On Thu, 8 Jun 2017 08:54:00 +0200 Peter Zijlstra <pet...@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 <pet...@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.