linux-kernel - CPU hotplug and IRQ affinity with 2.6.24-rt1

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Date:	Mon, 04 Feb 2008 15:35:13 -0800
From:	Max Krasnyanskiy <maxk@...lcomm.com>
To:	LKML <linux-kernel@...r.kernel.org>
CC:	Ingo Molnar <mingo@...e.hu>, Daniel Walker <dwalker@...sta.com>
Subject: CPU hotplug and IRQ affinity with 2.6.24-rt1

This is just an FYI. As part of the "Isolated CPU extensions" thread Daniel suggest for me
to check out latest RT kernels. So I did or at least tried to and immediately spotted a couple
of issues.

The machine I'm running it on is:
	HP xw9300, Dual Opteron, NUMA

It looks like with -rt kernel IRQ affinity masks are ignored on that system. ie I write 1 to 
lets say /proc/irq/23/smp_affinity but the interrupts keep coming to CPU1. 
Vanilla 2.6.24 does not have that issue.

Also the first thing I tried was to bring CPU1 off-line. Thats the fastest way to get irqs, 
soft-irqs, timers, etc of a CPU. But the box hung completely. It also managed to mess up my 
ext3 filesystem to the point where it required manual fsck (have not see that for a couple of
years now). 
I tried the same thing (ie echo 0 > /sys/devices/cpu/cpu1/online) from the console. It hang 
again with the message that looked something like:
	CPU1 is now off-line
	Thread IRQ-23 is on CPU1 ...

IRQ 23 is NVidia SATA. So I guess it has something to do with the borked affinity handling.
Vanilla 2.6.24 handles this just fine.

Anyway, like I said it's just an FYI, not an urgent issue.

Max

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