netdev - Re: question about softirqs

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Message-Id: <20090512.214427.193728136.davem@davemloft.net>
Date:	Tue, 12 May 2009 21:44:27 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	mingo@...e.hu
Cc:	a.p.zijlstra@...llo.nl, cfriesen@...tel.com, tglx@...utronix.de,
	rostedt@...dmis.org, linuxppc-dev@...abs.org, paulus@...ba.org,
	netdev@...r.kernel.org
Subject: Re: question about softirqs

From: Ingo Molnar <mingo@...e.hu>
Date: Tue, 12 May 2009 11:23:48 +0200

>> Wouldn't the even better solution be to get rid of softirqs 
>> all-together?
>> 
>> I see the recent work by Thomas to get threaded interrupts 
>> upstream as a good first step towards that goal, once the RX 
>> processing is moved to a thread (or multiple threads) one can 
>> priorize them in the regular sys_sched_setscheduler() way and its 
>> obvious that a FIFO task above the priority of the network tasks 
>> will have network starvation issues.
> 
> Yeah, that would be "nice". A single IRQ thread plus the process 
> context(s) doing networking might perform well.

Nice for -rt goals, but not for latency.

So we're going to regress in this area again?  I can't see how
that's so desirable, to be honest with you.

The fact that this discussion started about a task with a certain
priority not being able to make forward progress, even though it
was correct coded, just because softirqs are being processed in
a thread context, should be a big red flag that this is a buggered up
design.

I fully expected us to be, at this point, talking about putting the
pending softirq check back into the trap return path :-/
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