Re: [RFC][PATCH 5/5] powerpc: Remove SYNC from _switch

[Nicholas Piggin <npiggin@gmail.com>]

On Thu, 8 Jun 2017 08:54:00 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, Jun 08, 2017 at 10:32:44AM +1000, Nicholas Piggin wrote:
> > On Wed, 07 Jun 2017 18:15:06 +0200
> > Peter Zijlstra <peterz@infradead.org> wrote:
> >   
> > > Now that the scheduler's rq->lock is RCsc and thus provides full
> > > transitivity between scheduling actions. And since we cannot migrate
> > > current, a task needs a switch-out and a switch-in in order to
> > > migrate, in which case the RCsc provides all the ordering we need.  
> > 
> > Hi Peter,
> > 
> > I'm actually just working on removing this right now too, so
> > good timing.
> > 
> > I think we can't "just" remove it, because it is required to order
> > MMIO on powerpc as well.  
> 
> How is MMIO special? That is, there is only MMIO before we call into
> schedule() right? So the rq->lock should be sufficient to order that
> too.

MMIO uses different barriers. spinlock and smp_ type barriers do
not order it.

> > 
> > But what I have done is to comment that some other primitives are
> > already providing the hwsync for other, so we don't have to add
> > another one in _switch.  
> 
> Right, so this patch relies on the smp_mb__before_spinlock ->
> smp_mb__after_spinlock conversion that makes the rq->lock RCsc and
> should thus provide the required SYNC for migrations.

AFAIKS either one will do, so long as there is a hwsync there. The
point is just that I have added some commentary in the generic and
powerpc parts to make it clear we're relying on that behavior of
the primitive. smp_mb* is not guaranteed to order MMIO, it's just
that it does on powerpc.

> That said, I think you can already use the smp_mb__before_spinlock() as
> that is done with IRQs disabled, but its a more difficult argument. The
> rq->lock RCsc property should be more obvious.

This is what I got.

https://patchwork.ozlabs.org/patch/770154/

But I'm not sure if I followed I'm not sure why it's a more
difficult argument: any time a process moves it must first execute
a hwsync on the current CPU after it has performed all its access
there, and then it must execute hwsync on the new CPU before it
performs any new access.