On Thu, Jun 25, 2026 at 08:42:59PM -0400, Joel Fernandes wrote:
> The arming code in rcu_read_unlock_special() has two paths for
> deferring QS reporting: a softirq raise and an irq_work/set_need_resched
> combination.
>
> The irq_work path always calls set_need_resched_current() before
> queuing irq_work. The softirq path does not, relying solely on the
> softirq firing to do the rightthing.
>
> This results in a problem as follows:
>
> Consider 2 rcu_read_lock/unlock segments:
>
> rcu_read_lock(); // segment 1 starts
> // needs_exp becomes true
> preempt_disable();
> rcu_read_unlock(); // segment 1 ends; IRQs on, preempt
> // off, needs_exp=true =>
> // raise_softirq(RCU_SOFTIRQ);
> // arms defer_qs_pending.
> // Before this fix: no
> // set_need_resched_current().
> local_irq_disable();
> preempt_enable(); // softirq pending but IRQs disabled
> // hold it off.
> rcu_read_lock(); // segment 2 starts
> local_irq_enable(); // softirq fires: rcu_core runs, but
> // we are inside a reader (depth>0)
> // so no QS report; on softirq-exit
> // preempt-check finds no
> // need_resched -- still no nudge.
> preempt_disable();
> rcu_read_unlock(); // arming attempt suppressed
> // incorrectly: defer_qs_pending
> // already PENDING. Without this
> // fix, no fresh
> // set_need_resched_current() on
> // this path either.
> preempt_enable();
>
> Therefore, add set_need_resched_current() to the softirq deferral path to
> avoid long latencies in situations where GP needs to end sooner.
Hmmm... Doesn't this make some of the prior changes unnecessary?
Also, how often is this giving us unecessary context switches?
Finally, what exactly does (->defer_qs_pending == DEFER_QS_PENDING)
mean at this point?
Thanx, Paul
> Signed-off-by: Joel Fernandes <[email protected]>
> ---
> kernel/rcu/tree_plugin.h | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index c37d8cfeb714..f58ae29acdef 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -846,6 +846,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
> // Using softirq, safe to awaken, and either the
> // wakeup is free or there is either an expedited
> // GP in flight or a potential need to deboost.
> + set_need_resched_current();
> if (rdp->defer_qs_pending != DEFER_QS_PENDING) {
> rdp->defer_qs_pending = DEFER_QS_PENDING;
> raise_softirq_irqoff(RCU_SOFTIRQ);
> --
> 2.34.1
>
