Re: [PATCH RFC v3 03/13] rcu-tasks: Add a Tasks RCU implementation for reader-marked trampolines

[Paul E. McKenney]

On Thu, Sep 17, 2026 at 04:35:19PM +0000, Josef Bacik wrote:
> On Thu, 17 Sep 2026 08:40:10 -0700, Paul E. McKenney wrote:
> > On Thu, Sep 17, 2026 at 02:14:57PM +0200, Frederic Weisbecker wrote:
> > > You're right. So we'll indeed need to reuse the deferred qs points here.
> >
> > Except this is getting a bit involved.
> >
> > Don't get me wrong, if Josef is happy to take this on, far be it from me
> > to stand in his way.  But if not, we should be willing to treat this
> > optimization as a follow-on effort, whether by Josef or someone else.
> 
> Follow-on works for me. For what it's worth 03/13 is already fairly
> close to what Frederic describes, just outside the core flavor: no
> task-list scan (the GP waits per CPU for a pass through __schedule() or
> an EQS), the irq-exit preemption path checks the interrupted IP and
> queues the task as a holdout before the switch, and the holdout is keyed
> on where the task was interrupted so nothing is needed from the
> trampoline tail. Moving that IP check into the tick / rcu_exp_handler() /
> deferred-QS paths and reusing the blocked-tasks list is something I'm
> happy to look at once this has settled.
> 
> v4 will pick up Alexei's ask (reader emitted by the BPF JIT around the
> fentry and fexit regions rather than in the glue) and the idle-CPU hole
> Sashiko found.

Even better!  ;-)

							Thanx, Paul