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SubjectRe: [patch] Real-Time Preemption, -RT-2.6.10-rc1-mm2-V0.7.1

* Ingo Molnar <mingo@elte.hu> wrote:

> icmp/ping replies are handled by ksoftirqd. Once a networking request
> has been handed to ksoftirqd it cannot be redirected to another CPU,
> because softirq processing is fundamentally per-CPU. So if the network
> interrupt hits the CPU where the RT-task is running then the RT task
> will starve that ksoftirq instance (and hence the reply) even if
> another CPU in the system is idle.
>
> i agree that this is an SMP/RT artifact that should be fixed. hardirq
> workload can be redirected to other CPUs because it's single-threaded,
> but it's not that easy for softirq workload.
>
> i suspect the same phenomenon causes some of the other scheduling
> artifacts ('frozen' X) you've noticed.

does the ping phenomenon go away if you chrt both the networking IRQ
thread and both ksoftirqd's to above the RT task's priority? (this
doesnt solve the problem though - that task has RT priority for a reason
and on an SMP box the kernel should be able to schedule work without
getting into this starvation scenario.)

Ingo
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