h index 46be2fa7ac2..c2a812ebde8 100644 --- a/arch/x86/include/asm/msr.h +++ b/arch/x86/include/asm/msr.h @@ -108,9 +108,7 @@ static __always_inline unsigned long long __native_read_tsc(void) { DECLARE_ARGS(val, low, high); - rdtsc_barrier(); asm volatile("rdtsc" : EAX_EDX_RET(val, low, high)); - rdtsc_barrier(); return EAX_EDX_VAL(val, low, high); } diff --git a/arch/x86/include/asm/tsc.h b/arch/x86/include/asm/tsc.h index 38ae163cc91..9cd83a8e40d 100644 --- a/arch/x86/include/asm/tsc.h +++ b/arch/x86/include/asm/tsc.h @@ -34,6 +34,8 @@ static inline cycles_t get_cycles(void) static __always_inline cycles_t vget_cycles(void) { + cycles_t cycles; + /* * We only do VDSOs on TSC capable CPUs, so this shouldnt * access boot_cpu_data (which is not VDSO-safe): @@ -42,7 +44,11 @@ static __always_inline cycles_t vget_cycles(void) if (!cpu_has_tsc) return 0; #endif - return (cycles_t)__native_read_tsc(); + rdtsc_barrier(); + cycles = (cycles_t)__native_read_tsc(); + rdtsc_barrier(); + + return cycles; } extern void tsc_init(void);