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SubjectRe: [PATCH 1/2] IRQ: fix performance regression on large IA64 systems

* Jiri Slaby <jirislaby@gmail.com> 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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