[PATCH v4 2/2] watchdog/hardlockup: Avoid large stack frames in watchdog_hardlockup_check()

[Douglas Anderson]

After commit 77c12fc95980 ("watchdog/hardlockup: add a "cpu" param to
watchdog_hardlockup_check()") we started storing a `struct cpumask` on
the stack in watchdog_hardlockup_check(). On systems with
CONFIG_NR_CPUS set to 8192 this takes up 1K on the stack. That
triggers warnings with `CONFIG_FRAME_WARN` set to 1024.

We'll use the new trigger_allbutcpu_cpu_backtrace() to avoid needing
to use a CPU mask at all.

Fixes: 77c12fc95980 ("watchdog/hardlockup: add a "cpu" param to watchdog_hardlockup_check()")
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/r/202307310955.pLZDhpnl-lkp@xxxxxxxxx
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---

(no changes since v3)

Changes in v3:
- Rebase on change introducing trigger_allbutcpu_cpu_backtrace().

Changes in v2:
- Allocate space when userspace requests all cpus be backtraced.

 kernel/watchdog.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 085d7a78f62f..d145305d95fe 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -151,9 +151,6 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
 	 */
 	if (is_hardlockup(cpu)) {
 		unsigned int this_cpu = smp_processor_id();
-		struct cpumask backtrace_mask;
-
-		cpumask_copy(&backtrace_mask, cpu_online_mask);
 
 		/* Only print hardlockups once. */
 		if (per_cpu(watchdog_hardlockup_warned, cpu))
@@ -167,10 +164,8 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
 				show_regs(regs);
 			else
 				dump_stack();
-			cpumask_clear_cpu(cpu, &backtrace_mask);
 		} else {
-			if (trigger_single_cpu_backtrace(cpu))
-				cpumask_clear_cpu(cpu, &backtrace_mask);
+			trigger_single_cpu_backtrace(cpu);
 		}
 
 		/*
@@ -179,7 +174,7 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
 		 */
 		if (sysctl_hardlockup_all_cpu_backtrace &&
 		    !test_and_set_bit(0, &watchdog_hardlockup_all_cpu_dumped))
-			trigger_cpumask_backtrace(&backtrace_mask);
+			trigger_allbutcpu_cpu_backtrace(cpu);
 
 		if (hardlockup_panic)
 			nmi_panic(regs, "Hard LOCKUP");
-- 
2.41.0.585.gd2178a4bd4-goog