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

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Message-ID: <xm26wqc6z1c8.fsf@sword-of-the-dawn.mtv.corp.google.com>
Date:	Tue, 24 Jun 2014 10:03:51 -0700
From:	bsegall@...gle.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>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	"Mike Galbraith" <umgwanakikbuti@...il.com>,
	<khorenko@...allels.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> writes:

> 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>
> ---
>  kernel/sched/core.c |   15 +++++++++++----
>  kernel/sched/fair.c |   22 ++++++++++++++++++++++
>  2 files changed, 33 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7f3063c..707a3c5 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7842,11 +7842,18 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
>  		struct rq *rq = cfs_rq->rq;
>  
>  		raw_spin_lock_irq(&rq->lock);
> -		cfs_rq->runtime_enabled = runtime_enabled;
> -		cfs_rq->runtime_remaining = 0;
> +		/*
> +		 * Do not enable runtime on offline runqueues. We specially
> +		 * make it disabled in unthrottle_offline_cfs_rqs().
> +		 */
> +		if (cpu_online(i)) {
> +			cfs_rq->runtime_enabled = runtime_enabled;
> +			cfs_rq->runtime_remaining = 0;
> +
> +			if (cfs_rq->throttled)
> +				unthrottle_cfs_rq(cfs_rq);
> +		}

We can just do for_each_online_cpu, yes? Also we probably need
get_online_cpus/put_online_cpus, and/or want cpu_active_mask instead
right?
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