linux-kernel - Re: [PATCH 2/2] perf record: Replace 'overwrite' by 'flightrecorder' for better naming

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Message-ID: <dfc4dad3-75dc-6dcd-b5ee-45f030ddd414@huawei.com>
Date:   Wed, 1 Nov 2017 22:05:20 +0800
From:   "Wangnan (F)" <wangnan0@...wei.com>
To:     "Liang, Kan" <kan.liang@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "acme@...nel.org" <acme@...nel.org>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>
Subject: Re: [PATCH 2/2] perf record: Replace 'overwrite' by 'flightrecorder'
 for better naming

On 2017/11/1 21:26, Liang, Kan wrote:
>> The meaning of perf record's "overwrite" option and many "overwrite" in
>> source code are not clear. In perf's code, the 'overwrite' has 2 meanings:
>>   1. Make ringbuffer readonly (perf_evlist__mmap_ex's argument).
>>   2. Set evsel's "backward" attribute (in apply_config_terms).
>>
>> perf record doesn't use meaning 1 at all, but have a overwrite option, its
>> real meaning is setting backward.
>>
> I don't understand here.
> 'overwrite' has 2 meanings. perf record only support 1.
> It should be a bug, and need to be fixed.

Not a bug, but ambiguous.

Perf record doesn't need overwrite main channel (we have two channels:
evlist->mmap is main channel and evlist->backward_mmap is backward channel),
but some testcases require it, and your new patchset may require it.
'perf record --overwrite' doesn't set main channel overwrite. What it does
is moving all evsels to backward channel, and we can move some evsels back
to the main channel by /no-overwrite/ setting. This behavior is hard to
understand.

Thank you.