Re: [PATCH 1/2] IRQ: fix performance regression on large IA64systems

[Ingo Molnar]

* Jiri Slaby <jirislaby@xxxxxxxxx> 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.

What would be the effect/cost of enabling this by default? Seems 
desirable and eventually we'll hit those problems with regular 
systems too ...

	Ingo
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