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

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Date:   Mon, 22 Jun 2020 17:04:21 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Jiri Olsa <jolsa@...hat.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 22.06.2020 15:11, Jiri Olsa wrote:
> On Mon, Jun 22, 2020 at 01:50:03PM +0300, Alexey Budankov wrote:
>>
>> 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.
> 
> hum, I dont think this will be a problem:
> 
>     Allowing top 4% of low memory (per user) to be allocated in epoll watches,
>     we have:
> 
>     LOMEM    MAX_WATCHES (per user)
>     512MB    ~178000
>     1GB      ~356000
>     2GB      ~712000
> 
> my laptop has 19841945 allowed watches per user
> 
>>
>>>
>>>>
>>>>>
>>>>> 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?
> 
> apart from this one I still have a problem with that stat factoring
> having 1 complicated function deal with both fork and no fork processing,
> which I already commented on, but you ignored ;-)

Not an issue at all, lets split that func, dispatch_events() I suppose,
as you see it.

> 
> I'll try to go through that once more, and post some comments
> 
> jirka
> 

Thanks,
Alexey