[PATCH] 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.

Instead of putting this `struct cpumask` on the stack, let's declare
it as `static`. This has the downside of taking up 1K of memory all
the time on systems with `CONFIG_NR_CPUS` to 8192, but on systems with
smaller `CONFIG_NR_CPUS` it's not much emory (with 128 CPUs it's only
16 bytes of memory). Presumably anyone building a system with
`CONFIG_NR_CPUS=8192` can afford the extra 1K of memory.

NOTE: as part of this change, we no longer check the return value of
trigger_single_cpu_backtrace(). While we could do this and only call
cpumask_clear_cpu() if trigger_single_cpu_backtrace() didn't fail,
that's probably not worth it. There's no reason to believe that
trigger_cpumask_backtrace() will succeed at backtracing the CPU when
trigger_single_cpu_backtrace() failed.

Alternatives considered:
- Use kmalloc with GFP_ATOMIC to allocate. I decided against this
  since relying on kmalloc when the system is hard locked up seems
  like a bad idea.
- Change the arch_trigger_cpumask_backtrace() across all architectures
  to take an extra parameter to get the needed behavior. This seems
  like a lot of churn for a small savings.

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
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---

 kernel/watchdog.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index be38276a365f..19db2357969a 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);
 		}
 
 		/*
@@ -178,8 +173,13 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
 		 * hardlockups generating interleaving traces
 		 */
 		if (sysctl_hardlockup_all_cpu_backtrace &&
-		    !test_and_set_bit(0, &watchdog_hardlockup_all_cpu_dumped))
+		    !test_and_set_bit(0, &watchdog_hardlockup_all_cpu_dumped)) {
+			static struct cpumask backtrace_mask;
+
+			cpumask_copy(&backtrace_mask, cpu_online_mask);
+			cpumask_clear_cpu(cpu, &backtrace_mask);
 			trigger_cpumask_backtrace(&backtrace_mask);
+		}
 
 		if (hardlockup_panic)
 			nmi_panic(regs, "Hard LOCKUP");
-- 
2.41.0.487.g6d72f3e995-goog