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Message-ID: <202666b7-801c-44c7-9402-8c357abb8e5d@paulmck-laptop>
Date: Tue, 8 Jul 2025 17:06:48 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: linux-kernel@...r.kernel.org, Frederic Weisbecker <frederic@...nel.org>,
Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
Josh Triplett <josh@...htriplett.org>,
Boqun Feng <boqun.feng@...il.com>,
Uladzislau Rezki <urezki@...il.com>,
Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Zqiang <qiang.zhang@...ux.dev>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Clark Williams <clrkwllms@...nel.org>, rcu@...r.kernel.org,
linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH -rcu -next 3/7] rcu: Refactor expedited handling check in
rcu_read_unlock_special()
On Tue, Jul 08, 2025 at 10:22:20AM -0400, Joel Fernandes wrote:
> Extract the complex expedited handling condition in rcu_read_unlock_special()
> into a separate function rcu_unlock_needs_exp_handling() with detailed
> comments explaining each condition.
>
> This improves code readability. No functional change intended.
>
> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
> ---
> kernel/rcu/tree_plugin.h | 83 +++++++++++++++++++++++++++++++++++-----
> 1 file changed, 74 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index fa7b0d854833..e20c17163c13 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -646,6 +646,75 @@ static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
> local_irq_restore(flags);
> }
>
> +/*
> + * Check if expedited grace period processing during unlock is needed.
> + *
> + * This function determines whether expedited handling is required based on:
> + * 1. Task blocking an expedited grace period (based on a heuristic, could be
> + * false-positive, see below.)
> + * 2. CPU participating in an expedited grace period
> + * 3. Strict grace period mode requiring expedited handling
> + * 4. RCU priority deboosting needs when interrupts were disabled
> + *
> + * @t: The task being checked
> + * @rdp: The per-CPU RCU data
> + * @rnp: The RCU node for this CPU
> + * @irqs_were_disabled: Whether interrupts were disabled before rcu_read_unlock()
> + *
> + * Returns true if expedited processing of the rcu_read_unlock() is needed.
> + */
> +static bool rcu_unlock_needs_exp_handling(struct task_struct *t,
> + struct rcu_data *rdp,
> + struct rcu_node *rnp,
> + bool irqs_were_disabled)
> +{
> + /*
> + * Check if this task is blocking an expedited grace period. If the
> + * task was preempted within an RCU read-side critical section and is
> + * on the expedited grace period blockers list (exp_tasks), we need
> + * expedited handling to unblock the expedited GP. This is not an exact
> + * check because 't' might not be on the exp_tasks list at all - its
> + * just a fast heuristic that can be false-positive sometimes.
> + */
> + if (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_tasks))
> + return true;
> +
> + /*
> + * Check if this CPU is participating in an expedited grace period.
> + * The expmask bitmap tracks which CPUs need to check in for the
> + * current expedited GP. If our CPU's bit is set, we need expedited
> + * handling to help complete the expedited GP.
> + */
> + if (rdp->grpmask & READ_ONCE(rnp->expmask))
> + return true;
> +
> + /*
> + * In CONFIG_RCU_STRICT_GRACE_PERIOD=y kernels, all grace periods
> + * are treated as short for testing purposes even if that means
> + * disturbing the system more. Check if either:
> + * - This CPU has not yet reported a quiescent state, or
> + * - This task was preempted within an RCU critical section
> + * In either case, require expedited handling for strict GP mode.
> + */
> + if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
> + ((rdp->grpmask & READ_ONCE(rnp->qsmask)) || t->rcu_blocked_node))
> + return true;
> +
> + /*
> + * RCU priority boosting case: If a task is subject to RCU priority
> + * boosting and exits an RCU read-side critical section with interrupts
> + * disabled, we need expedited handling to ensure timely deboosting.
> + * Without this, a low-priority task could incorrectly run at high
> + * real-time priority for an extended period degrading real-time
> + * responsiveness. This applies to all CONFIG_RCU_BOOST=y kernels,
> + * not just to PREEMPT_RT.
> + */
> + if (IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled && t->rcu_blocked_node)
> + return true;
> +
> + return false;
> +}
> +
> /*
> * Handle special cases during rcu_read_unlock(), such as needing to
> * notify RCU core processing or task having blocked during the RCU
> @@ -665,18 +734,14 @@ static void rcu_read_unlock_special(struct task_struct *t)
> local_irq_save(flags);
> irqs_were_disabled = irqs_disabled_flags(flags);
> if (preempt_bh_were_disabled || irqs_were_disabled) {
> - bool expboost; // Expedited GP in flight or possible boosting.
> + bool needs_exp; // Expedited handling needed.
> struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> struct rcu_node *rnp = rdp->mynode;
>
> - expboost = (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_tasks)) ||
> - (rdp->grpmask & READ_ONCE(rnp->expmask)) ||
> - (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
> - ((rdp->grpmask & READ_ONCE(rnp->qsmask)) || t->rcu_blocked_node)) ||
> - (IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled &&
> - t->rcu_blocked_node);
> + needs_exp = rcu_unlock_needs_exp_handling(t, rdp, rnp, irqs_were_disabled);
> +
> // Need to defer quiescent state until everything is enabled.
> - if (use_softirq && (in_hardirq() || (expboost && !irqs_were_disabled))) {
> + if (use_softirq && (in_hardirq() || (needs_exp && !irqs_were_disabled))) {
> // 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.
> @@ -689,7 +754,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
> set_tsk_need_resched(current);
> set_preempt_need_resched();
> if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
> - expboost && rdp->defer_qs_iw_pending != DEFER_QS_PENDING &&
> + needs_exp && rdp->defer_qs_iw_pending != DEFER_QS_PENDING &&
> cpu_online(rdp->cpu)) {
> // Get scheduler to re-evaluate and call hooks.
> // If !IRQ_WORK, FQS scan will eventually IPI.
> --
> 2.34.1
>
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