Re: [RFC] per-cpu preempt_count

[Paul E. McKenney]

On Mon, Aug 12, 2013 at 10:35:48AM -0700, Linus Torvalds wrote:
> On Mon, Aug 12, 2013 at 4:51 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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 that "thread_info->preempt_count" variable would need to be
> renamed to "saved_preempt_count" or similar to make sure we catch any
> users. But the patch certainly looks simple otherwise.
> 
> I'm pretty sure I had a discussion about this with Paul McKenney some
> time ago (because the RCU readlock is the most noticeable user of the
> preempt count - the others tend to be hidden inside the out-of-line
> spinlock functions etc), and I thought he had tried this and had some
> problems. Maybe we've fixed things since, or maybe he missed some
> case..

I was doing something a bit different -- trying to put preemptible RCU's
nesting counter into a per-CPU variable.  I considered putting this
counter into thread_info, but got flummoxed by the save/restore code.
If Peter's approach works out, I will look into a similar approach for
RCU's nesting counter.

For whatever it is worth, with the current Kconfigs, RCU only invokes
preempt_enable() and preempt_disable() when CONFIG_PREEMPT=n, in which
case these two functions are nops.  So RCU never exercises the
conditional function call in preempt_enable().

However, preemptible RCU has a situation similar to preempt_disable()
and preempt_enable(): simple increment and (not so simple) decrement in
the common case, and rare conditional function call from rcu_read_unlock()
that is invoked only if the read-side critical section was preempted or
ran for a long time.

							Thanx, Paul

> But if the patch really is this simple, then we should just do it. Of
> course, we should double-check that the percpu preempt count is in a
> cacheline that is already accessed (preferably already dirtied) by the
> context switching code.  And I think this should be an
> architecture-specific thing, because using a percpu variable might be
> good on some architectures but not others. So I get the feeling that
> it should be in the x86 __switch_to(), rather than in the generic
> code. I think it would fit very well with the per-cpu "old_rsp" and
> "current_task" updates that we already do.
> 
> > 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.
> 
> And yeah, it could be two variables in the same cacheline or something.
> 
>                  Linus
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