Re: [RFC] per-cpu preempt_count

[Ingo Molnar <mingo@kernel.org>]

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Mon, Aug 12, 2013 at 4:51 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > The below boots to wanting to mount a root filesystem with
> > CONFIG_PREEMPT=y using kvm -smp 4.
> 
> But doesn't work in general? Or you just never tested?

(I think Peter never tested it on real hw - this is an RFC patch to show 
the concept .)

> > Adding TIF_NEED_RESCHED into the preempt count would allow a single 
> > test in preempt_check_resched() instead of still needing the TI. 
> > Removing PREEMPT_ACTIVE from preempt count should allow us to get rid 
> > of ti::preempt_count altogether.
> >
> > The only problem with TIF_NEED_RESCHED is that its cross-cpu which 
> > would make the entire thing atomic which would suck donkey balls so 
> > maybe we need two separate per-cpu variables?
> 
> Agreed. Making it atomic would suck, and cancel all advantages of the 
> better code generation to access it. Good point.

We could still have the advantages of NEED_RESCHED in preempt_count() by 
realizing that we only rarely actually set/clear need_resched and mostly 
read it from the highest freq user, the preempt_enable() check.

So we could have it atomic, but do atomic_read() in the preempt_enable() 
hotpath which wouldn't suck donkey balls, right?

That would allow a really sweet preempt_enable() fastpath, on x86 at 
least.

Thanks,

	Ingo