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author | Lai Jiangshan <laijs@cn.fujitsu.com> | 2008-06-12 10:42:58 +0200 |
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committer | Ingo Molnar <mingo@elte.hu> | 2008-06-12 14:23:55 +0200 |
commit | 2e084786f6fe052274f1dfa7c675fe4a02cacd6e (patch) | |
tree | 66190825236b1f3f59e88aaa83d2683bd121143d /kernel | |
parent | sched: fix TASK_WAKEKILL vs SIGKILL race (diff) | |
download | linux-2e084786f6fe052274f1dfa7c675fe4a02cacd6e.tar.xz linux-2e084786f6fe052274f1dfa7c675fe4a02cacd6e.zip |
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 <laijs@cn.fujitsu.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'kernel')
-rw-r--r-- | kernel/sched.c | 7 |
1 files changed, 5 insertions, 2 deletions
diff --git a/kernel/sched.c b/kernel/sched.c index 2c65bf29d133..6c1ecbdc0db9 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -312,12 +312,15 @@ static DEFINE_SPINLOCK(task_group_lock); #endif /* - * 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; #endif |