linux-kernel - Re: [PATCH v8 01/13] tools/libperf: avoid moving of fds at fdarray_

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Date:   Thu, 25 Jun 2020 22:32:29 +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 v8 01/13] tools/libperf: avoid moving of fds at
 fdarray__filter() call


On 25.06.2020 20:14, Jiri Olsa wrote:
> On Wed, Jun 24, 2020 at 08:19:32PM +0300, Alexey Budankov wrote:
>>
>> On 17.06.2020 11:35, Alexey Budankov wrote:
>>>
>>> Skip fds with zeroed revents field from count and avoid fds moving
>>> at fdarray__filter() call so fds indices returned by fdarray__add()
>>> call stay the same and can be used for direct access and processing
>>> of fd revents status field at entries array of struct fdarray object.
>>>
>>> Signed-off-by: Alexey Budankov <alexey.budankov@...ux.intel.com>
>>> ---
>>>  tools/lib/api/fd/array.c   | 11 +++++------
>>>  tools/perf/tests/fdarray.c | 20 ++------------------
>>>  2 files changed, 7 insertions(+), 24 deletions(-)
>>>
>>> diff --git a/tools/lib/api/fd/array.c b/tools/lib/api/fd/array.c
>>> index 58d44d5eee31..97843a837370 100644
>>> --- a/tools/lib/api/fd/array.c
>>> +++ b/tools/lib/api/fd/array.c
>>> @@ -93,22 +93,21 @@ int fdarray__filter(struct fdarray *fda, short revents,
>>>  		return 0;
>>>  
>>>  	for (fd = 0; fd < fda->nr; ++fd) {
>>> +		if (!fda->entries[fd].revents)
>>> +			continue;
>>> +
>>
>> So it looks like this condition also filters out non signaling events fds, not only
>> control and others fds, and this should be somehow avoided so such event related fds
>> would be counted. Several options have been proposed so far:
>>
>> 1) Explicit typing of fds via API extension and filtering based on the types:
>>    a) with separate fdarray__add_stat() call
>>    b) with type arg of existing fdarray__add() call
>>    c) various memory management design is possible
>>
>> 2) Playing tricks with fd positions inside entries and assumptions on fdarray API calls ordering
>>    - looks more like a hack than a designed solution
>>
>> 3) Rewrite of fdarray class to allocate separate object for every added fds
>>    - can be replaced with nonscrewing of fds by __filter()
>>
>> 4) Distinct between fds types at fdarray__filter() using .revents == 0 condition
>>    - seems to have corner cases and thus not applicable
>>
>> 5) Extension of fdarray__poll(, *arg_ptr, arg_size) with arg of fds array to atomically poll
>>    on fdarray_add()-ed fds and external arg fds and then external arg fds processing
>>
>> 6) Rewrite of fdarray class on epoll() call basis
>>    - introduces new scalability restrictions for Perf tool
> 
> hum, how many fds for polling do you expect in your workloads?

Currently it is several hundreds so default of 1K is easily hit and 
"Profile a Large Number of PMU Events on Multi-Core Systems" section [1]
recommends:

soft nofile 65535
hard nofile 65535

for for /etc/security/limits.conf settings.

~Alexey

[1] https://software.intel.com/content/www/us/en/develop/documentation/vtune-cookbook/top/configuration-recipes/profiling-hardware-without-sampling-drivers.html

> 
> jirka
>