linux-kernel - Re: [PATCH v2 12/15] perf record: introduce thread local variable for trace streaming

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Date:   Mon, 26 Oct 2020 17:11:30 +0300
From:   Alexei Budankov <budankov.lore@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        budankov.lore@...il.com
Subject: Re: [PATCH v2 12/15] perf record: introduce thread local variable for
 trace streaming


On 26.10.2020 13:34, Jiri Olsa wrote:
> On Mon, Oct 26, 2020 at 11:21:28AM +0300, Alexei Budankov wrote:
>>
>> On 24.10.2020 18:43, Jiri Olsa wrote:
>>> On Wed, Oct 21, 2020 at 07:07:00PM +0300, Alexey Budankov wrote:
>>>>
>>>> Introduce thread local variable and use it for threaded trace streaming.
>>>>
>>>> Signed-off-by: Alexey Budankov <alexey.budankov@...ux.intel.com>
>>>> ---
>>>>  tools/perf/builtin-record.c | 71 ++++++++++++++++++++++++++++++++-----
>>>>  1 file changed, 62 insertions(+), 9 deletions(-)
>>>>
>>>> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
>>>> index 89cb8e913fb3..3b7e9026f25b 100644
>>>> --- a/tools/perf/builtin-record.c
>>>> +++ b/tools/perf/builtin-record.c
>>>> @@ -101,6 +101,8 @@ struct thread_data {
>>>>  	u64		   bytes_written;
>>>>  };
>>>>  
>>>> +static __thread struct thread_data *thread;
>>>> +
>>>>  struct record {
>>>>  	struct perf_tool	tool;
>>>>  	struct record_opts	opts;
>>>> @@ -587,7 +589,11 @@ static int record__pushfn(struct mmap *map, void *to, void *bf, size_t size)
>>>>  		}
>>>>  	}
>>>>  
>>>> -	rec->samples++;
>>>> +	if (thread)
>>>> +		thread->samples++;
>>>> +	else
>>>> +		rec->samples++;
>>>
>>> this is really wrong, let's keep just single samples counter
>>> ditto for all the other places in this patch
>>
>> This does look like data parallelism [1] which is very true for
>> threaded trace streaming so your prototype design looks optimal.
>>
>> For this specific place incrementing global counter in memory is
>> less performant and faces scalability limitations as a number of
>> cores grow.
>>
>> Not sure why you have changed your mind.
> 
> I'm not sure I follow.. what I'm complaining about is to have
> 'samples' stat variable in separate locations for --threads
> and --no-threads mode

It is optimal to have samples variable as per thread one
and then sum up the total in the end of data collection.

Single global variable design has scalability and performance
drawbacks.

Why do you complain about per thread variable in this case?
It looks like ideally fits these specific needs.

Alexei

> 
> jirka
> 
>>
>> Alexei
>>
>> [1] https://en.wikipedia.org/wiki/Data_parallelism#:~:text=Data%20parallelism%20is%20parallelization%20across,on%20each%20element%20in%20parallel.
>>
>