linux-kernel - Re: [PATCH v2 1/2] Revert "[IA64] prevent ia64 from invoking irq handlers on offline CPUs"

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Date:	Mon, 9 Feb 2009 17:52:33 -0600
From:	Russ Anderson <rja@....com>
To:	Alex Chiang <achiang@...com>, tony.luck@...el.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, stable@...nel.org,
	linux-ia64@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Cc:	rja@....com
Subject: Re: [PATCH v2 1/2] Revert "[IA64] prevent ia64 from invoking irq handlers on offline CPUs"

On Mon, Feb 09, 2009 at 04:33:24PM -0700, Alex Chiang wrote:
> 
> I'm a little closer to understanding why the original revert
> survives my test though.
> 
> It seems that during ia64_process_pending_intr(), we will skip
> TLB flushes, and IPI reschedules.
> 
> Vectors lower than IA64_TIMER_VECTOR are masked (because we raise
> the TPR), meaning we won't see CMC/CPE interrupts or perfmon
> interrupts.
> 
> This leaves only IPIs and MCA above IA64_TIMER_VECTOR. The kernel
> doesn't actually send many IPIs to itself, so in practice, we
> almost never see those.  If we receive an MCA interrupt, well, we
> have more problems to worry about than taking a CPU offline (and
> whatever implications it may have on RCU). So I'm not concerned
> there.

Keep in mind there are recoverable MCAs on ia64.  It should
be a rare condition to have an MCA surface while taking a CPU
offline, but it could happen.

My main point is to make sure people do not assume that an MCA means
the system is going down.  

> The upshot is that in practice, we pretty much ever only need to
> handle the timer interrupt.

Thanks.
-- 
Russ Anderson, OS RAS/Partitioning Project Lead  
SGI - Silicon Graphics Inc          rja@....com
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