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perf: Optimize perf_output_begin()

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There's no point in re-doing the memory-barrier when we fail the
cmpxchg(). Also placing it after the space reservation loop makes it
clearer it only separates the userpage->tail read from the data
stores.

Signed-off-by: Peter Zijlstra <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Michael Neuling <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: [email protected]
Cc: Vince Weaver <[email protected]>
Cc: Victor Kaplansky <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Anton Blanchard <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>
This commit is contained in:
Peter Zijlstra 2013-10-31 17:25:38 +01:00 • committed by Ingo Molnar
commit 85f59edf96
1 changed files with 9 additions and 8 deletions
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17
kernel/events/ring_buffer.c
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@ -141,15 +141,7 @@ int perf_output_begin(struct perf_output_handle *handle,
perf_output_get_handle(handle);
do {
/*
* Userspace could choose to issue a mb() before updating the
* tail pointer. So that all reads will be completed before the
* write is issued.
*
* See perf_output_put_handle().
*/
tail = ACCESS_ONCE(rb->user_page->data_tail);
smp_mb();
offset = head = local_read(&rb->head);
if (!rb->overwrite &&
unlikely(CIRC_SPACE(head, tail, perf_data_size(rb)) < size))
@ -157,6 +149,15 @@ int perf_output_begin(struct perf_output_handle *handle,
head += size;
} while (local_cmpxchg(&rb->head, offset, head) != offset);
/*
* Separate the userpage->tail read from the data stores below.
* Matches the MB userspace SHOULD issue after reading the data
* and before storing the new tail position.
*
* See perf_output_put_handle().
*/
smp_mb();
if (unlikely(head - local_read(&rb->wakeup) > rb->watermark))
local_add(rb->watermark, &rb->wakeup);