linux-kernel - Re: [PATCH RESEND] x86/fixup_irq: Clean the offlining CPU from the irq affinity mask

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Message-ID: <1348679199.26695.455.camel@sbsiddha-desk.sc.intel.com>
Date:	Wed, 26 Sep 2012 10:06:39 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Chuansheng Liu <chuansheng.liu@...el.com>, tglx@...utronix.de,
	mingo@...hat.com, x86@...nel.org, linux-kernel@...r.kernel.org,
	yanmin_zhang@...ux.intel.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>
Subject: Re: [PATCH RESEND] x86/fixup_irq: Clean the offlining CPU from the
 irq affinity mask

On Wed, 2012-09-26 at 21:33 +0530, Srivatsa S. Bhat wrote:
> I have some fundamental questions here:
> 1. Why was the CPU never removed from the affinity masks in the original
> code? I find it hard to believe that it was just an oversight, because the
> whole point of fixup_irqs() is to affine the interrupts to other CPUs, IIUC.
> So, is that really a bug or is the existing code correct for some reason
> which I don't know of?

I am not aware of the history but my guess is that the affinity mask
which is coming from the user-space wants to be preserved. And
fixup_irqs() is fixing the underlying interrupt routing when the cpu
goes down with a hope that things will be corrected when the cpu comes
back online. But  as Liu noted, we are not correcting the underlying
routing when the cpu comes back online. I think we should fix that
rather than modifying the user-specified affinity.

> 2. In case this is indeed a bug, why are the warnings ratelimited when the
> interrupts can't be affined to other CPUs? Are they not serious enough to
> report? Put more strongly, why do we even silently return with a warning
> instead of reporting that the CPU offline operation failed?? Is that because
> we have come way too far in the hotplug sequence and we can't easily roll
> back? Or are we still actually OK in that situation?

Are you referring to the "cannot set affinity for irq" messages? That
happens only if the irq chip doesn't have the irq_set_affinity() setup.
But that is not common.

> 
> Suresh, I'd be grateful if you could kindly throw some light on these
> issues... I'm actually debugging an issue where an offline CPU gets apic timer
> interrupts (and in one case, I even saw a device interrupt), which I have
> reported in another thread at: https://lkml.org/lkml/2012/9/26/119
> But this issue in fixup_irqs() that Liu brought to light looks even more
> surprising to me..

These issues look different to me, will look into that.

thanks,
suresh

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