linux-kernel - Re: [PATCH v1] sched: fix nohz idle load balancer issues

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Message-ID: <1317284157.22581.1.camel@twins>
Date:	Thu, 29 Sep 2011 10:15:57 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Venki Pallipadi <venki@...gle.com>,
	Paul Turner <pjt@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] sched: fix nohz idle load balancer issues

On Wed, 2011-09-28 at 17:47 -0700, Suresh Siddha wrote:
> So ended up with using kick_process() and the scheduler_ipi() context to
> trigger the SCHED_SOFTIRQ instead of using smp call function vector
> sequence (which has a deadlock scenario in the context of heavy
> interrupts which I can explain in detail when I send the complete
> changelog). And also I am explicitly requesting for idle balance to
> address the stale idle_at_tick condition.

I'd be interested in hearing more about that deadlock, because when
allocating your own csd and not waiting for the result
__smp_call_function_single() should be deadlock free.

> @@ -2733,6 +2733,11 @@ void scheduler_ipi(void)
>         struct rq *rq = this_rq();
>         struct task_struct *list = xchg(&rq->wake_list, NULL);
>  
> +       if (unlikely((rq->idle == current) && rq->nohz_balance_kick)) {
> +               rq->idle_balance = 1;
> +               raise_softirq_irqoff(SCHED_SOFTIRQ);
> +       } 

This can end up being outside of irq_enter()/irq_exit(), which is
probably not what you want. See the somewhat large comment right below
here.
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