[RFC 1/3] sched/deadline: Skip the deadline bandwidth check if kexec_in_progress

[Pingfan Liu <piliu@redhat.com>]

During discussion of the scheduler deadline bug [1], Pierre Gondois
pointed out a potential issue during kexec: as CPUs are unplugged, the
available DL bandwidth of the root domain gradually decreases. At some
point, insufficient bandwidth triggers an overflow detection, causing
CPU hot-removal to fail and kexec to hang.

This can be reproduced by:
  chrt -d -T 1000000 -P 1000000 0 yes > /dev/null &
  kexec -e

This patch skips the DL bandwidth check if kexec is in progress.

[1]: https://lore.kernel.org/all/20250929133602.32462-1-piliu@redhat.com/

Reported-by: Pierre Gondois <pierre.gondois@arm.com>
Closes: https://lore.kernel.org/all/3408aca5-e6c9-434a-9950-82e9147fcbba@arm.com/
Signed-off-by: Pingfan Liu <piliu@redhat.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Pierre Gondois <pierre.gondois@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: Joel Granados <joel.granados@kernel.org>
To: kexec@lists.infradead.org
To: linux-kernel@vger.kernel.org
---
 kernel/kexec_core.c     | 6 ++++++
 kernel/sched/deadline.c | 7 +++++++
 2 files changed, 13 insertions(+)

diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index 31203f0bacafa..265de9d1ff5f5 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -1183,7 +1183,13 @@ int kernel_kexec(void)
 	} else
 #endif
 	{
+		/*
+		 * CPU hot-removal path refers to kexec_in_progress, it
+		 * requires a sync to ensure no in-flight hot-removing.
+		 */
+		cpu_hotplug_disable();
 		kexec_in_progress = true;
+		cpu_hotplug_enable();
 		kernel_restart_prepare("kexec reboot");
 		migrate_to_reboot_cpu();
 		syscore_shutdown();
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index a3a43baf4314e..cc864cc348b2c 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -18,6 +18,7 @@
 
 #include <linux/cpuset.h>
 #include <linux/sched/clock.h>
+#include <linux/kexec.h>
 #include <uapi/linux/sched/types.h>
 #include "sched.h"
 #include "pelt.h"
@@ -3502,6 +3503,12 @@ static int dl_bw_manage(enum dl_bw_request req, int cpu, u64 dl_bw)
 
 int dl_bw_deactivate(int cpu)
 {
+	/*
+	 * The system is shutting down and cannot roll back.  There is no point
+	 * in keeping track of bandwidth, which may fail hotplug.
+	 */
+	if (unlikely(kexec_in_progress))
+		return 0;
 	return dl_bw_manage(dl_bw_req_deactivate, cpu, 0);
 }
 
-- 
2.49.0