linux-kernel - Re: [PATCH] sched/fair: Rate limit calls to update_blocked

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Date:   Wed, 24 Mar 2021 09:05:45 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Paul McKenney <paulmck@...nel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Dietmar Eggeman <dietmar.eggemann@....com>,
        Qais Yousef <qais.yousef@....com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        Neeraj upadhyay <neeraj.iitr10@...il.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>
Subject: Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages()
 for NOHZ



On 3/24/21 6:44 AM, Vincent Guittot wrote:
> Hi Tim,

> 
> IIUC your problem, we call update_blocked_averages() but because of:
> 
> 		if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost) {
> 			update_next_balance(sd, &next_balance);
> 			break;
> 		}
> 
> the for_each_domain loop stops even before running load_balance on the 1st
> sched domain level which means that update_blocked_averages() was called
> unnecessarily. 
> 

That's right

> And this is even more true with a small sysctl_sched_migration_cost which allows newly
> idle LB for very small this_rq->avg_idle. We could wonder why you set such a low value 
> for sysctl_sched_migration_cost which is lower than the max_newidle_lb_cost of the
> smallest domain but that's probably because of task_hot().
> 
> if avg_idle is lower than the sd->max_newidle_lb_cost of the 1st sched_domain, we should
> skip spin_unlock/lock and for_each_domain() loop entirely
> 
> Maybe something like below:
> 

The patch makes sense.  I'll ask our benchmark team to queue this patch for testing.

Tim