linux-kernel - Re: [PATCH 4/5] sched/fair: Remove double_lock_balance() from active_load_balance_cpu

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Date:	Thu, 24 Jul 2014 17:04:11 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Kirill Tkhai <ktkhai@...allels.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Ingo Molnar <mingo@...nel.org>, Paul Turner <pjt@...gle.com>,
	tkhai@...dex.ru
Subject: Re: [PATCH 4/5] sched/fair: Remove double_lock_balance() from
 active_load_balance_cpu_stop()

On Tue, 2014-07-22 at 15:30 +0400, Kirill Tkhai wrote:
> Bad situation:
> 
> double_lock_balance() drops busiest_rq lock. The busiest_rq is *busiest*,
> and a lot of tasks and context switches there. We are dropping the lock
> and waiting for it again.
> 
> Let's just detach the task and once finally unlock it!
> 
> Warning: this admits unlocked using of can_migrate_task(), throttled_lb_pair(),
> and task_hot(). I added comments about that.
> 

Wonder if we should also consider removing double_lock_balance usage
from rt.c and deadline.c? Then those two schedulers will also not
lock both the source and destination queues at the same time
for load balancing.

Tim

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