linux-kernel - Re: [PATCH 3/5] perf core: Prepare writing into ring buffer from end

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Message-ID: <20160323192521.GA48575@ast-mbp.thefacebook.com>
Date:	Wed, 23 Mar 2016 12:25:24 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	"Wangnan (F)" <wangnan0@...wei.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com,
	linux-kernel@...r.kernel.org, He Kuang <hekuang@...wei.com>,
	Alexei Starovoitov <ast@...nel.org>,
	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>,
	Namhyung Kim <namhyung@...nel.org>,
	Zefan Li <lizefan@...wei.com>, pi3orama@....com
Subject: Re: [PATCH 3/5] perf core: Prepare writing into ring buffer from end

On Wed, Mar 23, 2016 at 06:08:41PM +0800, Wangnan (F) wrote:
> 
> 
> On 2016/3/23 17:50, Peter Zijlstra wrote:
> >On Mon, Mar 14, 2016 at 09:59:43AM +0000, Wang Nan wrote:
> >>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.
> >So while I agree that with __always_inline and constant propagation we
> >_should_ end up with the same code, we have:
> >
> >$ size defconfig-build/kernel/events/ring_buffer.o.{pre,post}
> >    text    data     bss     dec     hex filename
> >    3785       2       0    3787     ecb defconfig-build/kernel/events/ring_buffer.o.pre
> >    3673       2       0    3675     e5b defconfig-build/kernel/events/ring_buffer.o.post
> >
> >The patch actually makes the file shrink.
> >
> >So I think we still want to have some actual performance numbers.
> 
> There are some numbers. You can find them from:
> 
> http://lkml.iu.edu/hypermail/linux/kernel/1601.2/03966.html

Wang, when you respin, please add all perf analysis that you've
done and the reasons to do it this way to commit log
to make sure it stays in git history.

I've reviewed it in the past and looks like the patches didn't
change over the last months. So still ack, I believe
overwrite buffers is a great feature.