|author||Lai Jiangshan <firstname.lastname@example.org>||2008-06-12 16:42:58 +0800|
|committer||Ingo Molnar <email@example.com>||2008-06-12 14:23:55 +0200|
sched: fair group: fix overflow(was: fix divide by zero)
I found a bug which can be reproduced by this way:(linux-2.6.26-rc5, x86-64) (use 2^32, 2^33, ...., 2^63 as shares value) # mkdir /dev/cpuctl # mount -t cgroup -o cpu cpuctl /dev/cpuctl # cd /dev/cpuctl # mkdir sub # echo 0x8000000000000000 > sub/cpu.shares # echo $$ > sub/tasks oops here! divide by zero. This is because do_div() expects the 2th parameter to be 32 bits, but unsigned long is 64 bits in x86_64. Peter Zijstra pointed it out that the sane thing to do is limit the shares value to something smaller instead of using an even more expensive divide. Also, I found another bug about "the shares value is too large": pid1 and pid2 are set affinity to cpu#0 pid1 is attached to cg1 and pid2 is attached to cg2 if cg1/cpu.shares = 1024 cg2/cpu.shares = 2000000000 then pid2 got 100% usage of cpu, and pid1 0% if cg1/cpu.shares = 1024 cg2/cpu.shares = 20000000000 then pid2 got 0% usage of cpu, and pid1 100% And a weight of a cfs_rq is the sum of weights of which entities are queued on this cfs_rq, so the shares value should be limited to a smaller value. I think that (1UL << 18) is a good limited value: 1) it's not too large, we can create a lot of group before overflow 2) it's several times the weight value for nice=-19 (not too small) Signed-off-by: Lai Jiangshan <firstname.lastname@example.org> Acked-by: Peter Zijlstra <email@example.com> Signed-off-by: Ingo Molnar <firstname.lastname@example.org>
1 files changed, 5 insertions, 2 deletions
diff --git a/kernel/sched.c b/kernel/sched.c
index 2c65bf29d133..6c1ecbdc0db9 100644
@@ -312,12 +312,15 @@ static DEFINE_SPINLOCK(task_group_lock);
- * A weight of 0, 1 or ULONG_MAX can cause arithmetics problems.
+ * A weight of 0 or 1 can cause arithmetics problems.
+ * A weight of a cfs_rq is the sum of weights of which entities
+ * are queued on this cfs_rq, so a weight of a entity should not be
+ * too large, so as the shares value of a task group.
* (The default weight is 1024 - so there's no practical
* limitation from this.)
#define MIN_SHARES 2
-#define MAX_SHARES (ULONG_MAX - 1)
+#define MAX_SHARES (1UL << 18)
static int init_task_group_load = INIT_TASK_GROUP_LOAD;