Re: [PATCH v6 01/13] tools/libperf: introduce notion of static polled file descriptors

[Adrian Hunter]

On 3/06/20 3:52 pm, Alexey Budankov wrote:
> 
> On 03.06.2020 15:30, Hunter, Adrian wrote:
>>
>>
>>> -----Original Message-----
>>> From: Hunter, Adrian <adrian.hunter@xxxxxxxxx>
>>> Sent: Wednesday, June 3, 2020 3:24 PM
>>> To: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
>>> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>; Jiri Olsa
>>> <jolsa@xxxxxxxxxx>; Namhyung Kim <namhyung@xxxxxxxxxx>; Alexander
>>> Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>; Peter Zijlstra
>>> <peterz@xxxxxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>; Andi Kleen
>>> <ak@xxxxxxxxxxxxxxx>; linux-kernel <linux-kernel@xxxxxxxxxxxxxxx>
>>> Subject: Re: [PATCH v6 01/13] tools/libperf: introduce notion of static polled
>>> file descriptors
>>>
>>> On 3/06/20 3:01 pm, Alexey Budankov wrote:
>>>> Hi,
>>>>
>>>> On 03.06.2020 14:38, Adrian Hunter wrote:
>>>>> On 1/06/20 11:05 pm, Alexey Budankov wrote:
>>>>>>
>>>>>> Implement adding of file descriptors by fdarray__add_stat() to
>>>>>> fix-sized (currently 1) stat_entries array located at struct fdarray.
>>>>>> Append added file descriptors to the array used by poll() syscall
>>>>>> during fdarray__poll() call. Copy poll() result of the added
>>>>>> descriptors from the array back to the storage for separate analysis.
>>>>>
>>>>> Why not instead call evlist__add_pollfd() before other fds are added,
>>>>> so the fda->entries[] position is always fixed. Then this patch is not
>>> needed.
>>>>
>>>> It then will block event consumption loop, at least in record mode,
>>>> due to change sin initial assumptions behind fdarray__filter(). So
>>>> extension of the API with 'static' fds looks safer w.r.t. possible
>>>> functional regressions at the same time extending the API with ability
>>>> to atomically wait for (poll()) not only event fds but also any other fds
>>> during monitoring.
>>>
>>> So make fdarray__filter() return the number of filterable fds remaining.
>>>
>>
>>
>> Or perhaps simpler, compare the return value to the number of fds that are known not to be filterable
> 
> Well, various implementations are for sure possible but the proposed design
> avoids implicit code assumptions and dependency on API calls order as in API
> implementation as in client code and that is why it's been proposed this way.
> 

The dependencies are confined one function, namely __cmd_record(), it is
simpler, and avoids the inelegant changes to fdarray__poll().