Re: [PATCH 04/19] sched: Prepare for Core-wide rq->lock

[Peter Zijlstra]

On Thu, Apr 29, 2021 at 01:11:54PM -0700, Josh Don wrote:
> On Wed, Apr 28, 2021 at 2:13 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Tue, Apr 27, 2021 at 04:30:02PM -0700, Josh Don wrote:
> >
> > > Also, did you mean to have a preempt_enable_no_resched() rather than
> > > prempt_enable() in raw_spin_rq_trylock?
> >
> > No, trylock really needs to be preempt_enable(), because it can have
> > failed, at which point it will not have incremented the preemption count
> > and our decrement can hit 0, at which point we really should reschedule.
> 
> Ah yes, makes sense. Did you want to use preempt_enable_no_resched()
> at all then? No chance of preempt_count() being 1 at the point of
> enable, as you comment below.

preempt_enable_no_resched() avoids the branch which we know we'll never
take. It generates slightly saner code. It's not much, but every little
bit helps, right? ;-)

> > ( I'm suffering a cold and am really quite slow atm )
> 
> No worries, hope it's a mild one.

It's not the super popular one from '19, and the family seems to be
mostly recovered again, so all's well.

> > How's this then?
> 
> Looks good to me (other than the synchronize_sched()->synchronize_rcu()).
> 
> For these locking patches,
> Reviewed-by: Josh Don <joshdon@xxxxxxxxxx>

Thanks!, I'll go fold them into the proper place and update the repo.