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perf_counter: add an mmap method to allow userspace to read hardware counters

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Impact: new feature giving performance improvement

This adds the ability for userspace to do an mmap on a hardware counter
fd and get access to a read-only page that contains the information
needed to translate a hardware counter value to the full 64-bit
counter value that would be returned by a read on the fd.  This is
useful on architectures that allow user programs to read the hardware
counters, such as PowerPC.

The mmap will only succeed if the counter is a hardware counter
monitoring the current process.

On my quad 2.5GHz PowerPC 970MP machine, userspace can read a counter
and translate it to the full 64-bit value in about 30ns using the
mmapped page, compared to about 830ns for the read syscall on the
counter, so this does give a significant performance improvement.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Orig-LKML-Reference: <20090323172417.297057964@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Paul Mackerras
2009-03-23 18:22:08 +01:00
committed by Ingo Molnar
parent 96f6d44443
commit 37d8182838
3 changed files with 97 additions and 0 deletions
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6
arch/powerpc/kernel/perf_counter.c
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@@ -417,6 +417,8 @@ void hw_perf_restore(u64 disable)
atomic64_set(&counter->hw.prev_count, val);
counter->hw.idx = hwc_index[i] + 1;
write_pmc(counter->hw.idx, val);
if (counter->user_page)
perf_counter_update_userpage(counter);
}
mb();
cpuhw->mmcr[0] |= MMCR0_PMXE | MMCR0_FCECE;
@@ -572,6 +574,8 @@ static void power_perf_disable(struct perf_counter *counter)
ppmu->disable_pmc(counter->hw.idx - 1, cpuhw->mmcr);
write_pmc(counter->hw.idx, 0);
counter->hw.idx = 0;
if (counter->user_page)
perf_counter_update_userpage(counter);
break;
}
}
@@ -698,6 +702,8 @@ static void record_and_restart(struct perf_counter *counter, long val,
write_pmc(counter->hw.idx, val);
atomic64_set(&counter->hw.prev_count, val);
atomic64_set(&counter->hw.period_left, left);
if (counter->user_page)
perf_counter_update_userpage(counter);
/*
* Finally record data if requested.