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

[Liang, Kan]

> 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@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?
> 
> 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
>