Commit 85f59edf authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar

perf: Optimize perf_output_begin()

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: default avatarPeter Zijlstra <peterz@infradead.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Michael Ellerman <michael@ellerman.id.au>
Cc: Michael Neuling <mikey@neuling.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: james.hogan@imgtec.com
Cc: Vince Weaver <vince@deater.net>
Cc: Victor Kaplansky <VICTORK@il.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Anton Blanchard <anton@samba.org>
Link: http://lkml.kernel.org/n/tip-c19u6egfldyx86tpyc3zgkw9@git.kernel.orgSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent c72b42a3
...@@ -141,15 +141,7 @@ int perf_output_begin(struct perf_output_handle *handle, ...@@ -141,15 +141,7 @@ int perf_output_begin(struct perf_output_handle *handle,
perf_output_get_handle(handle); perf_output_get_handle(handle);
do { 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); tail = ACCESS_ONCE(rb->user_page->data_tail);
smp_mb();
offset = head = local_read(&rb->head); offset = head = local_read(&rb->head);
if (!rb->overwrite && if (!rb->overwrite &&
unlikely(CIRC_SPACE(head, tail, perf_data_size(rb)) < size)) unlikely(CIRC_SPACE(head, tail, perf_data_size(rb)) < size))
...@@ -157,6 +149,15 @@ int perf_output_begin(struct perf_output_handle *handle, ...@@ -157,6 +149,15 @@ int perf_output_begin(struct perf_output_handle *handle,
head += size; head += size;
} while (local_cmpxchg(&rb->head, offset, head) != offset); } 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)) if (unlikely(head - local_read(&rb->wakeup) > rb->watermark))
local_add(rb->watermark, &rb->wakeup); local_add(rb->watermark, &rb->wakeup);
......
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