Re: [RFC v2] rcu/tree: Try to invoke_rcu_core() if in_irq() during unlock

[Paul E. McKenney]

On Sun, Aug 18, 2019 at 05:49:48PM -0400, Joel Fernandes (Google) wrote:
> When we're in hard interrupt context in rcu_read_unlock_special(), we
> can still benefit from invoke_rcu_core() doing wake ups of rcuc
> threads when the !use_softirq parameter is passed.  This is safe
> to do so because:
> 
> 1. We avoid the scheduler deadlock issues thanks to the deferred_qs bit
> introduced in commit 23634ebc1d94 ("rcu: Check for wakeup-safe
> conditions in rcu_read_unlock_special()") by checking for the same in
> this patch.
> 
> 2. in_irq() implies in_interrupt() which implies raising softirq will
> not do any wake ups.
> 
> The rcuc thread which is awakened will run when the interrupt returns.
> 
> We also honor 25102de ("rcu: Only do rcu_read_unlock_special() wakeups
> if expedited") thus doing the rcuc awakening only when none of the
> following are true:
>   1. Critical section is blocking an expedited GP.
>   2. A nohz_full CPU.
> If neither of these cases are true (exp == false), then the "else" block
> will run to do the irq_work stuff.
> 
> This commit is based on a partial revert of d143b3d1cd89 ("rcu: Simplify
> rcu_read_unlock_special() deferred wakeups") with an additional in_irq()
> check added.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

OK, I will bite...  If it is safe to wake up an rcuc kthread, why
is it not safe to do raise_softirq()?

And from the nit department, looks like some whitespace damage on the
comments.

							Thanx, Paul

> ---
> v1->v2: Some minor character encoding issues in changelog corrected.
> 
> Note that I am still testing this patch, but I sent an early RFC for your
> feedback. Thanks!
> 
>  kernel/rcu/tree_plugin.h | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 2defc7fe74c3..f4b3055026dc 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -621,6 +621,11 @@ static void rcu_read_unlock_special(struct task_struct *t)
>  			// Using softirq, safe to awaken, and we get
>  			// no help from enabling irqs, unlike bh/preempt.
>  			raise_softirq_irqoff(RCU_SOFTIRQ);
> +		} else if (exp && in_irq() && !use_softirq &&
> +			   !t->rcu_read_unlock_special.b.deferred_qs) {
> +			// Safe to awaken rcuc kthread which will be
> +                       // scheduled in from the interrupt return path.
> +			invoke_rcu_core();
>  		} else {
>  			// Enabling BH or preempt does reschedule, so...
>  			// Also if no expediting or NO_HZ_FULL, slow is OK.
> -- 
> 2.23.0.rc1.153.gdeed80330f-goog
>