Re: [patch] sched: unlocked context-switches

[Ingo Molnar]

* Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:

> Well that does look like a pretty good cleanup. It certainly is the 
> final step in freeing complex architecture switching code from 
> entanglement with scheduler internal locking, and unifies the locking 
> scheme.
> 
> I did propose doing unconditionally unlocked switches a while back 
> when my patch first popped up - you were against it then, but I guess 
> you've had second thoughts?

the reordering of switch_to() and the switch_mm()-related logic was that 
made it really worthwile and clean. I.e. we pick a task atomically, we 
switch stacks, and then we switch the MM. Note that this setup still 
leaves the possibility open to move the stack-switching back under the 
irq-disabled section in a natural way.

> It does add an extra couple of stores to on_cpu, and a wmb() for 
> architectures that didn't previously need the unlocked switches. And 
> ia64 needs the extra interrupt disable / enable. Probably worth it?

it also removes extra stores to rq->prev_mm and other stores. I havent 
measured any degradation on x86.

If the irq disable/enable becomes widespread i'll do another patch to 
push the irq-enabling into switch_to() so the arch can do the 
stack-switch first and then enable interrupts and do the rest - but i 
didnt want to complicate things unnecessarily for now.

> Minor style request: I like that you're accessing ->on_cpu through 
> functions so the !SMP case doesn't clutter the code with ifdefs... but 
> can you do set_task_on_cpu(p) and clear_task_on_cpu(p) ?

yeah, i thought about these two variants and went for set_task_on_cpu() 
so that it's less encapsulated (it's really just a conditional 
assignment) and that it parallels set_task_cpu() use. But no strong 
feelings either way. Anyway, lets try what we have now, i'll do the rest 
in deltas.

	Ingo
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