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

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Message-ID: <20201027120130.GD2900849@krava>
Date:   Tue, 27 Oct 2020 13:01:30 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Alexei Budankov <budankov.lore@...il.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>
Subject: Re: [PATCH v2 12/15] perf record: introduce thread local variable
 for trace streaming

On Mon, Oct 26, 2020 at 05:11:30PM +0300, Alexei Budankov wrote:
> 
> 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.

I think there's misunderstanding.. I think we should move
samples to per thread 'thread' object and have just one
copy of that.. and do not increase separate variables for
thread and non-thread cases

jirka