Messages in this thread | | | Date | Fri, 11 Dec 2015 17:57:51 +0000 | From | Morten Rasmussen <> | Subject | Re: [PATCH] sched/fair: fix mul overflow on 32-bit systems |
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On Fri, Dec 11, 2015 at 05:00:01PM +0300, Andrey Ryabinin wrote: > > > On 12/11/2015 04:36 PM, Peter Zijlstra wrote: > > On Fri, Dec 11, 2015 at 02:25:51PM +0100, Peter Zijlstra wrote: > >> On Fri, Dec 11, 2015 at 03:55:18PM +0300, Andrey Ryabinin wrote: > >>> Make 'r' 64-bit type to avoid overflow in 'r * LOAD_AVG_MAX' > >>> on 32-bit systems: > >>> UBSAN: Undefined behaviour in kernel/sched/fair.c:2785:18 > >>> signed integer overflow: > >>> 87950 * 47742 cannot be represented in type 'int' > >>> > >>> Fixes: 9d89c257dfb9 ("sched/fair: Rewrite runnable load and utilization average tracking") > >>> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com> > >>> --- > >>> kernel/sched/fair.c | 4 ++-- > >>> 1 file changed, 2 insertions(+), 2 deletions(-) > >>> > >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > >>> index e3266eb..733f0b8 100644 > >>> --- a/kernel/sched/fair.c > >>> +++ b/kernel/sched/fair.c > >>> @@ -2780,14 +2780,14 @@ static inline int update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq) > >>> int decayed, removed = 0; > >>> > >>> if (atomic_long_read(&cfs_rq->removed_load_avg)) { > >>> - long r = atomic_long_xchg(&cfs_rq->removed_load_avg, 0); > >>> + s64 r = atomic_long_xchg(&cfs_rq->removed_load_avg, 0); > >>> sa->load_avg = max_t(long, sa->load_avg - r, 0); > >>> sa->load_sum = max_t(s64, sa->load_sum - r * LOAD_AVG_MAX, 0); > >> > >> This makes sense, because sched_avg::load_sum is u64.
A single removed nice=-20 task should be sufficient to cause the overflow.
> >> > >>> removed = 1; > >>> } > >>> > >>> if (atomic_long_read(&cfs_rq->removed_util_avg)) { > >>> - long r = atomic_long_xchg(&cfs_rq->removed_util_avg, 0); > >>> + s64 r = atomic_long_xchg(&cfs_rq->removed_util_avg, 0); > >>> sa->util_avg = max_t(long, sa->util_avg - r, 0); > >>> sa->util_sum = max_t(s32, sa->util_sum - r * LOAD_AVG_MAX, 0); > >>> } > >> > >> However sched_avg::util_sum is u32, so this is still wrecked. > > > > I seems to have wrecked that in: > > > > 006cdf025a33 ("sched/fair: Optimize per entity utilization tracking") > > > > maybe just make util_load u64 too?
It isn't as bad, but the optimization does increase the normal range (not guaranteed) for util_sum from 47742 to scale_down(SCHED_LOAD_SCALE)*47742 (= 1024*47742, unless you mess with the scaling).
> Is there any guarantee that the final result of expression 'util_sum - r * LOAD_AVG_MAX' always can be represented by s32? > > If yes, than we could just do this: > max_t(s32, (u64)sa->util_sum - r * LOAD_AVG_MAX, 0)
In most cases 'r' shouldn't exceed 1024 and util_sum not significantly exceed 1024*47742, but in extreme cases like spawning lots of new tasks it may potentially overflow 32 bit. Newly created tasks contribute 1024*47742 each to the rq util_sum, which means that more than ~87 new tasks on a single rq will get us in trouble I think.
Without Peter's optimization referenced above, that number should increase to ~87k tasks as each task only contributed 47742 before, but 'r' could still cause 32-bit overflow if we remove more than ~87 newly created tasks in one go. But I'm not sure if that is a situation we should worry about?
I think we have to either make util_sum u64 too or look at the optimizations again.
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