Re: [PATCH v4 1/2] sched: Fix UCLAMP_FLAG_IDLE setting

[Qais Yousef <qais.yousef@arm.com>]

On 07/19/21 17:16, Quentin Perret wrote:
> The UCLAMP_FLAG_IDLE flag is set on a runqueue when dequeueing the last
> active task to maintain the last uclamp.max and prevent blocked util
> from suddenly becoming visible.
> 
> However, there is an asymmetry in how the flag is set and cleared which
> can lead to having the flag set whilst there are active tasks on the rq.
> Specifically, the flag is cleared in the uclamp_rq_inc() path, which is
> called at enqueue time, but set in uclamp_rq_dec_id() which is called
> both when dequeueing a task _and_ in the update_uclamp_active() path. As
> a result, when both uclamp_rq_{dec,ind}_id() are called from
> update_uclamp_active(), the flag ends up being set but not cleared,
> hence leaving the runqueue in a broken state.
> 
> Fix this by clearing the flag in update_uclamp_active() as well.
> 
> Fixes: e496187da710 ("sched/uclamp: Enforce last task's UCLAMP_MAX")
> Reported-by: Rick Yiu <rickyiu@google.com>
> Signed-off-by: Quentin Perret <qperret@google.com>
> ---

I've put a note that handling of this flag needs to be improved for the future.
But for now and FWIW, this LGTM:

Reviewed-by: Qais Yousef <qais.yousef@arm.com>

Thanks!

--
Qais Yousef