linux-kernel - Re: [PATCH v10 6/7] sched: Provide runnable_load_avg back to cfs

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Date:	Mon, 27 Jul 2015 11:29:59 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	mingo@...nel.org, peterz@...radead.org,
	linux-kernel@...r.kernel.org, pjt@...gle.com, bsegall@...gle.com,
	morten.rasmussen@....com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, umgwanakikbuti@...il.com,
	len.brown@...el.com, rafael.j.wysocki@...el.com,
	arjan@...ux.intel.com, fengguang.wu@...el.com
Subject: Re: [PATCH v10 6/7] sched: Provide runnable_load_avg back to cfs_rq

On Mon, Jul 27, 2015 at 11:21:15AM +0800, Boqun Feng wrote:
> Hi Yuyang,
> 
> On Mon, Jul 27, 2015 at 02:43:25AM +0800, Yuyang Du wrote:
> > Hi Boqun,
> > 
> > On Tue, Jul 21, 2015 at 06:29:56PM +0800, Boqun Feng wrote:
> > > The point is that you have already tracked the sum of runnable_load_avg
> > > and blocked_load_avg in cfs_rq->avg.load_avg. If you're going to track
> > > part of the sum, you'd better track the one that's updated less
> > > frequently, right?
> > > 
> > > Anyway, this idea just comes into my mind. I wonder which is udpated
> > > less frequently myself too. ;-) So I ask to see whether there is
> > > something we can improve.
> > 
> > Actually, this is not the point.
> > 
> > 1) blocked load is more "difficult" to track, hint, migrate.
> > 
> > 2) r(t1) - b(t2) is not anything, hint, t1 != t2
> 
> Please consider this patch below, which is not tested yet, just for
> discussion. This patch is based on 1-5 in your patchset and going to
> replace patch 6. Hope this could make my point clear.
> 
> Thanks anyway for being patient with me ;-)
> 
> Regards,
> Boqun
> 
> ========================================================================
> 
> Subject: [PATCH] sched: lazy blocked load tracking
> 
> With this patch, cfs_rq_runnable_load_avg can be implemented as follow:
> 
> static inline unsigned long cfs_rq_runnable_load_avg(struct cfs_rq *cfs_rq)
> {
> 	u64 now = cfs_rq_clock_task(cfs_rq);
> 	decay_cfs_rq_blocked_load(now, cfs_rq);
> 
> 	return max_t(long, cfs_rq->avg.load_avg - cfs_rq->blocked_load_avg, 0);
> }
> 
> ---
>  kernel/sched/fair.c  | 41 +++++++++++++++++++++++++++++++++++++++++
>  kernel/sched/sched.h |  4 ++++
>  2 files changed, 45 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e977074..76beb81 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2625,6 +2625,20 @@ static __always_inline int __update_load_avg(u64 now, int cpu,
>  	return decayed;
>  }
>  
> +static inline u64 decay_cfs_rq_blocked_load(u64 now, struct cfs_rq *cfs_rq)
> +{
> +	u64 decays;
> +
> +	now = now >> 20;
> +	decays = now - cfs_rq->last_blocked_load_decays;
> +	
> +	cfs_rq->blocked_load_sum = decay_load(cfs_rq->blocked_load_sum, decays);
> +	cfs_rq->blocked_load_avg = div_u64(cfs->blocked_load_sum, LOAD_AVG_MAX);
> +	cfs_rq->last_blocked_load_update_time = now;

Sorry for the typo, should be last_blocked_load_decays here ;-)

Regards,
Boqun

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