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Date: Thu, 2 Nov 2017 00:00:28 +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 23:04, Liang, Kan wrote: >> On 2017/11/1 22:22, Liang, Kan wrote: >>>> 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. >> Then it introduces a main channel switching mechanism, and we need >> checking evlist->overwrite and another factor to determine which >> one is the main channel. Make things more complex. > We should check the evlist->overwrite. > Now, all perf tools force evlist->overwrite = false. I think it doesn’t make sense. > > What is another factor? If we support mixed channel as well as forward overwrite ring buffer, evlist->overwrite is not enough. > I don't think it will be too complex. > > In perf_evlist__mmap_ex, we just need to add a check. > If (!overwrite) > evlist->mmap = perf_evlist__alloc_mmap(evlist); > else > evlist->backward_mmap = perf_evlist__alloc_mmap(evlist); > > In perf_evlist__mmap_per_evsel, we already handle per-event overwrite. > It just need to add some similar codes to handler per-event nonoverwrite. Then the logic becomes: if (check write_backward) { maps = evlist->backward_mmap; if (!maps) { maps = perf_evlist__alloc_mmap(...); if (!maps) { /* error processing */ } evlist->backward_mmap = maps; if (evlist->bkw_mmap_state == BKW_MMAP_NOTREADY) perf_evlist__toggle_bkw_mmap(evlist, BKW_MMAP_RUNNING); } } else { maps = evlist->mmap; if (!maps) { maps = perf_evlist__alloc_mmap(...); if (!maps) { /* error processing */ } evlist->mmap = maps; .... } } > For other codes, they should already check evlist->mmap and evlist->backward_mmap. > So they probably don't need to change. > > > Thanks, > Kan > >
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