kernel/sys.c: get rid of expensive divides in groups_sort()

groups_sort() can be quite long if user loads a large gid table.

This is because GROUP_AT(group_info, some_integer) uses an integer divide.
So having to do XXX thousand divides during one syscall can lead to very
high latencies.  (NGROUPS_MAX=65536)

In the past (25 Mar 2006), an analog problem was found in groups_search()
(commit d74beb9f33a5f16d2965f11b275e401f225c949d ) and at that time I
changed some variables to unsigned int.

I believe that a more generic fix is to make sure NGROUPS_PER_BLOCK is
unsigned.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 9c13be3a21e8..7c8ca05c3cae 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -810,7 +810,7 @@ static inline int above_background_load(void)
 
 struct io_context;                      /* See blkdev.h */
 #define NGROUPS_SMALL           32
-#define NGROUPS_PER_BLOCK       ((int)(PAGE_SIZE / sizeof(gid_t)))
+#define NGROUPS_PER_BLOCK       ((unsigned int)(PAGE_SIZE / sizeof(gid_t)))
 struct group_info {
         int ngroups;
         atomic_t usage;