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Groups > linux.kernel > #1327787

[PATCH 21/54] perf core: Prepare writing into ring buffer from end

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Convert perf_output_begin to __perf_output_begin and make the later
function able to write records from the end of the ring buffer.
Following commits will utilize the 'backward' flag.

This patch doesn't introduce any extra performance overhead since we
use always_inline.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: He Kuang <hekuang@huawei.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
---
 kernel/events/ring_buffer.c | 42 ++++++++++++++++++++++++++++++++++++------
 1 file changed, 36 insertions(+), 6 deletions(-)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 22e1a47..37c11c6 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -102,8 +102,21 @@ out:
 	preempt_enable();
 }
 
-int perf_output_begin(struct perf_output_handle *handle,
-		      struct perf_event *event, unsigned int size)
+static bool __always_inline
+ring_buffer_has_space(unsigned long head, unsigned long tail,
+		      unsigned long data_size, unsigned int size,
+		      bool backward)
+{
+	if (!backward)
+		return CIRC_SPACE(head, tail, data_size) >= size;
+	else
+		return CIRC_SPACE(tail, head, data_size) >= size;
+}
+
+static int __always_inline
+__perf_output_begin(struct perf_output_handle *handle,
+		    struct perf_event *event, unsigned int size,
+		    bool backward)
 {
 	struct ring_buffer *rb;
 	unsigned long tail, offset, head;
@@ -146,9 +159,12 @@ int perf_output_begin(struct perf_output_handle *handle,
 	do {
 		tail = READ_ONCE(rb->user_page->data_tail);
 		offset = head = local_read(&rb->head);
-		if (!rb->overwrite &&
-		    unlikely(CIRC_SPACE(head, tail, perf_data_size(rb)) < size))
-			goto fail;
+		if (!rb->overwrite) {
+			if (unlikely(!ring_buffer_has_space(head, tail,
+							    perf_data_size(rb),
+							    size, backward)))
+				goto fail;
+		}
 
 		/*
 		 * The above forms a control dependency barrier separating the
@@ -162,9 +178,17 @@ int perf_output_begin(struct perf_output_handle *handle,
 		 * See perf_output_put_handle().
 		 */
 
-		head += size;
+		if (!backward)
+			head += size;
+		else
+			head -= size;
 	} while (local_cmpxchg(&rb->head, offset, head) != offset);
 
+	if (backward) {
+		offset = head;
+		head = (u64)(-head);
+	}
+
 	/*
 	 * We rely on the implied barrier() by local_cmpxchg() to ensure
 	 * none of the data stores below can be lifted up by the compiler.
@@ -206,6 +230,12 @@ out:
 	return -ENOSPC;
 }
 
+int perf_output_begin(struct perf_output_handle *handle,
+		      struct perf_event *event, unsigned int size)
+{
+	return __perf_output_begin(handle, event, size, false);
+}
+
 unsigned int perf_output_copy(struct perf_output_handle *handle,
 		      const void *buf, unsigned int len)
 {
-- 
1.8.3.4

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[PATCH 00/54] perf tools: Bugfix, BPF improvements and overwrite ring buffer support Wang Nan <wangnan0@huawei.com> - 2016-02-05 15:20 +0100
  [PATCH 08/54] perf record: Apply config to BPF objects before recording Wang Nan <wangnan0@huawei.com> - 2016-02-05 15:30 +0100
  [PATCH 19/54] perf core: Introduce new ioctl options to pause and resume ring buffer Wang Nan <wangnan0@huawei.com> - 2016-02-05 15:30 +0100
  [PATCH 09/54] perf tools: Enable passing event to BPF object Wang Nan <wangnan0@huawei.com> - 2016-02-05 15:30 +0100
  [PATCH 21/54] perf core: Prepare writing into ring buffer from end Wang Nan <wangnan0@huawei.com> - 2016-02-05 15:30 +0100
  [PATCH 05/54] perf data: Fix releasing event_class Wang Nan <wangnan0@huawei.com> - 2016-02-05 15:30 +0100
  [PATCH 02/54] perf tools: Fix symbols searching for offline module in buildid-cache Wang Nan <wangnan0@huawei.com> - 2016-02-05 15:30 +0100
  [PATCH 04/54] perf tools: Adjust symbol for shared objects Wang Nan <wangnan0@huawei.com> - 2016-02-05 15:30 +0100
  [PATCH 01/54] perf tools: Fix dangling pointers in parse_events__free_terms Wang Nan <wangnan0@huawei.com> - 2016-02-05 15:30 +0100
  [PATCH 03/54] perf tools: Record text offset in dso to calculate objdump address Wang Nan <wangnan0@huawei.com> - 2016-02-05 15:30 +0100

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