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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Message-Id: <200708091120.42281.dvhltc@us.ibm.com>
Date:	Thu, 9 Aug 2007 11:20:42 -0700
From:	Darren Hart <dvhltc@...ibm.com>
To:	"Robert de Vries" <robert.h.de.vries@...il.com>
Cc:	"Steven Rostedt" <rostedt@...dmis.org>,
	"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 Thursday 09 August 2007 02:49:27 Robert de Vries wrote:
> 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.

I think the goal is to try and have a generally robust default setup - but the 
interrupt shielding you are suggesting seems like it will become more and 
more relevant as the number of CPUs increases.

-- 
Darren Hart
IBM Linux Technology Center
Realtime Linux Team
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