perf bench mem: Always memset source before memcpy

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.

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: 47b5757bac03c338 ("perf bench mem: Move boilerplate memory allocation to the infrastructure")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: kernel@axis.com
Link: http://lore.kernel.org/lkml/20200810133404.30829-1-vincent.whitchurch@axis.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

diff --git a/tools/perf/bench/mem-functions.c b/tools/perf/bench/mem-functions.c
index 9235b76..19d45c3 100644 (file)
--- 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)