linux-kernel - [PATCH 6.16 1/2] genirq: Rebalance managed interrupts across multi-CPU hotplug

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Message-ID: <20250612183303.3433234-2-briannorris@chromium.org>
Date: Thu, 12 Jun 2025 11:32:51 -0700
From: Brian Norris <briannorris@...omium.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Douglas Anderson <dianders@...omium.org>,
	David Stevens <stevensd@...omium.org>,
	linux-kernel@...r.kernel.org,
	Tsai Sung-Fu <danielsftsai@...gle.com>,
	Aleksandrs Vinarskis <alex.vinarskis@...il.com>,
	Johan Hovold <johan@...nel.org>,
	Brian Norris <briannorris@...omium.org>
Subject: [PATCH 6.16 1/2] genirq: Rebalance managed interrupts across multi-CPU hotplug

Commit 788019eb559f ("genirq: Retain disable depth for managed
interrupts across CPU hotplug") intended to only decrement the disable
depth once per managed shutdown, but instead it decrements for each CPU
hotplug in the affinity mask, until its depth reaches a point where it
finally gets re-started.

For example, consider:

1. Interrupt is affine to CPU {M,N}
2. disable_irq() -> depth is 1
3. CPU M goes offline -> interrupt migrates to CPU N / depth is still 1
4. CPU N goes offline -> irq_shutdown() / depth is 2
5. CPU N goes online
    -> irq_restore_affinity_of_irq()
       -> irqd_is_managed_and_shutdown()==true
          -> irq_startup_managed() -> depth is 1
6. CPU M goes online
    -> irq_restore_affinity_of_irq()
       -> irqd_is_managed_and_shutdown()==true
          -> irq_startup_managed() -> depth is 0
          *** BUG: driver expects the interrupt is still disabled ***
             -> irq_startup() -> irqd_clr_managed_shutdown()
7. enable_irq() -> depth underflow / unbalanced enable_irq() warning

We should clear the managed-shutdown flag at step 6, so that further
hotplugs don't cause further imbalance.

Note: while it might be cleaner to also remove the
irqd_clr_managed_shutdown() from __irq_startup_managed() at the same
time, this is currently not possible because of
irq_update_affinity_desc() -- it sets IRQD_MANAGED_SHUTDOWN and expects
irq_startup() to clear it.

Fixes: 788019eb559f ("genirq: Retain disable depth for managed interrupts across CPU hotplug")
Signed-off-by: Brian Norris <briannorris@...omium.org>
---

 kernel/irq/chip.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/irq/chip.c b/kernel/irq/chip.c
index b0e0a7332993..2b274007e8ba 100644
--- a/kernel/irq/chip.c
+++ b/kernel/irq/chip.c
@@ -205,6 +205,14 @@ __irq_startup_managed(struct irq_desc *desc, const struct cpumask *aff,
 
 void irq_startup_managed(struct irq_desc *desc)
 {
+	struct irq_data *d = irq_desc_get_irq_data(desc);
+
+	/*
+	 * Clear managed-shutdown flag, so we don't repeat managed-startup for
+	 * multiple hotplugs, and cause imbalanced disable depth.
+	 */
+	irqd_clr_managed_shutdown(d);
+
 	/*
 	 * Only start it up when the disable depth is 1, so that a disable,
 	 * hotunplug, hotplug sequence does not end up enabling it during
-- 
2.50.0.rc1.591.g9c95f17f64-goog