linux-kernel - RE: [PATCH] Revert "sched/fair: Fix O(nr_cgroups) in the load balancing path"

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Date:   Wed, 30 Oct 2019 07:04:40 -0700
From:   "Doug Smythies" <dsmythies@...us.net>
To:     "'Vincent Guittot'" <vincent.guittot@...aro.org>
Cc:     "'linux-kernel'" <linux-kernel@...r.kernel.org>,
        "'open list:THERMAL'" <linux-pm@...r.kernel.org>,
        "'Peter Zijlstra'" <peterz@...radead.org>,
        "'Ingo Molnar'" <mingo@...nel.org>,
        "'Linus Torvalds'" <torvalds@...ux-foundation.org>,
        "'Thomas Gleixner'" <tglx@...utronix.de>,
        "'Sargun Dhillon'" <sargun@...gun.me>,
        "'Tejun Heo'" <tj@...nel.org>, "'Xie XiuQi'" <xiexiuqi@...wei.com>,
        <xiezhipeng1@...wei.com>,
        "'Srinivas Pandruvada'" <srinivas.pandruvada@...ux.intel.com>,
        "'Rik van Riel'" <riel@...riel.com>
Subject: RE: [PATCH] Revert "sched/fair: Fix O(nr_cgroups) in the load balancing path"

On 2019.10.29 09:02 Vincent Guittot wrote:

> Could you try the patch below ? It ensures that at least the root cfs rq stays
> in the list so each time update_blocked_averages is called, we will call update_cfs_rq_load_avg()
> for the root cfs_rq at least and even if everything already reach zero.
> This will ensure that cfs_rq_util_change is called even if nothing has
> changed.
>
> ---
>  kernel/sched/fair.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 151c0b7..ac0a549 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7552,6 +7552,8 @@ static inline void update_blocked_load_status(struct rq *rq, bool has_blocked) {
> 
>  static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
>  {
> +	struct rq *rq = rq_of(cfs_rq);
> +
>  	if (cfs_rq->load.weight)
> 		return false;
> 
> @@ -7564,6 +7566,9 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
> 	if (cfs_rq->avg.runnable_load_sum)
> 		return false;
> 
> +	if (cfs_rq == &rq->cfs)
> +		return false;
> +
> 	return true;
> }
> 
> -- 
> 2.7.4

Yes, this patch works and solves the long time
between calls of the intel_pstate CPU frequency scaling
driver issue.
I see you sent a formal patch a few hours ago.
I'll try it and report back.

... Doug