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perf/x86: Use local64_try_cmpxchg 

Use local64_try_cmpxchg instead of local64_cmpxchg (*ptr, old, new) == old.
x86 CMPXCHG instruction returns success in ZF flag, so this change saves a
compare after cmpxchg (and related move instruction in front of cmpxchg).

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails. There is no need to re-read the value in the loop.

No functional change intended.

Cc. "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20230706141720.2672-1-ubizjak@gmail.com
This commit is contained in:
Uros Bizjak 2023-07-06 16:16:48 +02:00 committed by Peter Zijlstra
parent 7c2128235e
commit 4c1c9dea20
4 changed files with 13 additions and 19 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
arch/x86/events/amd/ibs.c
View File

@ -156,8 +156,8 @@ perf_event_try_update(struct perf_event *event, u64 new_raw_count, int width)
* count to the generic event atomically:
*/
prev_raw_count = local64_read(&hwc->prev_count);
if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
new_raw_count) != prev_raw_count)
if (!local64_try_cmpxchg(&hwc->prev_count,
&prev_raw_count, new_raw_count))
return 0;
/*

10
arch/x86/events/core.c
View File

@ -129,13 +129,11 @@ u64 x86_perf_event_update(struct perf_event *event)
* exchange a new raw count - then add that new-prev delta
* count to the generic event atomically:
*/
again:
prev_raw_count = local64_read(&hwc->prev_count);
rdpmcl(hwc->event_base_rdpmc, new_raw_count);
if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
new_raw_count) != prev_raw_count)
goto again;
do {
rdpmcl(hwc->event_base_rdpmc, new_raw_count);
} while (!local64_try_cmpxchg(&hwc->prev_count,
&prev_raw_count, new_raw_count));
/*
* Now we have the new raw value and have updated the prev

10
arch/x86/events/intel/cstate.c
View File

@ -365,13 +365,11 @@ static void cstate_pmu_event_update(struct perf_event *event)
struct hw_perf_event *hwc = &event->hw;
u64 prev_raw_count, new_raw_count;
again:
prev_raw_count = local64_read(&hwc->prev_count);
new_raw_count = cstate_pmu_read_counter(event);
if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
new_raw_count) != prev_raw_count)
goto again;
do {
new_raw_count = cstate_pmu_read_counter(event);
} while (!local64_try_cmpxchg(&hwc->prev_count,
&prev_raw_count, new_raw_count));
local64_add(new_raw_count - prev_raw_count, &event->count);
}

8
arch/x86/events/msr.c
View File

@ -244,12 +244,10 @@ static void msr_event_update(struct perf_event *event)
s64 delta;
/* Careful, an NMI might modify the previous event value: */
again:
prev = local64_read(&event->hw.prev_count);
now = msr_read_counter(event);
if (local64_cmpxchg(&event->hw.prev_count, prev, now) != prev)
goto again;
do {
now = msr_read_counter(event);
} while (!local64_try_cmpxchg(&event->hw.prev_count, &prev, now));
delta = now - prev;
if (unlikely(event->hw.event_base == MSR_SMI_COUNT)) {