perf: builtin-record: Document and check that mmap_pages must be a power of two.

Now that we automatically point users at it, let's provide them some
guidance so that they hopefully don't just get mysterious EINVAL's
from the kernel.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1324301972-22740-4-git-send-email-nelhage@nelhage.comSigned-off-by: Nelson Elhage <nelhage@nelhage.com>
[ committer note: Made it work after 50a682ce ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-record.txt

@@ -89,7 +89,7 @@ OPTIONS
 
 -m::
 --mmap-pages=::
-	Number of mmap data pages.
+	Number of mmap data pages. Must be a power of two.
 
 -g::
 --call-graph::

tools/perf/builtin-record.c

@@ -279,6 +279,9 @@ static void perf_record__open(struct perf_record *rec)
 			    "/proc/sys/kernel/perf_event_mlock_kb,\n"
 			    "or try again with a smaller value of -m/--mmap_pages.\n"
 			    "(current value: %d)\n", opts->mmap_pages);
+		else if (!is_power_of_2(opts->mmap_pages))
+			die("--mmap_pages/-m value must be a power of two.");
+
 		die("failed to mmap with %d (%s)\n", errno, strerror(errno));
 	}
 

tools/perf/util/evlist.c

@@ -563,6 +563,8 @@ int perf_evlist__mmap(struct perf_evlist *evlist, unsigned int pages,
         /* 512 kiB: default amount of unprivileged mlocked memory */
         if (pages == UINT_MAX)
                 pages = (512 * 1024) / page_size;
+	else if (!is_power_of_2(pages))
+		return -EINVAL;
 
 	mask = pages * page_size - 1;
 

tools/perf/util/util.h

@@ -245,4 +245,15 @@ int readn(int fd, void *buf, size_t size);
 #define _STR(x) #x
 #define STR(x) _STR(x)
 
+/*
+ *  Determine whether some value is a power of two, where zero is
+ * *not* considered a power of two.
+ */
+
+static inline __attribute__((const))
+bool is_power_of_2(unsigned long n)
+{
+	return (n != 0 && ((n & (n - 1)) == 0));
+}
+
 #endif