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.com
Signed-off-by: Nelson Elhage <nelhage@nelhage.com>
[ committer note: Made it work after 50a682c ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

4 files changed, 17 insertions, 1 deletions

diff --git a/tools/perf/Documentation/perf-record.txt b/tools/perf/Documentation/perf-record.txt
index 5a520f825295..2937f7e14bb7 100644
--- a/tools/perf/Documentation/perf-record.txt
+++ b/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::
diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
index 56bb4476e3ba..e873ae2dd54c 100644
--- a/tools/perf/builtin-record.c
+++ b/tools/perf/builtin-record.c
@@ -279,6 +279,9 @@ try_again:
                             "/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));
         }
 
diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index 963d63dde457..fa1837088ca8 100644
--- a/tools/perf/util/evlist.c
+++ b/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;
 
diff --git a/tools/perf/util/util.h b/tools/perf/util/util.h
index 0128906bac88..37be34dff798 100644
--- a/tools/perf/util/util.h
+++ b/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