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

[Eric Dumazet <eric.dumazet@gmail.com>]

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.

Thanks.