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Date:   Wed, 12 Aug 2020 09:25:29 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>, kernel@...s.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf bench mem: Always memset source before memcpy

Em Mon, Aug 10, 2020 at 03:34:04PM +0200, Vincent Whitchurch escreveu:
> For memcpy, the source pages are memset to zero only when --cycles is
> used.  This leads to wildly different results with or without --cycles,
> since all sources pages are likely to be mapped to the same zero page
> without explicit writes.

Thanks, applied.

- Arnaldo
 
> Before this fix:
> 
> $ export cmd="./perf stat -e LLC-loads -- ./perf bench \
>   mem memcpy -s 1024MB -l 100 -f default"
> $ $cmd
> 
>          2,935,826      LLC-loads
>        3.821677452 seconds time elapsed
> 
> $ $cmd --cycles
> 
>        217,533,436      LLC-loads
>        8.616725985 seconds time elapsed
> 
> After this fix:
> 
> $ $cmd
> 
>        214,459,686      LLC-loads
>        8.674301124 seconds time elapsed
> 
> $ $cmd --cycles
> 
>        214,758,651      LLC-loads
>        8.644480006 seconds time elapsed
> 
> Fixes: 47b5757bac03c3387c ("perf bench mem: Move boilerplate memory allocation to the infrastructure")
> Signed-off-by: Vincent Whitchurch <vincent.whitchurch@...s.com>
> ---
>  tools/perf/bench/mem-functions.c | 21 +++++++++++----------
>  1 file changed, 11 insertions(+), 10 deletions(-)
> 
> diff --git a/tools/perf/bench/mem-functions.c b/tools/perf/bench/mem-functions.c
> index 9235b76501be..19d45c377ac1 100644
> --- a/tools/perf/bench/mem-functions.c
> +++ b/tools/perf/bench/mem-functions.c
> @@ -223,12 +223,8 @@ static int bench_mem_common(int argc, const char **argv, struct bench_mem_info *
>  	return 0;
>  }
>  
> -static u64 do_memcpy_cycles(const struct function *r, size_t size, void *src, void *dst)
> +static void memcpy_prefault(memcpy_t fn, size_t size, void *src, void *dst)
>  {
> -	u64 cycle_start = 0ULL, cycle_end = 0ULL;
> -	memcpy_t fn = r->fn.memcpy;
> -	int i;
> -
>  	/* Make sure to always prefault zero pages even if MMAP_THRESH is crossed: */
>  	memset(src, 0, size);
>  
> @@ -237,6 +233,15 @@ static u64 do_memcpy_cycles(const struct function *r, size_t size, void *src, vo
>  	 * to not measure page fault overhead:
>  	 */
>  	fn(dst, src, size);
> +}
> +
> +static u64 do_memcpy_cycles(const struct function *r, size_t size, void *src, void *dst)
> +{
> +	u64 cycle_start = 0ULL, cycle_end = 0ULL;
> +	memcpy_t fn = r->fn.memcpy;
> +	int i;
> +
> +	memcpy_prefault(fn, size, src, dst);
>  
>  	cycle_start = get_cycles();
>  	for (i = 0; i < nr_loops; ++i)
> @@ -252,11 +257,7 @@ static double do_memcpy_gettimeofday(const struct function *r, size_t size, void
>  	memcpy_t fn = r->fn.memcpy;
>  	int i;
>  
> -	/*
> -	 * We prefault the freshly allocated memory range here,
> -	 * to not measure page fault overhead:
> -	 */
> -	fn(dst, src, size);
> +	memcpy_prefault(fn, size, src, dst);
>  
>  	BUG_ON(gettimeofday(&tv_start, NULL));
>  	for (i = 0; i < nr_loops; ++i)
> -- 
> 2.25.1
> 

-- 

- Arnaldo

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