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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>
References: <5de4b954-24f0-1e8d-5a0d-7b12783b8218@linux.intel.com>
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Organization: Intel Corp.
Message-ID: <6a861ce4-3769-fc2a-5f9d-3003238a9736@linux.intel.com>
Date:   Mon, 8 Jun 2020 20:18:20 +0300
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On 08.06.2020 19:43, Alexey Budankov wrote:
> 
> On 08.06.2020 19:07, Jiri Olsa wrote:
>> On Mon, Jun 08, 2020 at 12:54:31PM +0300, 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>
>>>>>>>>> ---
>>>>>>>>>  tools/lib/api/fd/array.c                 | 42 +++++++++++++++++++++++-
>>>>>>>>>  tools/lib/api/fd/array.h                 |  7 ++++
>>>>>>>>>  tools/lib/perf/evlist.c                  | 11 +++++++
>>>>>>>>>  tools/lib/perf/include/internal/evlist.h |  2 ++
>>>>>>>>>  4 files changed, 61 insertions(+), 1 deletion(-)
>>>>>>>>>
>>>>>>>>> diff --git a/tools/lib/api/fd/array.c b/tools/lib/api/fd/array.c
>>>>>>>>> index 58d44d5eee31..b0027f2169c7 100644
>>>>>>>>> --- a/tools/lib/api/fd/array.c
>>>>>>>>> +++ b/tools/lib/api/fd/array.c
>>>>>>>>> @@ -11,10 +11,16 @@
>>>>>>>>>  
>>>>>>>>>  void fdarray__init(struct fdarray *fda, int nr_autogrow)
>>>>>>>>>  {
>>>>>>>>> +	int i;
>>>>>>>>> +
>>>>>>>>>  	fda->entries	 = NULL;
>>>>>>>>>  	fda->priv	 = NULL;
>>>>>>>>>  	fda->nr		 = fda->nr_alloc = 0;
>>>>>>>>>  	fda->nr_autogrow = nr_autogrow;
>>>>>>>>> +
>>>>>>>>> +	fda->nr_stat = 0;
>>>>>>>>> +	for (i = 0; i < FDARRAY__STAT_ENTRIES_MAX; i++)
>>>>>>>>> +		fda->stat_entries[i].fd = -1;
>>>>>>>>>  }
>>>>>>>>>  
>>>>>>>>>  int fdarray__grow(struct fdarray *fda, int nr)
>>>>>>>>> @@ -83,6 +89,20 @@ int fdarray__add(struct fdarray *fda, int fd, short revents)
>>>>>>>>>  	return pos;
>>>>>>>>>  }
>>>>>>>>>  
>>>>>>>>> +int fdarray__add_stat(struct fdarray *fda, int fd, short revents)
>>>>>>>>> +{
>>>>>>>>> +	int pos = fda->nr_stat;
>>>>>>>>> +
>>>>>>>>> +	if (pos >= FDARRAY__STAT_ENTRIES_MAX)
>>>>>>>>> +		return -1;
>>>>>>>>> +
>>>>>>>>> +	fda->stat_entries[pos].fd = fd;
>>>>>>>>> +	fda->stat_entries[pos].events = revents;
>>>>>>>>> +	fda->nr_stat++;
>>>>>>>>> +
>>>>>>>>> +	return pos;
>>>>>>>>> +}
>>>>>>>>> +
>>>>>>>>>  int fdarray__filter(struct fdarray *fda, short revents,
>>>>>>>>>  		    void (*entry_destructor)(struct fdarray *fda, int fd, void *arg),
>>>>>>>>>  		    void *arg)
>>>>>>>>> @@ -113,7 +133,27 @@ int fdarray__filter(struct fdarray *fda, short revents,
>>>>>>>>>  
>>>>>>>>>  int fdarray__poll(struct fdarray *fda, int timeout)
>>>>>>>>>  {
>>>>>>>>> -	return poll(fda->entries, fda->nr, timeout);
>>>>>>>>> +	int nr, i, pos, res;
>>>>>>>>> +
>>>>>>>>> +	nr = fda->nr;
>>>>>>>>> +
>>>>>>>>> +	for (i = 0; i < fda->nr_stat; i++) {
>>>>>>>>> +		if (fda->stat_entries[i].fd != -1) {
>>>>>>>>> +			pos = fdarray__add(fda, fda->stat_entries[i].fd,
>>>>>>>>> +					   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);
>>
>> I think there's solution without having filterable type,
> 
> and still making the atomic fdarray__poll()?

How is it about design like this?

    int fdarray__poll(struct fdarray *fda, int timeout)

with additional external array of fds to simultaneously poll() on:

    int fdarray__poll(struct fdarray *fda, int timeout,
                      int *fds, size_t fds_size)

fds would be added to array just prior poll() call.

That wouldn't cause fdarray class extension with fd types and wouldn't change 
fdarray__filter() keeping the types processing outside of fdarray and libperf.

Adoption of new fdarray__poll() signature in the tool via macro (or inline function):

#define fdarray__poll(fda, timeout) \
	fdarray__poll_ext(fda, timeout, NULL, 0)
int fdarray__poll_ext(struct fdarray *fda, int timeout,
                      int *fds, size_t fds_size)

~Alexey