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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: <be40edeb-0cb9-5e11-2a22-8392316cdced@linux.intel.com>
 <49eca46e-4d0e-2ae5-d7d9-e37a4d680270@linux.intel.com>
 <20200608084344.GA1520715@krava>
 <2d80a43a-54cf-3d12-92fd-066217c95d76@linux.intel.com>
 <20200608160758.GD1558310@krava>
 <bde9bcc3-9ec0-6e37-26f6-139b038ad3de@linux.intel.com>
 <20200615123048.GB2088119@krava>
 <8b29e324-eb8d-2266-562b-ca46aec76a3e@linux.intel.com>
 <20200615165802.GD2088119@krava>
 <8351b3ee-d345-a394-d687-443f2d2f7ec4@linux.intel.com>
 <20200622102142.GA2583819@krava>
From:   Alexey Budankov <alexey.budankov@linux.intel.com>
Organization: Intel Corp.
Message-ID: <cad3d9a6-da28-c627-de73-17169a7c36a1@linux.intel.com>
Date:   Mon, 22 Jun 2020 13:50:03 +0300
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On 22.06.2020 13:21, Jiri Olsa wrote:
> On Mon, Jun 22, 2020 at 12:47:19PM +0300, Alexey Budankov wrote:
> 
> SNIP
> 
>>>>>>>> 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
>>>>>>
>>>>>> Friendly reminder.
>>>>>
>>>>> hm? I believe we discussed this in here:
>>>>>   https://lore.kernel.org/lkml/20200609145611.GI1558310@krava/
>>>>
>>>> Do you want it to be implemented like in the patch posted by the link?
>>>
>>> no idea.. looking for good solution ;-)
>>>
>>> how about switching completely to epoll? I tried and it
>>> does not look that bad
>>
>> Well, epoll() is perhaps possible but why does it want switching to epoll()?
>> What are the benefits and/or specific task being solved by this switch? 
> 
> epoll change fixes the same issues as the patch you took in v8
> 
> on top of it it's not a hack and wil make polling more user
> friendly because of the clear interface

Clear. The opposite thing is /proc/sys/fs/epoll/max_user_watches limit that
will affect Perf tool usage additionally to the current process limit on 
a number of simultaneously open file descriptors (ulimit -n). So move to 
epoll() will impose one limit what can affect Perf tool scalability.

> 
>>
>>>
>>> there might be some loose ends (interface change), but
>>> I think this would solve our problems with fdarray
>>
>> Your first patch accomodated in v8 actually avoids fds typing
>> and solves pos (=fdarray__add()) staleness issue with fdarray.
> 
> yea, it was a change meant for discussion (which never happened),
> and I considered it to be more a hack than a solution
> 
> I suppose we can live with that for a while, but I'd like to
> have clean solution for polling as well

I wouldn't treat it as a hack but more as a fix because returned
pos is now a part of interface that can be safely used in callers.
Can we go with this fix for the patch set?

Thanks,
Alexey