linux-kernel - Re: [PATCH 1/2] timer: Avoid waking up an idle-core by migrate running timer

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Date:	Thu, 23 Apr 2015 14:45:56 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	viresh kumar <viresh.kumar@...aro.org>,
	Ingo Molnar <mingo@...hat.com>, linaro-kernel@...ts.linaro.org,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Miao <realmz6@...il.com>, shashim@...eaurora.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, cl@...nel.org
Subject: Re: [PATCH 1/2] timer: Avoid waking up an idle-core by migrate
 running timer

On Wed, 22 Apr 2015, Eric Dumazet wrote:
> On Wed, 2015-04-22 at 23:56 +0200, Thomas Gleixner wrote:
> 
> > -int get_nohz_timer_target(int pinned)
> > +int get_nohz_timer_target(void)
> >  {
> > -	int cpu = smp_processor_id();
> > -	int i;
> > +	int i, cpu = smp_processor_id();
> >  	struct sched_domain *sd;
> >  
> > -	if (pinned || !get_sysctl_timer_migration() || !idle_cpu(cpu))
> > +	if (!idle_cpu(cpu))
> >  		return cpu;
> 
> Maybe also test in_serving_softirq() ?
> 
> if (in_serving_softirq() || !idle_cpu(cpu))
> 	return cpu;
> 
> There is a fundamental problem with networking load : Many cpus appear
> to be idle from scheduler perspective because no user/kernel task is running.
> 
> CPUs servicing NIC queues can be very busy handling thousands of packets
> per second, yet have no user/kernel task running.
> 
> idle_cpu() return code is : this cpu is idle.    hmmmm, really ?
> 
> cpus are busy, *and* have to access alien data/locks to activate timers
> that hardly fire anyway.
> 
> When idle_cpu() finally gives the right indication, it is too late :
> ksoftirqd might be running on the wrong cpu. Innocent cpus, overwhelmed
> by a sudden timer load and locked into a service loop.
> 
> This cannot resist to a DOS, and even with non malicious traffic, the
> overhead is high.

You definitely have a point from the high throughput networking
perspective.

Though in a power optimizing scenario with minimal network traffic
this might be the wrong decision. We have to gather data from the
power maniacs whether this matters or not. The FULL_NO_HZ camp might
be pretty unhappy about the above.

Thanks,

	tglx


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