Messages in this thread | | | Date | Wed, 8 Jun 2022 16:29:27 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 20/21] rcu/context_tracking: Merge dynticks counter and context tracking states |
| |
On Tue, May 31, 2022 at 06:15:36PM +0200, nicolas saenz julienne wrote: > On Tue, 2022-05-31 at 16:23 +0200, Frederic Weisbecker wrote: > > But idle at least is an exception and CONTEXT_IDLE will remain during the > > interrupt handling. It's not that trivial to handle the idle case because > > ct_irq_exit() needs to know that it is called between ct_idle_enter() and > > ct_idle_exit(). > > Just for the record, this behaviour was already here regardless of this series, > so it's not something it needs to fix.
Right.
> > Something like this should work, right? > > ct_idle_enter() > //IRQ or NMI > if (__ct_state() == CONTEXT_IDLE) > ct_idle_exit()
Right but that's one more costly operation (atomic_add_return())
> ct_irq_enter()
Ideally this should increment by RCU_DYNTICKS_IDX - CONTEXT_IDLE
> ... > ct_irq_exit()
And this should increment by RCU_DYNTICKS_IDX + CONTEXT_IDLE
I guess the CONTEXT_IDLE state should be remembered on some per cpu variable somewhere.
BTW one interesting optimization to do when an idle interrupt leads to setting need_resched() would be to have:
idle_loop() { while (!need_resched) { rcu_idle_enter(); mwait(); //IRQ { rcu_irq_enter(); do_irq()... //set need_resched() rcu_irq_exit() // but no need to do the atomic_add_return() here // since we want to keep RCU watching as we'll // escape from idle } rcu_idle_exit() // and no need to do the atomic_add_return() here either
That's two expensive operations spared for a pretty common event.
> if (needs_update_state()) //using irqentry_state_t for ex. > ct_idle_entry() > ct_idle_exit() > > Note that it's not a big issue as we can work around this behaviour by checking > through dynticks whether a CPU is really idle. > > Do you think it's worth fixing nonetheless?
Nothing urgent for sure.
> > Regards, > Nicolas
| |