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From: Leo Yan <leo.yan@linaro.org>
To: Arnaldo Carvalho de Melo <acme@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Ingo Molnar <mingo@redhat.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Mathieu Poirier <mathieu.poirier@linaro.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Mike Leach <mike.leach@linaro.org>,
	linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org
Cc: Leo Yan <leo.yan@linaro.org>
Subject: [PATCH v3 01/10] perf/ring_buffer: Add comment for barriers on AUX ring buffer
Date: Sun,  4 Jul 2021 15:16:35 +0800	[thread overview]
Message-ID: <20210704071644.107397-2-leo.yan@linaro.org> (raw)
In-Reply-To: <20210704071644.107397-1-leo.yan@linaro.org>

AUX ring buffer applies almost the same barriers as perf ring buffer,
but there has an exception for ordering between writing the AUX trace
data and updating user_page::aux_head.

This patch adds comment for how to use the barriers on AUX ring buffer,
and gives comment to ask the drivers to flush the trace data into AUX
ring buffer prior to updating user_page::aux_head.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/events/ring_buffer.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 52868716ec35..5cf6579be05e 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -509,6 +509,15 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
 		perf_event_aux_event(handle->event, aux_head, size,
 				     handle->aux_flags);
 
+	/*
+	 * See perf_output_put_handle(), AUX ring buffer applies the same
+	 * barrier pairing as the perf ring buffer; except for B, since
+	 * AUX ring buffer is written by hardware trace, we cannot simply
+	 * use the generic memory barrier (like smp_wmb()) prior to update
+	 * user_page::aux_head, the hardware trace driver takes the
+	 * responsibility to ensure the trace data has been flushed into
+	 * the AUX buffer before calling perf_aux_output_end().
+	 */
 	WRITE_ONCE(rb->user_page->aux_head, rb->aux_head);
 	if (rb_need_aux_wakeup(rb))
 		wakeup = true;
-- 
2.25.1


  reply	other threads:[~2021-07-04  7:17 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-04  7:16 [PATCH v3 00/10] perf: Refine barriers for " Leo Yan
2021-07-04  7:16 ` Leo Yan [this message]
2021-07-04  7:16 ` [PATCH v3 02/10] coresight: tmc-etr: Add barrier after updating " Leo Yan
2021-07-04  7:16 ` [PATCH v3 03/10] coresight: tmc-etf: Add comment for store ordering Leo Yan
2021-07-04  7:16 ` [PATCH v3 04/10] perf/x86: Add barrier after updating bts Leo Yan
2021-07-04  7:16 ` [PATCH v3 05/10] perf auxtrace: Drop legacy __sync functions Leo Yan
2021-07-10 12:34   ` Adrian Hunter
2021-07-11  7:41     ` Leo Yan
2021-07-04  7:16 ` [PATCH v3 06/10] perf auxtrace: Remove auxtrace_mmap__read_snapshot_head() Leo Yan
2021-07-04  7:16 ` [PATCH v3 07/10] perf: Cleanup for HAVE_SYNC_COMPARE_AND_SWAP_SUPPORT Leo Yan
2021-07-10 12:36   ` Adrian Hunter
2021-07-11  8:08     ` Leo Yan
2021-07-04  7:16 ` [PATCH v3 08/10] perf auxtrace: Use WRITE_ONCE() for updating aux_tail Leo Yan
2021-07-04  7:16 ` [PATCH v3 09/10] perf env: Set kernel bit mode Leo Yan
2021-07-04  7:16 ` [PATCH v3 10/10] perf auxtrace: Add compat_auxtrace_mmap__{read_head|write_tail} Leo Yan

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