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 <mingo@elte.hu>
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@ -108,9 +108,7 @@ static __always_inline unsigned long long __native_read_tsc(void)
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{
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DECLARE_ARGS(val, low, high);
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rdtsc_barrier();
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asm volatile("rdtsc" : EAX_EDX_RET(val, low, high));
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rdtsc_barrier();
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return EAX_EDX_VAL(val, low, high);
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}
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@ -34,6 +34,8 @@ static inline cycles_t get_cycles(void)
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static __always_inline cycles_t vget_cycles(void)
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{
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cycles_t cycles;
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/*
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* We only do VDSOs on TSC capable CPUs, so this shouldnt
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* access boot_cpu_data (which is not VDSO-safe):
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@ -42,7 +44,11 @@ static __always_inline cycles_t vget_cycles(void)
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if (!cpu_has_tsc)
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return 0;
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#endif
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return (cycles_t)__native_read_tsc();
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rdtsc_barrier();
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cycles = (cycles_t)__native_read_tsc();
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rdtsc_barrier();
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return cycles;
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}
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extern void tsc_init(void);
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