linux-kernel - Re: [PATCH 03/10] perf tool: new iterfaces to read event from ring buffer

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Date:   Wed, 11 Oct 2017 03:10:37 +0800
From:   "Wangnan (F)" <wangnan0@...wei.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
CC:     "Liang, Kan" <kan.liang@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jolsa@...nel.org" <jolsa@...nel.org>,
        "hekuang@...wei.com" <hekuang@...wei.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "Hunter, Adrian" <adrian.hunter@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: Re: [PATCH 03/10] perf tool: new iterfaces to read event from ring
 buffer



On 2017/10/11 3:00, Arnaldo Carvalho de Melo wrote:
> Em Tue, Oct 10, 2017 at 03:36:28PM -0300, Arnaldo Carvalho de Melo escreveu:
>> Em Tue, Oct 10, 2017 at 03:34:55PM -0300, Arnaldo Carvalho de Melo escreveu:
>>> Em Tue, Oct 10, 2017 at 06:28:18PM +0000, Liang, Kan escreveu:
>>>>> Em Tue, Oct 10, 2017 at 10:20:16AM -0700, kan.liang@...el.com escreveu:
>>>>>> From: Kan Liang <kan.liang@...el.com>
>>>>>>
>>>>>> The perf_evlist__mmap_read only support forward mode. It needs a
>>>>>> common function to support both forward and backward mode.
>>>>>> The perf_evlist__mmap_read_backward is buggy.
>>>>> So, what is the bug? You state that it is buggy, but don't spell out the bug,
>>>>> please do so.
>>>>>
>>>> union perf_event *perf_evlist__mmap_read_backward(struct perf_evlist *evlist, int idx)
>>>> {
>>>> 	struct perf_mmap *md = &evlist->mmap[idx];  <--- it should be backward_mmap
>>>>
>>>>> If it fixes an existing bug, then it should go separate from this patchkit, right?
>>>> There is no one use perf_evlist__mmap_read_backward. So it doesn't trigger any issue.
>>> There is no one at the end of your patchkit? Or no user _right now_? If
>>> there is a user now, lemme see... yeah, no user right now, so _that_ is
>>> yet another bug, i.e. it should be used, no? If this is just a left
>>> over, then we should just throw it away, now, its a cleanup.
>> Wang, can you take a look at these two issues?
> So it looks leftover that should've been removed by the following cset, right Wang?
>
> - Arnaldo
>
> commit a0c6f451f90204847ce5f91c3268d83a76bde1b6
> Author: Wang Nan <wangnan0@...wei.com>
> Date:   Thu Jul 14 08:34:41 2016 +0000
>
>      perf evlist: Drop evlist->backward
>      
>      Now there's no real user of evlist->backward. Drop it. We are going to
>      use evlist->backward_mmap as a container for backward ring buffer.
>

Yes, it should be removed, but then there will be no corresponding
function to perf_evlist__mmap_read(), which read an record from forward
ring buffer.

I think Kan wants to become the first user of this function because
he is trying to make 'perf top' utilizing backward ring buffer. It needs
perf_evlist__mmap_read_backward(), and he triggers the bug use his
unpublished patch set.

I think we can remove it now, let Kan fix and add it back in his 'perf top'
patch set.

Thank you.