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Subject: Re: [PATCH v7 01/13] tools/libperf: introduce notion of static polled
 file descriptors
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>
 <3c92a0ad-d7d3-4e78-f0b8-1d3a7122c69e@linux.intel.com>
 <20200605105051.GA1404794@krava>
From:   Alexey Budankov <alexey.budankov@linux.intel.com>
Organization: Intel Corp.
Message-ID: <b1934951-1dc2-a6b8-ddb3-25643ba37a00@linux.intel.com>
Date:   Fri, 5 Jun 2020 14:50:22 +0300
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On 05.06.2020 13:50, 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?

Whatever stat fd which is not -1 is added. If static fd is -1 then it is skipped for
poll() call. Complete stat slot (fd == -1) isn't expected to be reused but reuse could
be supported by simple change at fdarray__add_stat()
and probably bring required generality.

> 
> 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

It would complicate without a reason. If it really matters I would add possibility of
realloc of stat entries in fdarray__add_stat() and that would make stat entries usage
more similar to filterable ones without dramatic change and risk of regressions.

~Alexey

> 
> 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
> 
> how about we make the 'pos indexes' being stable by allocating
> separate object for each added descriptor and each poll call
> would create pollfd array from current objects, and entries
> would keep pointer to its pollfd entry
> 
>   struct fdentry *entry {
>        int              fd;
>        int              events;
>        struct pollfd   *pollfd;
>   }
> 
>   entry1 = fdarray__add(a, fd1 ...);
>   entry2 = fdarray__add(a, fd2 ...);
>   entry3 = fdarray__add(a, fd3 ...);
> 
>   fdarray__poll(a);
> 
>   struct pollfd *fdarray__entry_pollfd(a, entry1);
> 
> or smoething like that ;-)
> 
> jirka
>