Re: [PATCH v2 4/5] perf record: add meta-data support for pipe-mode

From: David Ahern
Date: Tue May 15 2012 - 23:34:24 EST

On 5/15/12 5:28 AM, Stephane Eranian wrote:
This patch adds the meta-data header support for perf record
when used in pipe mode: perf record -o -

Up until now, meta-data was only available when perf record
was used in "regular" mode, i.e., generating a file.
For users depending on pipe mode, no host, event header information
was gathered. This patch addresses this limitation.

The difficulty in pipe mode is that information needs to be written
sequentially to the pipe. Meta data headers are usually generated
(and also expected) at the beginning of the file (or piped output).
To solve this problem, we introduce new synthetic record types,
one for each meta-data type. The approach is similar to what
is already used for BUILD_ID and TRACING_DATA.

We have modified util/header.c such that the same routines are used
to generate and read the meta-data information regardless of pipe-mode
vs. regular mode. To make this work, we added a new struct called
feat_fd which encapsulates all the information necessary to read or
write meta-data information to a file/pipe or from a file/pipe.

It should be noted that there is a limitation with the current
perf in terms of endianess in pipe mode. Perf assumes the records
are generated using the same endianess during collection and
analysis. That is always the case with the example shown below.
However, one could also do:
$ perf record -o - noploop 2 | perf inject -b>
$ cat | perf report -i -

With this patch, it is possible to get:
$ perf record -o - noploop 2 | perf inject -b | perf report -i -
# ========
# captured on: Fri Jan 20 18:13:55 2012
# ========
# hostname : quad
# os release : 3.2.0-rc7-tip
# perf version : 3.2.0
# arch : x86_64
# nrcpus online : 4
# nrcpus avail : 4
# cpudesc : Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz
# cpuid : GenuineIntel,6,15,11
# total memory : 8092884 kB
# HEADER_CPU_TOPOLOGY info available, use -I to display
noploop for 2 seconds
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.084 MB - (~3677 samples) ]
99.80% noploop noploop [.] noploop
0.19% noploop [kernel.kallsyms] [k] radix_tree_gang_lookup

Signed-off-by: Stephane Eranian<eranian@xxxxxxxxxx>

I love the feature and it works nicely, but there has been push back on adding more synthesized events. Also the size of the patch is a bit much to take in. If there is no objection to the synthesized can you break the patch up -- e.g., introduce the events in one, synthesis and processing functions in another, plug into the commands, ...

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