linux-kernel - Re: [PATCH v7 01/13] tools/libperf: introduce notion of static polled file descriptors

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Date:   Mon, 8 Jun 2020 10:43:44 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 01/13] tools/libperf: introduce notion of static
 polled file descriptors

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@...ux.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?

jirka