perf bench: Fix memcpy benchmark for large sizes

The glibc calloc() function 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.
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Hitoshi Mitake <h.mitake@gmail.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Link: http://lkml.kernel.org/n/tip-pzz2qrdq9eymxda0y8yxdn33@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/bench/mem-memcpy.c

@@ -117,6 +117,8 @@ static void alloc_mem(void **dst, void **src, size_t length)
 	*src = zalloc(length);
 	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)