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

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Message-ID: <37D7C6CF3E00A74B8858931C1DB2F077537DC27E@SHSMSX103.ccr.corp.intel.com>
Date:   Wed, 1 Nov 2017 14:22:27 +0000
From:   "Liang, Kan" <kan.liang@...el.com>
To:     "Wangnan (F)" <wangnan0@...wei.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
> evlist->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.
> 

As my understanding, the 'main channel' should depends on what user sets.
If --overwrite is applied, then evlist->backward_mmap should be the
'main channel'. evlist->overwrite should be set to true as well.

/no-overwrite/ setting is per-event setting.
Only when we finish the global setting, then the per-event setting will be
considered. You may refer to apply_config_terms.

Thanks,
Kan