linux-kernel - Re: [PATCH 2/2] perf record: Replace 'overwrite' by 'flightrecorder' for better naming

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Date:   Thu, 2 Nov 2017 00:00:28 +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>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>
Subject: Re: [PATCH 2/2] perf record: Replace 'overwrite' by 'flightrecorder'
 for better naming



On 2017/11/1 23:04, Liang, Kan wrote:
>> On 2017/11/1 22:22, Liang, Kan wrote:
>>>> On 2017/11/1 21:26, Liang, Kan wrote:
>>>>>> The meaning of perf record's "overwrite" option and many "overwrite"
>>>>>> in source code are not clear. In perf's code, the 'overwrite' has 2
>> meanings:
>>>>>>     1. Make ringbuffer readonly (perf_evlist__mmap_ex's argument).
>>>>>>     2. Set evsel's "backward" attribute (in apply_config_terms).
>>>>>>
>>>>>> perf record doesn't use meaning 1 at all, but have a overwrite
>>>>>> option, its real meaning is setting backward.
>>>>>>
>>>>> I don't understand here.
>>>>> 'overwrite' has 2 meanings. perf record only support 1.
>>>>> It should be a bug, and need to be fixed.
>>>> Not a bug, but ambiguous.
>>>>
>>>> Perf record doesn't need overwrite main channel (we have two channels:
>>>> evlist->mmap is main channel and evlist->backward_mmap is backward
>>>> evlist->channel),
>>>> but some testcases require it, and your new patchset may require it.
>>>> 'perf record --overwrite' doesn't set main channel overwrite. What it does
>> is
>>>> moving all evsels to backward channel, and we can move some evsels
>> back to
>>>> the main channel by /no-overwrite/ setting. This behavior is hard to
>>>> understand.
>>>>
>>> As my understanding, the 'main channel' should depends on what user sets.
>>> If --overwrite is applied, then evlist->backward_mmap should be the
>>> 'main channel'. evlist->overwrite should be set to true as well.
>> Then it introduces a main channel switching mechanism, and we need
>> checking evlist->overwrite and another factor to determine which
>> one is the main channel. Make things more complex.
> We should check the evlist->overwrite.
> Now, all perf tools force evlist->overwrite = false. I think it doesn’t make sense.
>
> What is another factor?

If we support mixed channel as well as forward overwrite ring buffer,
evlist->overwrite is not enough.

> I don't think it will be too complex.
>
> In perf_evlist__mmap_ex, we just need to add a check.
> If (!overwrite)
> 	evlist->mmap = perf_evlist__alloc_mmap(evlist);
> else
> 	evlist->backward_mmap = perf_evlist__alloc_mmap(evlist);
>
> In perf_evlist__mmap_per_evsel, we already handle per-event overwrite.
> It just need to add some similar codes to handler per-event nonoverwrite.

Then the logic becomes:

  if (check write_backward) {
     maps = evlist->backward_mmap;
     if (!maps) {
       maps = perf_evlist__alloc_mmap(...);
       if (!maps) {
           /* error processing */
       }
       evlist->backward_mmap = maps;
       if (evlist->bkw_mmap_state == BKW_MMAP_NOTREADY)
         perf_evlist__toggle_bkw_mmap(evlist, BKW_MMAP_RUNNING);
     }
  } else {
     maps = evlist->mmap;
     if (!maps) {
       maps = perf_evlist__alloc_mmap(...);
       if (!maps) {
           /* error processing */
       }
       evlist->mmap = maps;
       ....
     }
  }

> For other codes, they should already check evlist->mmap and evlist->backward_mmap.
> So they probably don't need to change.
>
>
> Thanks,
> Kan
>
>