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From: Thomas Gleixner <tglx@linutronix.de>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	viresh kumar <viresh.kumar@linaro.org>,
	Ingo Molnar <mingo@redhat.com>,
	linaro-kernel@lists.linaro.org,
	LKML <linux-kernel@vger.kernel.org>,
	Steven Miao <realmz6@gmail.com>,
	shashim@codeaurora.org, Frederic Weisbecker <fweisbec@gmail.com>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	cl@kernel.org
Subject: Re: [PATCH 1/2] timer: Avoid waking up an idle-core by migrate running timer
Date: Thu, 23 Apr 2015 14:45:56 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.11.1504231439300.13914@nanos> (raw)
In-Reply-To: <1429772251.22254.35.camel@edumazet-glaptop2.roam.corp.google.com>

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



  reply	other threads:[~2015-04-23 12:45 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-31  6:55 [PATCH 0/2] timer: Migrate running timers Viresh Kumar
2015-03-31  6:55 ` [PATCH 1/2] timer: Avoid waking up an idle-core by migrate running timer Viresh Kumar
2015-03-31 14:53   ` Peter Zijlstra
2015-04-14 23:13   ` Thomas Gleixner
2015-04-17  8:12     ` viresh kumar
2015-04-17  8:32       ` Ingo Molnar
2015-04-21 21:32       ` Thomas Gleixner
2015-04-21 21:54         ` Eric Dumazet
2015-04-22 15:29           ` Peter Zijlstra
2015-04-22 16:02             ` Eric Dumazet
2015-04-22 18:56               ` Thomas Gleixner
2015-04-22 19:59                 ` Eric Dumazet
2015-04-22 21:56                   ` Thomas Gleixner
2015-04-23  6:57                     ` Eric Dumazet
2015-04-23 12:45                       ` Thomas Gleixner [this message]
2015-04-25 18:37                         ` Eric Dumazet
2015-05-05 13:00                           ` Thomas Gleixner
2015-05-06 16:33                             ` Eric Dumazet
2015-04-15 15:54   ` Thomas Gleixner
2015-03-31  6:55 ` [PATCH 2/2] timer: Replace base-> 'running_timer' with 'busy' Viresh Kumar
2015-03-31 15:01 ` [PATCH 0/2] timer: Migrate running timers Viresh Kumar

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