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SubjectRe: [PATCH -mm 1/2] irq_work, Use llist in irq_work
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On Thu, 2011-09-01 at 09:46 +0800, Huang Ying wrote:

> You mean we should not use cpu_relax before the first cmpxchg?

Yeah, that's just wasting time for no reason..

> You suggest something as follow?
>
> void llist_add(struct llist_node *new, struct llist_head *head)
> {
> struct llist_node *entry, *old_entry;
>
> #ifndef CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG
> BUG_ON(in_nmi());
> #endif
>
> entry = head->first;
> for (;;) {
> old_entry = entry;
> new->next = entry;
> entry = cmpxchg(&head->first, old_entry, new);
> if (entry == old_entry)
> break;
> cpu_relax();
> }
> }

If you insist on having cpu_relax(), then yes that's lots better. Also
avoids the assignment in your conditional. Thing with cpu_relax() is
that its only beneficial in the highly contended case and degrade
light/un-contended loads.

Also, just noticed, why do you have different list_head/list_node
structures? They're the same, a single pointer.

> > and loose the get/put
> > cpu muck? The existing preempt_disable/enable() are already superfluous
> > and could be removed, you just made all this way more horrid than need
> > be.
>
> Will it cause race condition to remove preempt_disable/enable?
> Considering something as follow:
>
> - get irq_work_list of CPU A
> - queue irq_work into irq_work_list of CPU A
> - preempted and resumed execution on CPU B
> - arch_irq_work_raise on CPU B
>
> irq_work_run on CPU B will do nothing. While irq_work need to wait for
> next timer interrupt. Isn't it an issue?

Yes that's unfortunate, the current version would work just fine without
preempt but that's because of the this_cpu_* ops foo.

Not sure it would make sense to add a special this_cpu_llist_add() or
so.. esp seeing that this_cpu_* is basically x86-only.


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