[PATCH] perf, tools, bench: Fix memcpy benchmark for large sizes v2

[Andi Kleen]

From: Andi Kleen <ak@xxxxxxxxxxxxxxx>

glibc calloc() has an optimization to not explicitely memset()
very large calloc allocations that just came from mmap(),
because they are known to be zero.

This could result in the perf memcpy benchmark reading only from
the zero page, which gives unrealistic results.

Always call memset explicitly on the source area to avoid this problem.

Cc: h.mitake@xxxxxxxxx
Cc: kirill.shutemov@xxxxxxxxxxxxxxx
v2: Actually memset the right area and also fix the NULL check before.
Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
---
 tools/perf/bench/mem-memcpy.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/perf/bench/mem-memcpy.c b/tools/perf/bench/mem-memcpy.c
index 93c83e3..0887b46 100644
--- a/tools/perf/bench/mem-memcpy.c
+++ b/tools/perf/bench/mem-memcpy.c
@@ -115,8 +115,10 @@ static void alloc_mem(void **dst, void **src, size_t length)
 		die("memory allocation failed - maybe length is too large?\n");
 
 	*src = zalloc(length);
-	if (!src)
+	if (!*src)
 		die("memory allocation failed - maybe length is too large?\n");
+	/* Make sure to always replace the zero pages even if MMAP_THRESH is crossed */
+	memset(*src, 0, length);
 }
 
 static u64 do_memcpy_cycle(memcpy_t fn, size_t len, bool prefault)
-- 
1.8.1.4

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