[RFC 3/3] kexec_core: Promote the kexec to DL task

[Pingfan Liu <piliu@redhat.com>]

The previous patch lifted the deadline bandwidth check during the kexec
process. As a result, DL tasks may be crowded onto the kexec CPU, which
may starve the kexec task. At this point, the kexec task is the only
task needed to make the reboot proceed, hence promoting it to a deadline
task prevents this starvation.

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>
To: kexec@lists.infradead.org
To: linux-kernel@vger.kernel.org
---
 kernel/kexec_core.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index 265de9d1ff5f5..0960bea1a8bab 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -41,6 +41,7 @@
 #include <linux/objtool.h>
 #include <linux/kmsg_dump.h>
 #include <linux/dma-map-ops.h>
+#include <uapi/linux/sched/types.h>
 
 #include <asm/page.h>
 #include <asm/sections.h>
@@ -1183,6 +1184,20 @@ int kernel_kexec(void)
 	} else
 #endif
 	{
+		struct sched_attr attr = {
+			.size		= sizeof(struct sched_attr),
+			.sched_policy	= SCHED_DEADLINE,
+			.sched_nice	= 0,
+			.sched_priority	= 0,
+			/*
+			 * Fake (unused) bandwidth; workaround to "fix"
+			 * priority inheritance.
+			 */
+			.sched_runtime	= NSEC_PER_MSEC,
+			.sched_deadline = 10 * NSEC_PER_MSEC,
+			.sched_period	= 10 * NSEC_PER_MSEC,
+		};
+
 		/*
 		 * CPU hot-removal path refers to kexec_in_progress, it
 		 * requires a sync to ensure no in-flight hot-removing.
@@ -1202,6 +1217,13 @@ int kernel_kexec(void)
 		 */
 		cpu_hotplug_enable();
 		pr_notice("Starting new kernel\n");
+		/*
+		 * During hot-removing cpu, all DL tasks will be migrated to
+		 * this cpu.  To prevent this task from starving, promoting it
+		 * to DL task. And soon, local interrupt will be disabled in
+		 * machine_kexec().
+		 */
+		sched_setattr_nocheck(current, &attr);
 		machine_shutdown();
 	}
 
-- 
2.49.0