linux-kernel - Re: [PATCH RFC 04/14] rcu: Promote blocked tasks from per-CPU to rnp lists

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Message-ID: <20260109035209.GB1105379@joelbox2>
Date: Thu, 8 Jan 2026 22:52:09 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Joel Fernandes <joelagnelf@...dia.com>, linux-kernel@...r.kernel.org,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang@...ux.dev>, Uladzislau Rezki <urezki@...il.com>,
	rcu@...r.kernel.org
Subject: Re: [PATCH RFC 04/14] rcu: Promote blocked tasks from per-CPU to rnp
 lists

On Mon, Jan 05, 2026 at 10:59:31AM -0500, Steven Rostedt wrote:
> On Fri,  2 Jan 2026 19:23:33 -0500
> Joel Fernandes <joelagnelf@...dia.com> wrote:
> 
> > +#ifdef CONFIG_RCU_PER_CPU_BLOCKED_LISTS
> > +/*
> > + * Promote blocked tasks from a single CPU's per-CPU list to the rnp list.
> > + *
> > + * If there are no tracked blockers (gp_tasks NULL) and this CPU
> > + * is still blocking the corresponding GP (bit set in qsmask), set
> > + * the pointer to ensure the GP machinery knows about the blocking task.
> > + * This handles late promotion during QS reporting, where tasks may have
> > + * blocked after rcu_gp_init() or sync_exp_reset_tree() ran their scans.
> > + */
> > +static void rcu_promote_blocked_tasks_rdp(struct rcu_data *rdp,
> > +					  struct rcu_node *rnp)
> > +{
> > +	struct task_struct *t, *tmp;
> > +
> > +	raw_lockdep_assert_held_rcu_node(rnp);
> > +
> > +	raw_spin_lock(&rdp->blkd_lock);
> > +	list_for_each_entry_safe(t, tmp, &rdp->blkd_list, rcu_rdp_entry) {
> 
> How big can this list be? This would be considered an unbounded latency for
> PREEMPT_RT. If this is needed, then we need to disable this when PREEMPT_RT
> is enabled.

Steve, thanks. This is still quite a bit in the experimental/RFC phase, but
if we ever were to do this, we could splice the list of tasks into O(1)
instead of O(N) I am doing here. Great point.

Thanks for the suggestions about the guards as well on the other patch, I
shall use that where possible in any of my new code.

thanks,

 - Joel