Messages in this thread Patch in this message | | | Date | Wed, 19 Dec 2007 01:14:33 +0100 | From | Eric Dumazet <> | Subject | [PATCH] kernel/sys.c : Get rid of expensive divides in groups_sort() |
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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> diff --git a/include/linux/sched.h b/include/linux/sched.h index ac3d496..725a491 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -775,7 +775,7 @@ 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; diff --git a/kernel/sys.c b/kernel/sys.c index d1fe71e..091e58f 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -1148,7 +1148,7 @@ static int groups_to_user(gid_t __user *grouplist, int count = group_info->ngroups; for (i = 0; i < group_info->nblocks; i++) { - int cp_count = min(NGROUPS_PER_BLOCK, count); + int cp_count = min_t(int, NGROUPS_PER_BLOCK, count); int off = i * NGROUPS_PER_BLOCK; int len = cp_count * sizeof(*grouplist); @@ -1168,7 +1168,7 @@ static int groups_from_user(struct group_info *group_info, int count = group_info->ngroups; for (i = 0; i < group_info->nblocks; i++) { - int cp_count = min(NGROUPS_PER_BLOCK, count); + int cp_count = min_t(int, NGROUPS_PER_BLOCK, count); int off = i * NGROUPS_PER_BLOCK; int len = cp_count * sizeof(*grouplist); | |