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Date:   Mon, 8 Jun 2020 18:07:58 +0200
From:   Jiri Olsa <jolsa@redhat.com>
To:     Alexey Budankov <alexey.budankov@linux.intel.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>
Subject: Re: [PATCH v7 01/13] tools/libperf: introduce notion of static
 polled file descriptors
Message-ID: <20200608160758.GD1558310@krava>
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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,
I'm not sure why you think this is needed

I'm busy with other things this week, but I think I can
come up with some patch early next week if needed

jirka