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

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Message-ID: <20080927194424.GG18619@elte.hu>
Date:	Sat, 27 Sep 2008 21:44:24 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Yinghai Lu <yhlu.kernel@...il.com>
Subject: Re: Should irq_chip->mask disable percpu interrupts to all cpus,
	or just to this cpu?


* Eric W. Biederman <ebiederm@...ssion.com> wrote:

> Jeremy Fitzhardinge <jeremy@...p.org> writes:
> 
> > I found handle_percpu_irq() which addresses my concerns.  It doesn't
> > attempt to mask the interrupt, takes no locks, and doesn't set or test
> > IRQ_INPROGRESS in desc->status, so it will scale perfectly across
> > multiple cpus.  It makes no changes to the desc structure, so there
> > isn't even any cacheline bouncing.
> 
> kstat_irqs.  Is arguably part of the irq structure.
> And kstat_irqs is a major pain in my book.
> 
> And for a rare event you have a cacheline read.
> I don't think we are quite there yet but we really want to allocate
> irq_desc on the right NUMA node in a multi socket system, to reduce
> the cache miss times.

note that we already do _almost_ that in tip/irq/sparseirq. dyn_array[] 
will extend itself in a NUMA-aware fashion. (normal device irq_desc 
entries will be allocated via kmalloc)

what would be needed is to deallocate/reallocate irq_desc when the IRQ 
affinity is changed? (i.e. when a device is migrated to a specific NUMA 
node)

> Is it a big deal?  Probably not.  But I think it would be a bad idea 
> to increasingly use infrastructure that will make it hard to optimize 
> the code.
> 
> Especially since the common case in high performance drivers is going 
> to be, individually routable irq sources.  Having one queue per cpu 
> and one irq per queue.  Which sounds like the same case you have.

agreed - the kstat_irqs cacheline bounce would show up in Xen benchmarks 
i'm sure.

	Ingo
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