Re: [PATCH v2] sched/uclamp: Fix race condition leading to imbalanced uclamp_rq

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Oct 26, 2026 at 11:24:31AM +0000, Qais Yousef wrote:
> On 10/26/26 10:12, Peter Zijlstra wrote:
> 
> > > +static inline bool __uclamp_is_imbalanced(struct rq *rq, enum uclamp_id clamp_id)
> > > +{
> > > +	struct uclamp_bucket *bucket = rq->uclamp[clamp_id].bucket;
> > > +	int bucket_id = UCLAMP_BUCKETS - 1;
> > > +
> > > +	/*
> > > +	 * On idle all bucket.tasks must be 0, or we have hit imbalance
> > > +	 * problem.
> > > +	 */
> > > +	for ( ; bucket_id >= 0; bucket_id--)
> > > +		if (bucket[bucket_id].tasks)
> > > +			return true;
> > > +
> > > +	return false;
> > > +}
> > > +
> > > +static inline bool uclamp_is_imbalanced(struct rq *rq)
> > > +{
> > > +	enum uclamp_id clamp_id;
> > > +
> > > +	for_each_clamp_id(clamp_id)
> > > +		if(__uclamp_is_imbalanced(rq, clamp_id))
> > > +			return true;
> > > +
> > > +	return false;
> > > +}
> > 
> > Anyway; I was thinking, since we have a hole in struct uclamp_rq; and
> > also uclamp_flags. Can we fold uclamp_flags into the hold, and also use
> > UCLAMP_FLAG_IDLE to detect if we need this bucket scan at all?
> 
> Actually maybe we can just check if the flag is not set then this means we
> have a corruption without scanning the buckets. I didn't like the scan, but
> seemed safer for a backport. Let me try that and update.
> 
> I can fold the uclamp_flags into buckets, I don't think this will grow and I
> do have a path to help us get rid off uclamp_rq (hopefully) in the future.
> 
> Okay to do this as a separate follow up patch? Since this will go to LTS, I
> think safer keep it minimalist and do the folding bit in a separate patch.

Sure; I just noticed these separate things while trying to re acquaint
myself with uclamp ;-)