Commit ec3232bd authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar

perf_counter: x86: More accurate counter update

Take the counter width into account instead of assuming 32 bits.

In particular Nehalem has 44 bit wide counters, and all
arithmetics should happen on a 44-bit signed integer basis.

[ Impact: fix rare event imprecision, warning message on Nehalem ]
Signed-off-by: default avatarPeter Zijlstra <>
Cc: Paul Mackerras <>
Cc: Corey Ashford <>
LKML-Reference: <new-submission>
Signed-off-by: default avatarIngo Molnar <>
parent 1a853e36
......@@ -138,7 +138,9 @@ static u64
x86_perf_counter_update(struct perf_counter *counter,
struct hw_perf_counter *hwc, int idx)
u64 prev_raw_count, new_raw_count, delta;
int shift = 64 - x86_pmu.counter_bits;
u64 prev_raw_count, new_raw_count;
s64 delta;
* Careful: an NMI might modify the previous counter value.
......@@ -161,9 +163,10 @@ x86_perf_counter_update(struct perf_counter *counter,
* (counter-)time and add that to the generic counter.
* Careful, not all hw sign-extends above the physical width
* of the count, so we do that by clipping the delta to 32 bits:
* of the count.
delta = (u64)(u32)((s32)new_raw_count - (s32)prev_raw_count);
delta = (new_raw_count << shift) - (prev_raw_count << shift);
delta >>= shift;
atomic64_add(delta, &counter->count);
atomic64_sub(delta, &hwc->period_left);
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