Commit c48b6053 authored Mar 01, 2012 by Vince Weaver Committed by Ingo Molnar Jun 06, 2012

perf/x86: Use rdpmc() rather than rdmsr() when possible in the kernel



The rdpmc instruction is faster than the equivelant rdmsr call,
so use it when possible in the kernel.

The perfctr kernel patches did this, after extensive testing showed
rdpmc to always be faster (One can look in etc/costs in the perfctr-2.6
package to see a historical list of the overhead).

I have done some tests on a 3.2 kernel, the kernel module I used
was included in the first posting of this patch:

                   rdmsr           rdpmc
 Core2 T9900:      203.9 cycles     30.9 cycles
 AMD fam0fh:        56.2 cycles      9.8 cycles
 Atom 6/28/2:      129.7 cycles     50.6 cycles

The speedup of using rdpmc is large.

[ It's probably possible (and desirable) to do this without
  requiring a new field in the hw_perf_event structure, but
  the fixed events make this tricky. ]

Signed-off-by: Vince Weaver <vweaver1@eecs.utk.edu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/alpine.DEB.2.00.1203011724030.26934@cl320.eecs.utk.edu


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 1ff4d58a

arch/x86/kernel/cpu/perf_event.c

+3 −1

Original line number	Diff line number	Diff line
		@@ -75,7 +75,7 @@ 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);
		rdpmcl(hwc->event_base_rdpmc, new_raw_count);

		if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
		new_raw_count) != prev_raw_count)
		@@ -819,9 +819,11 @@ 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);
		}
		}

include/linux/perf_event.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -677,6 +677,7 @@ struct hw_perf_event {
		u64 last_tag;
		unsigned long config_base;
		unsigned long event_base;
		int event_base_rdpmc;
		int idx;
		int last_cpu;

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