linux-kernel - Re: [PATCH v2 1/3] sched/fair: Disable runtime

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Message-ID: <20140626110811.GA20048@linux.vnet.ibm.com>
Date:	Thu, 26 Jun 2014 16:38:11 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Kirill Tkhai <ktkhai@...allels.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, tkhai@...dex.ru,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	khorenko@...allels.com, Ben Segall <bsegall@...gle.com>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v2 1/3] sched/fair: Disable runtime_enabled on dying rq

* Kirill Tkhai <ktkhai@...allels.com> [2014-06-24 11:53:52]:

> We kill rq->rd on the CPU_DOWN_PREPARE stage:
> 
> 	cpuset_cpu_inactive -> cpuset_update_active_cpus -> partition_sched_domains ->
> 	-> cpu_attach_domain -> rq_attach_root -> set_rq_offline
> 
> This unthrottles all throttled cfs_rqs.
> 
> But the cpu is still able to call schedule() till
> 
> 	take_cpu_down->__cpu_disable()
> 
> is called from stop_machine.
> 
> This case the tasks from just unthrottled cfs_rqs are pickable
> in a standard scheduler way, and they are picked by dying cpu.
> The cfs_rqs becomes throttled again, and migrate_tasks()
> in migration_call skips their tasks (one more unthrottle
> in migrate_tasks()->CPU_DYING does not happen, because rq->rd
> is already NULL).
> 
> Patch sets runtime_enabled to zero. This guarantees, the runtime
> is not accounted, and the cfs_rqs won't exceed given
> cfs_rq->runtime_remaining = 1, and tasks will be pickable
> in migrate_tasks(). runtime_enabled is recalculated again
> when rq becomes online again.
> 
> Ben Segall also noticed, we always enable runtime in
> tg_set_cfs_bandwidth(). Actually, we should do that for online
> cpus only. To fix that, we check if a cpu is online when
> its rq is locked. This guarantees we do not have races with
> set_rq_offline(), which also requires rq->lock.
> 
> v2: Fix race with tg_set_cfs_bandwidth().
>     Move cfs_rq->runtime_enabled=0 above unthrottle_cfs_rq().
> 
> Signed-off-by: Kirill Tkhai <ktkhai@...allels.com>
> CC: Konstantin Khorenko <khorenko@...allels.com>
> CC: Ben Segall <bsegall@...gle.com>
> CC: Paul Turner <pjt@...gle.com>
> CC: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> CC: Mike Galbraith <umgwanakikbuti@...il.com>
> CC: Peter Zijlstra <peterz@...radead.org>
> CC: Ingo Molnar <mingo@...nel.org>

looks good to me.

Reviewed-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>

-- 
Thanks and Regards
Srikar Dronamraju

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