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 11:34:26 +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 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

jirka

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