[RFC PATCH] perf/core: Put size of a sample at the end of it

[Wang Nan]

This is an RFC patch which is for overwrite mode ring buffer. I'd like
to discuss the correctness of this new idea for retriving as many
events as possible from overwrite mode ring buffer. If there's no
fundamental problem, I'll start perf side work.

The biggest problem for overwrite ring buffer is that it is hard to find
the start position of valid record. [1] and [2] tries to solve this
problem by introducing 'tail' and 'next_tail' into metadata page, and
update them each time the ring buffer is half full. Which adds more
instructions to event output code path, hurt performance. In addition,
even with them we still unable to recover all possible records. For
example:

             data_tail          head
                |                 |
                V                 V
 +------+-------+----------+------+---+
 |  A   |   B   |   C      |  D   |   |
 +------+-------+----------+------+---+

If a record written at head pointer and it overwrites record A:

   head      data_tail
    |           |
    V           V
 +--+---+-------+----------+------+---+
 |E |...|   B   |   C      |  D   | E |
 +--+---+-------+----------+------+---+

Record B is still valid but we can't get it through data_tail.

This patch suggests a different solution for this problem that, by
appending the length of a record at the end of it, user program is
possible to get every possible record in a backward manner, don't
need saving tail pointer.

For example:

   head
    |
    V
 +--+---+-------+----------+------+---+
 |E6|...|   B  8|   C    11|  D  7|E..|
 +--+---+-------+----------+------+---+

In this case, from the 'head' pointer provided by kernel, user program
can first see '6' by (*(head - sizeof(u16))), then it can get the start
pointer of record 'E', then it can read size and find start position
of record D, C, B in similar way.

Kernel side implementation is easy: simply adding a PERF_SAMPLE_SIZE
for the size output.

This sloution requires user program (perf) do more things. At least
following things and limitations should be considered:

 1. Before reading such ring buffer, perf must ensure all events which
    may output to it is already stopped, so the 'head' pointer it get
    is the end of the last record.

 2. We must ensure all events attached this ring buffer has
    'PERF_SAMPLE_SIZE' selected.

 3. There must no tracking events output to this ring buffer.

 4. 2 bytes extra space is required for each record.

Further improvement can be taken:

 1. If PERF_SAMPLE_SIZE is selected, we can avoid outputting the event
    size in header. Which eliminate extra space cose;

 2. We can find a way to append size information for tracking events
    also.

[1] http://lkml.kernel.org/r/20130708121557.GA17211@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
[2] http://lkml.kernel.org/r/20151023151205.GW11639@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: David Ahern <dsahern@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Yunlong Song <yunlong.song@xxxxxxxxxx>
---
 include/uapi/linux/perf_event.h | 3 ++-
 kernel/events/core.c            | 6 ++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 1afe962..c4066da 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -139,8 +139,9 @@ enum perf_event_sample_format {
 	PERF_SAMPLE_IDENTIFIER			= 1U << 16,
 	PERF_SAMPLE_TRANSACTION			= 1U << 17,
 	PERF_SAMPLE_REGS_INTR			= 1U << 18,
+	PERF_SAMPLE_SIZE			= 1U << 19,
 
-	PERF_SAMPLE_MAX = 1U << 19,		/* non-ABI */
+	PERF_SAMPLE_MAX = 1U << 20,		/* non-ABI */
 };
 
 /*
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 5854fcf..bbbacec 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5473,6 +5473,9 @@ void perf_output_sample(struct perf_output_handle *handle,
 		}
 	}
 
+	if (sample_type & PERF_SAMPLE_SIZE)
+		perf_output_put(handle, header->size);
+
 	if (!event->attr.watermark) {
 		int wakeup_events = event->attr.wakeup_events;
 
@@ -5592,6 +5595,9 @@ void perf_prepare_sample(struct perf_event_header *header,
 
 		header->size += size;
 	}
+
+	if (sample_type & PERF_SAMPLE_SIZE)
+		header->size += sizeof(u16);
 }
 
 void perf_event_output(struct perf_event *event,
-- 
1.8.3.4

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