linux-kernel - Re: Should irq_chip->mask disable percpu interrupts to all cpus, or just to this cpu?

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Date:	Wed, 24 Sep 2008 02:54:16 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Should irq_chip->mask disable percpu interrupts to all cpus, or just to this cpu?

Ingo Molnar <mingo@...e.hu> writes:

> * Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>
>> Hi,
>> 
>> I'm reworking Xen's interrupt handling to isolate it a bit from the 
>> workings of the apic-based code, as Eric suggested a while back.
>> 
>> As I've mentioned before, Xen represents interrupts as event channels. 
>> There are two major classes of event channels: per-cpu and, erm, not 
>> percpu.  Per-cpu event channels are for things like timers and IPI 
>> function calls which are inherently per-cpu; it's meaningless to 
>> consider, for example, migrating them from cpu to cpu.  I guess 
>> they're analogous to the the local apic vectors.
>> 
>> (Non-percpu event channels can be bound to a particular cpu, and 
>> rebound at will; I'm not worried about them here.)
>> 
>> Previously I allocated an irq per percpu event channel per cpu.  This 
>> was pretty wasteful, since I need about 5-6 of them per cpu, so the 
>> number of interrupts increases quite quickly as cpus does.  There's no 
>> deep problem with that, but it gets fairly ugly in /proc/interrupts, 
>> and there's some tricky corners to manage in suspend/resume.

Every high performance device wants one irq per cpu.
So if it gets ugly in /proc/interrupts we should look at fixing
/proc/interrupts.

It looked like in Xen each of those interrupts were delivered
to different event channels.  Did I misread that code?

I really hate the notion of sharing a single irq_desc across
multiple cpus as a preferred mode of operation.  As NUMA comes
into play it guarantees we will have cross cpu memory fetches
on a fast path for irq handling.

Other than the beautiful way we print things in /proc/interrupts
IRQ_PER_CPU feels like a really bad idea.  Especially in that
it enshrines the nasty per cpu irq counters that scale horribly.

Eric
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