On Mon, Aug 17, 2020 at 11:16:33AM +0200, pet...@infradead.org wrote: > On Mon, Aug 17, 2020 at 11:03:25AM +0200, pet...@infradead.org wrote: > > On Thu, Jul 23, 2020 at 09:14:11AM -0700, Paul E. McKenney wrote: > > > > --- a/kernel/rcu/tree.c > > > > +++ b/kernel/rcu/tree.c > > > > @@ -1287,8 +1287,6 @@ static int rcu_implicit_dynticks_qs(stru > > > > if (IS_ENABLED(CONFIG_IRQ_WORK) && > > > > !rdp->rcu_iw_pending && rdp->rcu_iw_gp_seq != > > > > rnp->gp_seq && > > > > (rnp->ffmask & rdp->grpmask)) { > > > > - init_irq_work(&rdp->rcu_iw, rcu_iw_handler); > > > > > > We are actually better off with the IRQ_WORK_INIT_HARD() here rather > > > than unconditionally at boot. > > > > Ah, but there isn't an init_irq_work() variant that does the HARD thing. > > Ah you meant doing: > > rdp->rcu_iw = IRQ_WORK_INIT_HARD(rcu_iw_handler) > > But then it is non-obvious how that doesn't trample state. I suppose > that rcu_iw_pending thing ensures that... I'll think about it.
Yes, this is what I had in mind. And you are right, the point of the !rdp->rcu_iw_pending check is to prevent initialization while still in use. > > > The reason for this is that we get here only if a single grace > > > period extends beyond 10.5 seconds (mainline) or beyond 30 seconds > > > (many distribution kernels). Which almost never happens. And yes, > > > rcutree_prepare_cpu() is also invoked as each CPU that comes online, > > > not that this is all that common outside of rcutorture and boot time. ;-) > > > > What do you mean 'also' ? Afaict this is CPU bringup only code (initial > > and hotplug). We really don't care about code there. It's the slowest > > possible path we have in the kernel. The "also" was acknowledging that a workload with lots of CPU hotplug would also needlessly invoke IRQ_WORK_INIT_HARD() multiple times. Thanx, Paul > > > > - atomic_set(&rdp->rcu_iw.flags, > > > > IRQ_WORK_HARD_IRQ); > > > > rdp->rcu_iw_pending = true; > > > > rdp->rcu_iw_gp_seq = rnp->gp_seq; > > > > irq_work_queue_on(&rdp->rcu_iw, rdp->cpu); > >