linux-kernel - Re: [PATCH RT] Only run softirqs from the irq thread if the irq affinity is set to 1 CPU

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Date:	Thu, 9 Aug 2007 11:49:27 +0200
From:	"Robert de Vries" <robert.h.de.vries@...il.com>
To:	"Steven Rostedt" <rostedt@...dmis.org>
Cc:	"Darren Hart" <dvhltc@...ibm.com>, "Ingo Molnar" <mingo@...e.hu>,
	RT <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"john stultz" <johnstul@...ibm.com>
Subject: Re: [PATCH RT] Only run softirqs from the irq thread if the irq affinity is set to 1 CPU

On 8/8/07, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Wed, 2007-08-08 at 14:16 -0700, Darren Hart wrote:
>
> > It seems to me that this patch will reduce the frequency of irqd/softirqd
> > starvation, but the core problem still exists: softirq tasks can't migrate to
> > other CPUs to perform their work if a higher priority task preempts them.
> > I'm wondering if we want to keep special casing things to minimize the
> > problem or not - seems to me the worst case is still the same - and isn't the
> > worst case the only case that matters (for -rt)?
> >
>
> softirq tasks should never migrate to other CPUs. A softirq exists in
> every CPU. So if you trigger a softirq on CPU1 it will only run on CPU1.
> If a high priority task preempts it, that same softirq can still run on
> other CPUS. Only the thread that was preempted wont switch. But that's
> the characteristic of softirqs, and that's how people who use them in
> development expect them to work.

Wouldn't a developer of a real-time system configure the system so
that interrupts do not interfere with the real-time tasks running on a
specific CPU?
In other words, is this problem not simply a misconfiguration of the system?
I personally redirect all interrupts away from the CPU's where my
real-time tasks run and only allow the interrupts that I want to
handle in my application on the CPU's where I handle them so as to
minimize latency.

    Robert
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