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sched: improve sched_clock() performance

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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 <mingo@elte.hu>
This commit is contained in:
Ingo Molnar 2008-11-08 16:19:55 +01:00
parent 52c642f33b
commit 0d12cdd5f8
2 changed files with 7 additions and 3 deletions
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2
arch/x86/include/asm/msr.h
View File

@ -108,9 +108,7 @@ static __always_inline unsigned long long __native_read_tsc(void)
{ {
DECLARE_ARGS(val, low, high); DECLARE_ARGS(val, low, high);
rdtsc_barrier();
asm volatile("rdtsc" : EAX_EDX_RET(val, low, high)); asm volatile("rdtsc" : EAX_EDX_RET(val, low, high));
rdtsc_barrier();
return EAX_EDX_VAL(val, low, high); return EAX_EDX_VAL(val, low, high);
} }

8
arch/x86/include/asm/tsc.h
View File

@ -34,6 +34,8 @@ static inline cycles_t get_cycles(void)
static __always_inline cycles_t vget_cycles(void) static __always_inline cycles_t vget_cycles(void)
{ {
cycles_t cycles;
/* /*
* We only do VDSOs on TSC capable CPUs, so this shouldnt * We only do VDSOs on TSC capable CPUs, so this shouldnt
* access boot_cpu_data (which is not VDSO-safe): * 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) if (!cpu_has_tsc)
return 0; return 0;
#endif #endif
return (cycles_t)__native_read_tsc(); rdtsc_barrier();
cycles = (cycles_t)__native_read_tsc();
rdtsc_barrier();
return cycles;
} }
extern void tsc_init(void); extern void tsc_init(void);