Messages in this thread | | | Date | Thu, 29 Apr 2021 15:02:58 +0000 | From | Quentin Perret <> | Subject | Re: [PATCH] sched: Fix out-of-bound access in uclamp |
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On Thursday 29 Apr 2021 at 14:34:14 (+0200), Dietmar Eggemann wrote: > On 28/04/2021 19:27, Quentin Perret wrote: > > Util-clamp places tasks in different buckets based on their clamp values > > for performance reasons. However, the size of buckets is currently > > computed using a rounding division, which can lead to an off-by-one > > error in some configurations. > > > > For instance, with 20 buckets, the bucket size will be 1024/20=51.2, > > rounded to the closest value: 51. Now, a task with a clamp of 1024 (as > > is the default for the min clamp of RT tasks) will be mapped to bucket > > id 1024/51=20 as we're now using a standard integer division. Sadly, > > correct indexes are in range [0,19], hence leading to an out of bound > > memory access. > > > > Fix this by using a rounding-up division when computing the bucket size. > > But in case you use e.g. 16 buckets, wouldn't you still end up with this > task mapped into bucket_id=16? > > 1024/16=64 > > 1024/64=16
Hrmpf, you're right ...
So I guess the following will do:
#define UCLAMP_BUCKET_DELTA (SCHED_CAPACITY_SCALE / UCLAMP_BUCKETS + 1)
Thanks, Quentin
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