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Date: Wed, 1 Nov 2017 15:04:40 +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 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? 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. 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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