Re: [PATCH] kernel/sys.c : Get rid of expensive divides in groups_sort()

[Eric Dumazet]

Andrew Morton a écrit :
> On Wed, 19 Dec 2007 01:14:33 +0100
> Eric Dumazet <dada1@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
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;