linux-kernel - Re: [PATCH] perf bench sched pipe: fix enforced blocking reads in worker

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Message-ID: <Z-Bat1oBCQqT5mt5@gmail.com>
Date: Sun, 23 Mar 2025 20:02:15 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Dirk Gouders <dirk@...ders.net>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-perf-users@...r.kernel.org
Subject: Re: [PATCH] perf bench sched pipe: fix enforced blocking reads in
 worker_thread


* Dirk Gouders <dirk@...ders.net> wrote:

> The function worker_thread() is programmed in a way that roughly
> doubles the number of expectable context switches, because it enforces
> blocking reads:
> 
>  Performance counter stats for 'perf bench sched pipe':
> 
>          2,000,004      context-switches
> 
>       11.859548321 seconds time elapsed
> 
>        0.674871000 seconds user
>        8.076890000 seconds sys
> 
> The result of this behavior is that the blocking reads by far dominate
> the performance analysis of 'perf bench sched pipe':
> 
> Samples: 78K of event 'cycles:P', Event count (approx.): 27964965844
> Overhead  Command     Shared Object         Symbol
>   25.28%  sched-pipe  [kernel.kallsyms]     [k] read_hpet
>    8.11%  sched-pipe  [kernel.kallsyms]     [k] retbleed_untrain_ret
>    2.82%  sched-pipe  [kernel.kallsyms]     [k] pipe_write
> 
> From the code, it is unclear if that behavior is wanted but the log
> says that at least Ingo Molnar aims to mimic lmbench's lat_ctx, that
> doesn't handle the pipe ends that way
> (https://sourceforge.net/p/lmbench/code/HEAD/tree/trunk/lmbench2/src/lat_ctx.c)
> 
> Fix worker_thread() by always first feeding the write ends of the pipes
> and then trying to read.
> 
> This roughly halves the context switches and runtime of pure
> 'perf bench sched pipe':
> 
>  Performance counter stats for 'perf bench sched pipe':
> 
>          1,005,770      context-switches
> 
>        6.033448041 seconds time elapsed
> 
>        0.423142000 seconds user
>        4.519829000 seconds sys
> 
> And the blocking reads do no longer dominate the analysis at the above
> extreme:
> 
> Samples: 40K of event 'cycles:P', Event count (approx.): 14309364879
> Overhead  Command     Shared Object         Symbol
>   12.20%  sched-pipe  [kernel.kallsyms]     [k] read_hpet
>    9.23%  sched-pipe  [kernel.kallsyms]     [k] retbleed_untrain_ret
>    3.68%  sched-pipe  [kernel.kallsyms]     [k] pipe_write
> 
> Signed-off-by: Dirk Gouders <dirk@...ders.net>
> ---
>  tools/perf/bench/sched-pipe.c | 15 ++++-----------
>  1 file changed, 4 insertions(+), 11 deletions(-)
> 
> diff --git a/tools/perf/bench/sched-pipe.c b/tools/perf/bench/sched-pipe.c
> index e2562677df96..70139036d68f 100644
> --- a/tools/perf/bench/sched-pipe.c
> +++ b/tools/perf/bench/sched-pipe.c
> @@ -204,17 +204,10 @@ static void *worker_thread(void *__tdata)
>  	}
>  
>  	for (i = 0; i < loops; i++) {
> -		if (!td->nr) {
> -			ret = read_pipe(td);
> -			BUG_ON(ret != sizeof(int));
> -			ret = write(td->pipe_write, &m, sizeof(int));
> -			BUG_ON(ret != sizeof(int));
> -		} else {
> -			ret = write(td->pipe_write, &m, sizeof(int));
> -			BUG_ON(ret != sizeof(int));
> -			ret = read_pipe(td);
> -			BUG_ON(ret != sizeof(int));
> -		}
> +		ret = write(td->pipe_write, &m, sizeof(int));
> +		BUG_ON(ret != sizeof(int));
> +		ret = read_pipe(td);
> +		BUG_ON(ret != sizeof(int));

Yeah, this was unintended:

Acked-by: Ingo Molnar <mingo@...nel.org>

Thanks,

	Ingo