Commit-ID: 85f59edf9684603026c64c902791748116d29478 Gitweb: http://git.kernel.org/tip/85f59edf9684603026c64c902791748116d29478 Author: Peter Zijlstra <pet...@infradead.org> AuthorDate: Thu, 31 Oct 2013 17:25:38 +0100 Committer: Ingo Molnar <mi...@kernel.org> CommitDate: Wed, 6 Nov 2013 12:34:20 +0100
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: Peter Zijlstra <pet...@infradead.org> Cc: Benjamin Herrenschmidt <b...@kernel.crashing.org> Cc: Frederic Weisbecker <fweis...@gmail.com> Cc: Mathieu Desnoyers <mathieu.desnoy...@polymtl.ca> Cc: Michael Ellerman <mich...@ellerman.id.au> Cc: Michael Neuling <mi...@neuling.org> Cc: "Paul E. McKenney" <paul...@linux.vnet.ibm.com> Cc: james.ho...@imgtec.com Cc: Vince Weaver <vi...@deater.net> Cc: Victor Kaplansky <vict...@il.ibm.com> Cc: Oleg Nesterov <o...@redhat.com> Cc: Anton Blanchard <an...@samba.org> Link: http://lkml.kernel.org/n/tip-c19u6egfldyx86tpyc3zg...@git.kernel.org Signed-off-by: Ingo Molnar <mi...@kernel.org> --- kernel/events/ring_buffer.c | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-) diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c index 383cde4..6ed16ec 100644 --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -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); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
