Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
IronPort-SDR: xEINcCMnRLNZ9Bo/qOOmYOIJ2jdDF3O6WMA0iZNcmM7VbzKIgrmDcLDVgD6uQzghdHpEk6Rpkq
 AiBZEpbh+P2A==
IronPort-SDR: pd1ihpN8R/97XNixh/WG3gwIGZTJUULrvKUoVbWitmqsDlfRxFWjNBfqFy9HpdxpR8SIv6lJSV
 ds3g2nAE09rQ==
Subject: Re: [PATCH v8 01/13] tools/libperf: avoid moving of fds at
 fdarray__filter() call
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: <0781a077-aa82-5b4a-273e-c17372a72b93@linux.intel.com>
 <3d36dc7a-4249-096c-7554-80e6d290eac5@linux.intel.com>
 <fada6325-2e6a-0de4-918f-0bc7d1410c52@linux.intel.com>
 <20200625171405.GL2719003@krava>
 <688910f3-289e-d63e-79e3-0a17a6df0e9e@linux.intel.com>
 <20200626093745.GM2719003@krava>
From:   Alexey Budankov <alexey.budankov@linux.intel.com>
Organization: Intel Corp.
Message-ID: <6582201a-9570-709f-f6e9-5a644296f49d@linux.intel.com>
Date:   Fri, 26 Jun 2020 13:06:51 +0300
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101
 Thunderbird/68.9.0
MIME-Version: 1.0
In-Reply-To: <20200626093745.GM2719003@krava>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 26.06.2020 12:37, Jiri Olsa wrote:
> On Thu, Jun 25, 2020 at 10:32:29PM +0300, Alexey Budankov wrote:
>>
>> 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@linux.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
> 
> I'm confused, are you talking about file descriptors limit now?
> this wont be affected by epoll change.. what do I miss?

Currently there is already uname -n limit on the amount of open file descriptors
and Perf tool process is affected by that limit.

Moving to epoll() will impose one more max_user_watches limit and that can additionally
confine Perf applicability even though default value on some machines currently
is high enough.

~Alexey

> 
> I thought your concern was fs.epoll.max_user_watches, which has
> default value that seems to be enough:
> 
> 	$ cat /proc/sys/fs/epoll/max_user_watches
> 	3169996
> 
> jirka
> 
> 
>>
>> 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
>>>
>>
>