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SubjectRe: [PATCH] perf_event use rdpmc rather than rdmsr when possible in kernel
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On Mon, 2012-02-20 at 17:38 -0500, Vince Weaver wrote:

> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index 5adce10..5550047 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -85,7 +85,7 @@ u64 x86_perf_event_update(struct perf_event *event)
> */
> again:
> prev_raw_count = local64_read(&hwc->prev_count);
> - rdmsrl(hwc->event_base, new_raw_count);
> + new_raw_count=native_read_pmc(hwc->event_base_rdpmc);

That really wants to be rdpmc(), bypassing paravirt like that isn't
nice.

>
> if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
> new_raw_count) != prev_raw_count)
> @@ -768,9 +768,11 @@ static inline void x86_assign_hw_event(struct perf_event *event,
> } else if (hwc->idx >= X86_PMC_IDX_FIXED) {
> hwc->config_base = MSR_ARCH_PERFMON_FIXED_CTR_CTRL;
> hwc->event_base = MSR_ARCH_PERFMON_FIXED_CTR0 + (hwc->idx - X86_PMC_IDX_FIXED);
> + hwc->event_base_rdpmc = (hwc->idx - X86_PMC_IDX_FIXED) | 1<<30;
> } else {
> hwc->config_base = x86_pmu_config_addr(hwc->idx);
> hwc->event_base = x86_pmu_event_addr(hwc->idx);
> + hwc->event_base_rdpmc = x86_pmu_addr_offset(hwc->idx);
> }
> }
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index abb2776..432ac69 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -562,6 +562,7 @@ struct hw_perf_event {
> u64 last_tag;
> unsigned long config_base;
> unsigned long event_base;
> + unsigned long event_base_rdpmc;

We could make that unsigned int, the SDM explicitly states
rdmsr/wrmsr/rdpmc take ECX (and ignore the upper 32bits RCX).

> int idx;
> int last_cpu;
> struct hw_perf_event_extra extra_reg;

But yeah, avoiding the extra variable will add a conditional and extra
instructions to the rdpmc path.


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