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Date: Wed, 1 Nov 2017 20:10:49 +0800 From: "Wangnan (F)" <wangnan0@...wei.com> To: Namhyung Kim <namhyung@...nel.org> CC: <linux-kernel@...r.kernel.org>, <kan.liang@...el.com>, <acme@...nel.org>, <jolsa@...hat.com>, <kernel-team@....com> Subject: Re: [PATCH 1/2] perf mmap: Fix perf backward recording On 2017/11/1 20:00, Namhyung Kim wrote: > On Wed, Nov 01, 2017 at 06:32:50PM +0800, Wangnan (F) wrote: >> >> On 2017/11/1 17:49, Namhyung Kim wrote: >>> Hi, >>> >>> On Wed, Nov 01, 2017 at 05:53:26AM +0000, Wang Nan wrote: >>>> perf record backward recording doesn't work as we expected: it never >>>> overwrite when ring buffer full. >>>> >>>> Test: >>>> >>>> (Run a busy printing python task background like this: >>>> >>>> while True: >>>> print 123 >>>> >>>> send SIGUSR2 to perf to capture snapshot.) >> [SNIP] >> >>>> Signed-off-by: Wang Nan <wangnan0@...wei.com> >>>> --- >>>> tools/perf/util/evlist.c | 8 +++++++- >>>> 1 file changed, 7 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c >>>> index c6c891e..4c5daba 100644 >>>> --- a/tools/perf/util/evlist.c >>>> +++ b/tools/perf/util/evlist.c >>>> @@ -799,22 +799,28 @@ perf_evlist__should_poll(struct perf_evlist *evlist __maybe_unused, >>>> } >>>> static int perf_evlist__mmap_per_evsel(struct perf_evlist *evlist, int idx, >>>> - struct mmap_params *mp, int cpu_idx, >>>> + struct mmap_params *_mp, int cpu_idx, >>>> int thread, int *_output, int *_output_backward) >>>> { >>>> struct perf_evsel *evsel; >>>> int revent; >>>> int evlist_cpu = cpu_map__cpu(evlist->cpus, cpu_idx); >>>> + struct mmap_params *mp; >>>> evlist__for_each_entry(evlist, evsel) { >>>> struct perf_mmap *maps = evlist->mmap; >>>> + struct mmap_params rdonly_mp; >>>> int *output = _output; >>>> int fd; >>>> int cpu; >>>> + mp = _mp; >>>> if (evsel->attr.write_backward) { >>>> output = _output_backward; >>>> maps = evlist->backward_mmap; >>>> + rdonly_mp = *_mp; >>>> + rdonly_mp.prot &= ~PROT_WRITE; >>>> + mp = &rdonly_mp; >>>> if (!maps) { >>>> maps = perf_evlist__alloc_mmap(evlist); >>>> -- >>> What about this instead (not tested)? >>> >>> >>> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c >>> index c6c891e154a6..27ebe355e794 100644 >>> --- a/tools/perf/util/evlist.c >>> +++ b/tools/perf/util/evlist.c >>> @@ -838,6 +838,11 @@ static int perf_evlist__mmap_per_evsel(struct perf_evlist *evlist, int idx, >>> if (*output == -1) { >>> *output = fd; >>> + if (evsel->attr.write_backward) >>> + mp->prot = PROT_READ; >>> + else >>> + mp->prot = PROT_READ | PROT_WRITE; >>> + >> If evlist->overwrite is true, PROT_WRITE should be unset even if >> write_backward is >> not set. If you want to delay the setting of mp->prot, you need to consider >> both evlist->overwrite and evsel->attr.write_backward. > I thought evsel->attr.write_backward should be set when > evlist->overwrite is set. Do you mean following case? > > perf record --overwrite -e 'cycles/no-overwrite/' > No. evlist->overwrite is unrelated to '--overwrite'. This is why I said the concept of 'overwrite' and 'backward' is ambiguous. perf record never sets 'evlist->overwrite'. What '--overwrite' actually does is setting write_backward. Some testcases needs overwrite evlist. Thank you.
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