linux-kernel - Re: [PATCH] perf tools: fix: Force backward ring buffer mapped readonly

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Date:   Thu, 12 Oct 2017 09:03:25 +0800
From:   "Wangnan (F)" <wangnan0@...wei.com>
To:     "Liang, Kan" <kan.liang@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "acme@...nel.org" <acme@...nel.org>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        "Alexander Shishkin" <alexander.shishkin@...ux.intel.com>,
        "Hunter, Adrian" <adrian.hunter@...el.com>,
        Andi Kleen <ak@...ux.intel.com>, Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH] perf tools: fix: Force backward ring buffer mapped
 readonly



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@...wei.com>
>> Cc: Liang Kan <kan.liang@...el.com>
>> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Ingo Molnar <mingo@...nel.org>
>> Cc: Jiri Olsa <jolsa@...nel.org>
>> Cc: Namhyung Kim <namhyung@...nel.org>
>> Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
>> Cc: Adrian Hunter <adrian.hunter@...el.com>
>> Cc: Andi Kleen <ak@...ux.intel.com>
>> Cc: Li Zefan <lizefan@...wei.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.

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