linux-kernel - [tip: smp/core] cpu/hotplug: Rework sparse_irq locking in bringup

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Message-ID: <168422819974.404.13412319672677019398.tip-bot2@tip-bot2>
Date:   Tue, 16 May 2023 09:09:59 -0000
From:   "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Helge Deller <deller@....de>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: smp/core] cpu/hotplug: Rework sparse_irq locking in bringup_cpu()

The following commit has been merged into the smp/core branch of tip:

Commit-ID:     22b612e2d53f6e13ce7b55ed565a104512f0eb00
Gitweb:        https://git.kernel.org/tip/22b612e2d53f6e13ce7b55ed565a104512f0eb00
Author:        Thomas Gleixner <tglx@...utronix.de>
AuthorDate:    Fri, 12 May 2023 23:07:21 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Mon, 15 May 2023 13:44:54 +02:00

cpu/hotplug: Rework sparse_irq locking in bringup_cpu()

There is no harm to hold sparse_irq lock until the upcoming CPU completes
in cpuhp_online_idle(). This allows to remove cpu_online() synchronization
from architecture code.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Tested-by: Michael Kelley <mikelley@...rosoft.com>
Tested-by: Oleksandr Natalenko <oleksandr@...alenko.name>
Tested-by: Helge Deller <deller@....de> # parisc
Tested-by: Guilherme G. Piccoli <gpiccoli@...lia.com> # Steam Deck
Link: https://lore.kernel.org/r/20230512205256.263722880@linutronix.de
---
 kernel/cpu.c | 34 ++++++++++++++++++++++++----------
 1 file changed, 24 insertions(+), 10 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index c0d859c..df8f137 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -558,7 +558,7 @@ static int cpuhp_kick_ap(int cpu, struct cpuhp_cpu_state *st,
 	return ret;
 }
 
-static int bringup_wait_for_ap(unsigned int cpu)
+static int bringup_wait_for_ap_online(unsigned int cpu)
 {
 	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
 
@@ -579,15 +579,12 @@ static int bringup_wait_for_ap(unsigned int cpu)
 	 */
 	if (!cpu_smt_allowed(cpu))
 		return -ECANCELED;
-
-	if (st->target <= CPUHP_AP_ONLINE_IDLE)
-		return 0;
-
-	return cpuhp_kick_ap(cpu, st, st->target);
+	return 0;
 }
 
 static int bringup_cpu(unsigned int cpu)
 {
+	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
 	struct task_struct *idle = idle_thread_get(cpu);
 	int ret;
 
@@ -600,16 +597,33 @@ static int bringup_cpu(unsigned int cpu)
 	/*
 	 * Some architectures have to walk the irq descriptors to
 	 * setup the vector space for the cpu which comes online.
-	 * Prevent irq alloc/free across the bringup.
+	 *
+	 * Prevent irq alloc/free across the bringup by acquiring the
+	 * sparse irq lock. Hold it until the upcoming CPU completes the
+	 * startup in cpuhp_online_idle() which allows to avoid
+	 * intermediate synchronization points in the architecture code.
 	 */
 	irq_lock_sparse();
 
 	/* Arch-specific enabling code. */
 	ret = __cpu_up(cpu, idle);
-	irq_unlock_sparse();
 	if (ret)
-		return ret;
-	return bringup_wait_for_ap(cpu);
+		goto out_unlock;
+
+	ret = bringup_wait_for_ap_online(cpu);
+	if (ret)
+		goto out_unlock;
+
+	irq_unlock_sparse();
+
+	if (st->target <= CPUHP_AP_ONLINE_IDLE)
+		return 0;
+
+	return cpuhp_kick_ap(cpu, st, st->target);
+
+out_unlock:
+	irq_unlock_sparse();
+	return ret;
 }
 
 static int finish_cpu(unsigned int cpu)