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Commit 0d12cdd5 authored by Ingo Molnar's avatar Ingo Molnar
Browse files

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: default avatarIngo Molnar <mingo@elte.hu>
parent 52c642f3

arch/x86/include/asm/msr.h

+0 −2
Original line number Diff line number Diff line
@@ -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);

}

arch/x86/include/asm/tsc.h

+7 −1
Original line number Diff line number Diff line
@@ -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);