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Subject[tip:perf/core] perf: Optimize perf_output_*() by avoiding local_xchg()
Commit-ID:  6d1acfd5c6bfd5231c13a8f2858d7f2afbaa1b62
Gitweb: http://git.kernel.org/tip/6d1acfd5c6bfd5231c13a8f2858d7f2afbaa1b62
Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
AuthorDate: Tue, 18 May 2010 11:12:48 +0200
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Tue, 18 May 2010 18:35:49 +0200

perf: Optimize perf_output_*() by avoiding local_xchg()

Since the x86 XCHG ins implies LOCK, avoid the use by
using a sequence count instead.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
include/linux/perf_event.h | 1 +
kernel/perf_event.c | 3 ++-
2 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index ce76676..fe50347 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -804,6 +804,7 @@ struct perf_output_handle {
struct perf_mmap_data *data;
unsigned long head;
unsigned long offset;
+ unsigned long wakeup;
int nmi;
int sample;
};
diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 1f98c78..7e3bcf1 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -2917,6 +2917,7 @@ static void perf_output_get_handle(struct perf_output_handle *handle)

preempt_disable();
local_inc(&data->nest);
+ handle->wakeup = local_read(&data->wakeup);
}

static void perf_output_put_handle(struct perf_output_handle *handle)
@@ -2950,7 +2951,7 @@ again:
goto again;
}

- if (local_xchg(&data->wakeup, 0))
+ if (handle->wakeup != local_read(&data->wakeup))
perf_output_wakeup(handle);

preempt_enable();

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