linux-kernel - Re: [PATCH 0/6] perf core: Read from overwrite ring buffer

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Message-Id: <A9CFD1D0-AD05-44C0-86E0-880B77FD1216@163.com>
Date:	Tue, 19 Jan 2016 22:14:06 +0800
From:	pi3orama <pi3orama@....com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Wang Nan <wangnan0@...wei.com>, peterz@...radead.org,
	ast@...nel.org, linux-kernel@...r.kernel.org,
	He Kuang <hekuang@...wei.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Brendan Gregg <brendan.d.gregg@...il.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Zefan Li <lizefan@...wei.com>
Subject: Re: [PATCH 0/6] perf core: Read from overwrite ring buffer



发自我的 iPhone

> 在 2016年1月19日，下午9:58，Namhyung Kim <namhyung@...nel.org> 写道：
> 
> Hi,
> 
>> On Tue, Jan 19, 2016 at 11:16:44AM +0000, Wang Nan wrote:
>> This patchset introduces two methods to support reading from overwrite.
>> 
>> 1) Tailsize: write the size of an event at the end of it
>> 2) Backward writing: write the ring buffer from the end of it to the
>>    beginning.
> 
> So both of two methods should be used together?
> 

They are separated, we should use only one
of them. I prefer backward writing. But if we
select them both they should work.

Both of them have drawback. Tailsize
method adds 8 bytes to each record
(needs more cycles to write and consume
more space), backward writing may causes
more cache misses since processor doesn't
support backward cache prefetching. Tomorrow I will compare their performance penalty to see which one is better.

Thank you.

> Thanks,
> Namhyung
> 
> 
>> 
>> Patch 1/6 introduces a new ioctl operation to pause and resume ring
>> buffer since reading from a overwrite ring buffer is not reliable.
>> 
>> To reduce overhead as much as possible, force setting overflow_handler
>> and create specific function for backward writing and onward writing.
>> 
>> Cc: He Kuang <hekuang@...wei.com>
>> Cc: Alexei Starovoitov <ast@...nel.org>
>> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
>> Cc: Brendan Gregg <brendan.d.gregg@...il.com>
>> Cc: Jiri Olsa <jolsa@...nel.org>
>> Cc: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
>> Cc: Namhyung Kim <namhyung@...nel.org>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Zefan Li <lizefan@...wei.com>
>> Cc: pi3orama@....com
>> 
>> Wang Nan (6):
>>  perf core: Introduce new ioctl options to pause and resume ring buffer
>>  perf core: Set event's default overflow_handler
>>  perf core: Prepare writing into ring buffer from end
>>  perf core: Add backwork attribute to perf event
>>  perf core: Reduce perf event output overhead by setting overwrite
>>    handler
>>  perf/core: Put size of a sample at the end of it by
>>    PERF_SAMPLE_TAILSIZE
>> 
>> include/linux/perf_event.h      |  39 +++++++---
>> include/uapi/linux/perf_event.h |   7 +-
>> kernel/events/core.c            | 155 +++++++++++++++++++++++++++++++---------
>> kernel/events/internal.h        |  11 +++
>> kernel/events/ring_buffer.c     |  65 ++++++++++++++---
>> 5 files changed, 223 insertions(+), 54 deletions(-)
>> 
>> -- 
>> 1.8.3.4
>>