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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/