linux-kernel - Re: [PATCH 1/2] IRQ: fix performance regression on large IA64 systems

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Date:	Sun, 5 Jul 2009 21:26:11 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jiri Slaby <jirislaby@...il.com>
cc:	mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	yinghai@...nel.org, linux-kernel@...r.kernel.org,
	Bernhard Walle <bernhard.walle@....de>
Subject: Re: [PATCH 1/2] IRQ: fix performance regression on large IA64
 systems

On Fri, 3 Jul 2009, Jiri Slaby wrote:
> Commit b60c1f6ffd88850079ae419aa933ab0eddbd5535
> (drop note_interrupt() for per-CPU for proper scaling) removed call to
> note_interrupt() in __do_IRQ(). Commit
> d85a60d85ea5b7c597508c1510c88e657773d378
> (Add IRQF_IRQPOLL flag (common code)) added it again, because it's needed
> for irqpoll.
> 
> This patch now introduces a new parameter 'only_fixup' for
> note_interrupt(). This parameter determines two cases:
> 
>   TRUE  => The function should be only executed when irqfixup is set.
>            Either 'irqpoll' or 'irqfixup' directly set that.
> 
>   FALSE => Just the behaviour as note_interrupt() always had.
>
> Now the patch converts all calls of note_interrupt() to only_fixup=FALSE,
> except the call that has been removed by b60c1f6ffd.
> So that call is always done, but the body is only executed when either
> 'irqpoll' or 'irqfixup' are specified.
> 
> This is needed because __do_IRQ() calls note_interrupt() to record IRQ
> statistics. It ends up creating serious cache line contention,
> enough that a 1024p system live locks under the crushing weight of the
> timer tick.
> 
> The note_interrupt() call modifies fields in the irq_desc_t structure.
> For PER_CPU timer interrupts (on ia64 machines) this causes cacheline
> contention.
> 
> Systems with 1024 processors take an extremely long time to boot up, as
> most of the time is spent attempting to service timer interrupts.  With
> noirqdebug added to the boot line, the system boots in close to the normal
> amount of time.

Hmm. I'm not really happy about that patch. It's all about percpu
interrupts which happen to have the same irq number. I think the
correct thing to do is to use the handle_percpu_irq() handler and
modify handle_percpu_irq to call note_interrupt() only when the return
value of the action handler is IRQ_NONE. Otherwise we can leave
everything untouched.

Thanks,

	tglx


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