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

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Date:   Thu, 12 Oct 2017 22:35:24 +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 2017/10/12 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@...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.
> 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 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