Re: [PATCH] sched: Fix out-of-bound access in uclamp

[Quentin Perret <qperret@google.com>]

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