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

[Ingo Molnar]

* Mark_H_Johnson@xxxxxxxxxxxx <Mark_H_Johnson@xxxxxxxxxxxx> wrote:

> Let me follow up briefly on the regression I noticed yesterday on ping
> responses from an SMP system with one real time task running. [...]

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.

	Ingo
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