Messages in this thread Patch in this message | | | Date | Wed, 19 Dec 2007 01:36:07 +0100 | From | Eric Dumazet <> | Subject | Re: [PATCH] kernel/sys.c : Get rid of expensive divides in groups_sort() |
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Andrew Morton a écrit : > On Wed, 19 Dec 2007 01:14:33 +0100 > Eric Dumazet <dada1@cosmosbay.com> wrote: > >> 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. >> > >> 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))) > > That was a dopey thing to do. Both PAGE_SIZE and sizeof() are unsigned and > we went and cast it to a signed thing, even though it is a nonsense to ever > consider a negative value of this.
Maybe the author intention was to cast it to an "int", instead of "long", I dont know...
> >> +#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); >> > > We now have a jumble of signed and unsigned, stuck together with min_t > sticky tape. > > Please, take a closer look in there and see if there's anything which > *needs* to be signed: is there anything which can actually, sensibly have a > negative quantity? I bet there isn't, and I bet the code would be cleaner, > faster and more obviously correct if it was converted to unsigned throughout. > > I blame C. Negative quantities are rare, and C's default of treating > scalars as signed was a mistake. Oh well. > >
Yes I agree that 'unsigned' should be the default...
Well, I was trying to keep patch small, but as you have some time, I will make it bigger :)
[PATCH] 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> 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..d632352 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -1145,16 +1145,16 @@ static int groups_to_user(gid_t __user *grouplist, struct group_info *group_info) { int i; - int count = group_info->ngroups; + unsigned int count = group_info->ngroups; for (i = 0; i < group_info->nblocks; i++) { - int cp_count = min(NGROUPS_PER_BLOCK, count); - int off = i * NGROUPS_PER_BLOCK; - int len = cp_count * sizeof(*grouplist); + unsigned int cp_count = min(NGROUPS_PER_BLOCK, count); + unsigned int len = cp_count * sizeof(*grouplist); - if (copy_to_user(grouplist+off, group_info->blocks[i], len)) + if (copy_to_user(grouplist, group_info->blocks[i], len)) return -EFAULT; + grouplist += NGROUPS_PER_BLOCK; count -= cp_count; } return 0; @@ -1165,16 +1165,16 @@ static int groups_from_user(struct group_info *group_info, gid_t __user *grouplist) { int i; - int count = group_info->ngroups; + unsigned int count = group_info->ngroups; for (i = 0; i < group_info->nblocks; i++) { - int cp_count = min(NGROUPS_PER_BLOCK, count); - int off = i * NGROUPS_PER_BLOCK; - int len = cp_count * sizeof(*grouplist); + unsigned int cp_count = min(NGROUPS_PER_BLOCK, count); + unsigned int len = cp_count * sizeof(*grouplist); - if (copy_from_user(group_info->blocks[i], grouplist+off, len)) + if (copy_from_user(group_info->blocks[i], grouplist, len)) return -EFAULT; + grouplist += NGROUPS_PER_BLOCK; count -= cp_count; } return 0; | |