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Subject: Re: [PATCH v7 01/13] tools/libperf: introduce notion of static polled
 file descriptors
From:   Alexey Budankov <alexey.budankov@linux.intel.com>
To:     Jiri Olsa <jolsa@redhat.com>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        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>
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Organization: Intel Corp.
Message-ID: <ebbfc433-5691-2f48-e07e-b1e086472218@linux.intel.com>
Date:   Mon, 8 Jun 2020 18:05:51 +0300
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On 08.06.2020 12:54, Alexey Budankov wrote:
> 
> On 08.06.2020 11:43, Jiri Olsa wrote:
>> On Mon, Jun 08, 2020 at 11:08:56AM +0300, Alexey Budankov wrote:
>>>
>>> On 05.06.2020 19:15, Alexey Budankov wrote:
>>>>
>>>> On 05.06.2020 14:38, Jiri Olsa wrote:
>>>>> On Fri, Jun 05, 2020 at 12:50:54PM +0200, Jiri Olsa wrote:
>>>>>> On Wed, Jun 03, 2020 at 06:52:59PM +0300, 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 analysis.
>>>>>>>
>>>>>>> Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
>>>>>>> ---
<SNIP>
>>>>>>> +					   fda->stat_entries[i].events);
>>>>>>
>>>>>> so every call to fdarray__poll will add whatever is
>>>>>> in stat_entries to entries? how is it removed?
>>>>>>
>>>>>> I think you should either follow what Adrian said
>>>>>> and put 'static' descriptors early and check for
>>>>>> filter number to match it as an 'quick fix'
>>>>>>
>>>>>> or we should fix it for real and make it generic
>>>>>>
>>>>>> so currently the interface is like this:
>>>>>>
>>>>>>   pos1 = fdarray__add(a, fd1 ... );
>>>>>>   pos2 = fdarray__add(a, fd2 ... );
>>>>>>   pos3 = fdarray__add(a, fd2 ... );
>>>>>>
>>>>>>   fdarray__poll(a);
>>>>>>
>>>>>>   num = fdarray__filter(a, revents, destructor, arg);
>>>>>>
>>>>>> when fdarray__filter removes some of the fds the 'pos1,pos2,pos3'
>>>>>> indexes are not relevant anymore
>>>>
>>>> and that is why the return value of fdarray__add() should be converted
>>>> to bool (added/not added). Currently the return value is used as bool
>>>> only allover the calling code.
>>>>
>>>> fdarray__add_fixed() brings the notion of fd with fixed pos which is
>>>> valid after fdarray__add_fixed() call so the pos could be used to access
>>>> pos fd poll status after poll() call.
>>>>
>>>> pos = fdarray__add_fixed(array, fd);
>>>> fdarray_poll(array);
>>>> revents = fdarray_fixed_revents(array, pos);
>>>> fdarray__del(array, pos);
>>>
>>> So how is it about just adding _revents() and _del() for fixed fds with
>>> correction of retval to bool for fdarray__add()?
>>
>> I don't like the separation for fixed and non-fixed fds,
>> why can't we make generic?
> 
> Usage models are different but they want still to be parts of the same class
> for atomic poll(). The distinction is filterable vs. not filterable.
> The distinction should be somehow provided in API. Options are:
> 1. expose separate API calls like __add_nonfilterable(), __del_nonfilterable();
>    use nonfilterable quality in __filter() and __poll() and, perhaps, other internals;
> 2. extend fdarray__add(, nonfilterable) with the nonfilterable quality
>    use the type in __filter() and __poll() and, perhaps, other internals;
>    expose less API calls in comparison with option 1
> 
> Exposure of pos for filterable fds should be converted to bool since currently
> the returned pos can become stale and there is no way in API to check its state.
> So it could look like this:
> 
> fdkey = fdarray__add(array, fd, events, type)
> type: filterable, nonfilterable, somthing else
> revents = fdarray__get_revents(fdkey);
> fdarray__del(array, fdkey);

Are there any thoughts regarding this?

~Alexey