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SubjectRE: [PATCH] perf tools: fix: Force backward ring buffer mapped readonly
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> On 2017/10/12 20:56, Liang, Kan wrote:
> >> On 2017/10/11 21:16, Liang, Kan wrote:
> >>>> perf record's --overwrite option doesn't work as we expect.
> >>>> For example:
> >> [SNIP]
> >>
> >>>> In the above example we get same records from the backward ring
> >>>> buffer all the time. Overwriting is not triggered.
> >>>>
> >>>> This commit maps backward ring buffers readonly, make it overwritable.
> >>>> It is safe because we assume backward ring buffer always overwritable
> >>>> in other part of code.
> >>>>
> >>>> After applying this patch:
> >>>>
> >>>> $ ~/linux/tools/perf$ sudo ./perf record -m 4 -e raw_syscalls:*
> >>>> -g -- overwrite \
> >>>> --switch-output=1s --tail-synthesize dd
> >>>> if=/dev/zero of=/dev/null
> >> [SNIP]
> >>
> >>>>
> >>>> Signed-off-by: Wang Nan <wangnan0@huawei.com>
> >>>> Cc: Liang Kan <kan.liang@intel.com>
> >>>> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> >>>> Cc: Peter Zijlstra <peterz@infradead.org>
> >>>> Cc: Ingo Molnar <mingo@kernel.org>
> >>>> Cc: Jiri Olsa <jolsa@kernel.org>
> >>>> Cc: Namhyung Kim <namhyung@kernel.org>
> >>>> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> >>>> Cc: Adrian Hunter <adrian.hunter@intel.com>
> >>>> Cc: Andi Kleen <ak@linux.intel.com>
> >>>> Cc: Li Zefan <lizefan@huawei.com>
> >>>> ---
> >>>> tools/perf/util/evlist.c | 7 ++++++-
> >>>> 1 file changed, 6 insertions(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> >>>> index c6c891e..a86b0d2 100644
> >>>> --- a/tools/perf/util/evlist.c
> >>>> +++ b/tools/perf/util/evlist.c
> >>>> @@ -799,12 +799,14 @@ 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 = _mp;
> >>>> + struct mmap_params backward_mp;
> >>>>
> >>>> evlist__for_each_entry(evlist, evsel) {
> >>>> struct perf_mmap *maps = evlist->mmap; @@ -815,6 +817,9
> >> @@ static
> >>>> int perf_evlist__mmap_per_evsel(struct
> >>>> perf_evlist *evlist, int idx,
> >>>> if (evsel->attr.write_backward) {
> >>>> output = _output_backward;
> >>>> maps = evlist->backward_mmap;
> >>>> + backward_mp = *mp;
> >>>> + backward_mp.prot &= ~PROT_WRITE;
> >>>> + mp = &backward_mp;
> >>>>
> >>>> if (!maps) {
> >>>> maps = perf_evlist__alloc_mmap(evlist);
> >>> So it's trying to support per-event overwrite.
> >>> How about the global --overwrite option?
> >> Not only the per-event overwrite. See the example above. The overall --
> >> overwrite option is also respected. In perf_evsel__config, per-event evsel
> >> 'backward' setting is set based on overall '--overwrite' and per-event
> >> '/overwrite/' setting.
> > But how about evlist->overwrite? I think it still keeps the wrong setting.
> > The overwrite is implicitly applied. Some settings are inconsistent.
> >
> > Is there any drawback if you use opts->overwrite for
> perf_evlist__mmap_ex?
>
> We will always face such inconsistency, because we have
> an /no-overwrite/ option which can be set per-evsel.
> Setting evlist->overwrite won't make things more consistent,
> because in a evlist, different evsel can have different
> overwrite setting. A simple solution is making evlist
> non-overwrite by default, and watch all overwrite evsels
> a special cases. Then we have only 2 cases to consider:
>
> 1. overwrite evsel in a non-overwrite evlist.
> 2. non-overwrite evsel in a non-overwrite evlist.
>

If evlist->overwrite is always non-overwrite, why not remove it?

> If we reset evlist->overwrite according to --overwrite, we
> will have 4 cases to consider:
>
> 1. overwrite evsel in a overwrite evlist.
> 2. non-overwrite evsel in a overwrite evlist.
> 3. overwrite evsel in a non-overwrite evlist.
> 4. non-overwrite evsel in a non-overwrite evlist.
>
> The real problem is: there's 'overwrite' and 'backward'
> concepts in our code, but these two concepts are neither
> independent nor identical.
>
> Thank you.
>
>
> > Thanks,
> > Kan
> >>> I think we should use opts->overwrite to replace the hard code 'false'
> >>> for perf_evlist__mmap_ex as well.
> >>>
> >>> Thanks,
> >>> Kan
>

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