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

From: Alexey Budankov <alexey.budankov@linux.intel.com>
To: Adrian Hunter <adrian.hunter@intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Andi Kleen <ak@linux.intel.com>,
	linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v6 01/13] tools/libperf: introduce notion of static polled file descriptors
Date: Wed, 3 Jun 2020 20:00:12 +0300	[thread overview]
Message-ID: <5a01eace-e502-c78c-7ab8-a298b0e18c08@linux.intel.com> (raw)
In-Reply-To: <2d264b14-7f60-9d43-2d52-3709c7295c4a@intel.com>

On 03.06.2020 18:53, Adrian Hunter wrote:
> 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@intel.com>
>>>> Sent: Wednesday, June 3, 2020 3:24 PM
>>>> To: Alexey Budankov <alexey.budankov@linux.intel.com>
>>>> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>; Jiri Olsa
>>>> <jolsa@redhat.com>; Namhyung Kim <namhyung@kernel.org>; Alexander
>>>> Shishkin <alexander.shishkin@linux.intel.com>; Peter Zijlstra
>>>> <peterz@infradead.org>; Ingo Molnar <mingo@redhat.com>; Andi Kleen
>>>> <ak@linux.intel.com>; linux-kernel <linux-kernel@vger.kernel.org>
>>>> 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().

As far the changes don't cause regressions and serve their purpose well
they are acceptable.