Re: [PATCH] sched: Fix UCLAMP_FLAG_IDLE setting

[Qais Yousef]

On 06/09/21 14:33, 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 task on the rq.
> Specifically, the flag is set in the uclamp_rq_inc() path, which is
> called at enqueue time, but cleared in the uclamp_rq_dec_id() which is

It is actually the other way around. It is cleared in uclamp_rq_inc() and
set in uclamp_rq_dec_id().

> called both when dequeueing and task _and_ during cgroup migrations.

Is it cgroup migrations or when cgroup uclamp values are updated you mean?

It is worth being direct and mention that uclamp_update_active() will perform
uclamp_rq_dec/inc_id() when the cgroup uclamp values are updated, which would
end up with the flag set but not cleared in this path.

> 
> Fix this by setting the flag in the uclamp_rq_inc_id() path to ensure
> things remain symmetrical.
> 
> Reported-by: Rick Yiu <rickyiu@xxxxxxxxxx>
> Signed-off-by: Quentin Perret <qperret@xxxxxxxxxx>
> ---

With the commit message fixed.

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

Thanks!

--
Qais Yousef

>  kernel/sched/core.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 5226cc26a095..3b213402798e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -980,6 +980,7 @@ static inline void uclamp_idle_reset(struct rq *rq, enum uclamp_id clamp_id,
>  	if (!(rq->uclamp_flags & UCLAMP_FLAG_IDLE))
>  		return;
>  
> +	rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
>  	WRITE_ONCE(rq->uclamp[clamp_id].value, clamp_value);
>  }
>  
> @@ -1252,10 +1253,6 @@ static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p)
>  
>  	for_each_clamp_id(clamp_id)
>  		uclamp_rq_inc_id(rq, p, clamp_id);
> -
> -	/* Reset clamp idle holding when there is one RUNNABLE task */
> -	if (rq->uclamp_flags & UCLAMP_FLAG_IDLE)
> -		rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
>  }
>  
>  static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p)
> -- 
> 2.32.0.272.g935e593368-goog
>