__do_softirq and real-time execution context

__do_softirq and real-time execution context

[Kostya B]

Hello,

Please consider the following:

__do_softirq could be run from several checkpoints in kernel and even can employ userspace process context. 
For example, under heavy network traffic such process could be blocked for a relatively large period until ksoftirqd wakes up and takes control for Rx/Tx. 

In the case of regular userspace process the ksoftirqd solves the kernel/user trade-off problem pretty well.
However, in the case of _real-time_ process, such behavior may yield severe outcomes. Real-time (e.g. NTPL FIFO thread) might require short responses and low latency.

The proposal is that  __do_softirq should not run its pending tasks in the context of RT process.  Just check has_rt_policy() and immediately wake up ksoftirqd.

Regards,
- Kostya


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Re: __do_softirq and real-time execution context

[Peter Zijlstra]

On Mon, 2008-08-04 at 07:02 +0000, Kostya B wrote:
> Hello,
> 
> Please consider the following:
> 
> __do_softirq could be run from several checkpoints in kernel and even can employ userspace process context. 
> For example, under heavy network traffic such process could be blocked for a relatively large period until ksoftirqd wakes up and takes control for Rx/Tx. 
> 
> In the case of regular userspace process the ksoftirqd solves the kernel/user trade-off problem pretty well.
> However, in the case of _real-time_ process, such behavior may yield severe outcomes. Real-time (e.g. NTPL FIFO thread) might require short responses and low latency.
> 
> The proposal is that  __do_softirq should not run its pending tasks in the context of RT process.  Just check has_rt_policy() and immediately wake up ksoftirqd.

Please look at the -rt patches.