RE: [PATCH] perf tools: fix: Force backward ring buffer mapped readonly

[Liang, Kan]

> 
> 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@xxxxxxxxxx>
> >> Cc: Liang Kan <kan.liang@xxxxxxxxx>
> >> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> >> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> >> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> >> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> >> Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> >> Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> >> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> >> Cc: Li Zefan <lizefan@xxxxxxxxxx>
> >> ---
> >>   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?

Thanks,
Kan
> 
> > I think we should use opts->overwrite to replace the hard code 'false'
> > for perf_evlist__mmap_ex as well.
> >
> > Thanks,
> > Kan
>