[PATCH] BUG: sysrq: Reset the watchdog during the polling of sysrq-t processing tasks

[PATCH] BUG: sysrq: Reset the watchdog during the polling of sysrq-t processing tasks

[cx19970]

When the number of processes in the system is extremely large,
manually execute the "sysrq-t" command and print the task list
information on the serial port. The kernel will experience a soft lockup.
You need to manually press the keys to input the "sysrq-t" command,
that is: the "BREAK" key + the "t" key.
For X86 devices with the 8250 serial port, the baud rate is 115200,
and the response duration of "sysrq-t" will exceed 30 minutes.

The key factors causing the soft lock problem:
1.Too many processes.
2.Serial port printing is too slow.
3.The default timeout of the kernel watchdog is 20 seconds, which is too short.
4.The kernel task list "task" is too long.
5.The kernel scheduling queue "cfs_rq" is too long.
6.Exclusive CPU core during sysrq-t.
7.There is too much content to be printed during sysrq-t.
8.Reading cpuinfo requires inter-core communication.

Solution: Add the operation of resetting the watchdog when printing the
task list "task" and the scheduling queue "cfs_rq" using sysrq-t.

Signed-off-by: cx19970 <cx19970@qq.com>
---
 kernel/sched/debug.c | 2 ++
 kernel/sched/fair.c  | 5 ++++-
 kernel/sched/rt.c    | 5 ++++-
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 02e16b70a790..f3d8df4f8490 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -789,6 +789,8 @@ static void print_rq(struct seq_file *m, struct rq *rq, int rq_cpu)
 		if (task_cpu(p) != rq_cpu)
 			continue;
 
+		touch_nmi_watchdog();
+		touch_all_softlockup_watchdogs();
 		print_task(m, rq, p);
 	}
 	rcu_read_unlock();
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5b752324270b..0e93054d2c46 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13672,8 +13672,11 @@ void print_cfs_stats(struct seq_file *m, int cpu)
 	struct cfs_rq *cfs_rq, *pos;
 
 	rcu_read_lock();
-	for_each_leaf_cfs_rq_safe(cpu_rq(cpu), cfs_rq, pos)
+	for_each_leaf_cfs_rq_safe(cpu_rq(cpu), cfs_rq, pos) {
+		touch_nmi_watchdog();
+		touch_all_softlockup_watchdogs();
 		print_cfs_rq(m, cpu, cfs_rq);
+	}
 	rcu_read_unlock();
 }
 
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 7936d4333731..46ac5ddc5071 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2932,7 +2932,10 @@ void print_rt_stats(struct seq_file *m, int cpu)
 	struct rt_rq *rt_rq;
 
 	rcu_read_lock();
-	for_each_rt_rq(rt_rq, iter, cpu_rq(cpu))
+	for_each_rt_rq(rt_rq, iter, cpu_rq(cpu)) {
+		touch_nmi_watchdog();
+		touch_all_softlockup_watchdogs();
 		print_rt_rq(m, cpu, rt_rq);
+	}
 	rcu_read_unlock();
 }
-- 
2.36.1.windows.1

Re: [PATCH] BUG: sysrq: Reset the watchdog during the polling of sysrq-t processing tasks

[kernel test robot]

Hi cx19970,

kernel test robot noticed the following build errors:

[auto build test ERROR on tip/sched/core]
[also build test ERROR on linus/master v6.18-rc6 next-20251114]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/cx19970/BUG-sysrq-Reset-the-watchdog-during-the-polling-of-sysrq-t-processing-tasks/20251117-130012
base:   tip/sched/core
patch link:    https://lore.kernel.org/r/tencent_B7DDA306E3B6A8CDF3195E4C083C9BD37E07%40qq.com
patch subject: [PATCH] BUG: sysrq: Reset the watchdog during the polling of sysrq-t processing tasks
config: i386-allnoconfig (https://download.01.org/0day-ci/archive/20251117/202511171515.bQfhqZLh-lkp@intel.com/config)
compiler: gcc-14 (Debian 14.2.0-19) 14.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20251117/202511171515.bQfhqZLh-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202511171515.bQfhqZLh-lkp@intel.com/

All errors (new ones prefixed by >>):

   kernel/sched/fair.c: In function 'print_cfs_stats':
>> kernel/sched/fair.c:13940:17: error: implicit declaration of function 'touch_nmi_watchdog' [-Wimplicit-function-declaration]
   13940 |                 touch_nmi_watchdog();
         |                 ^~~~~~~~~~~~~~~~~~
>> kernel/sched/fair.c:13941:17: error: implicit declaration of function 'touch_all_softlockup_watchdogs' [-Wimplicit-function-declaration]
   13941 |                 touch_all_softlockup_watchdogs();
         |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
--
   In file included from kernel/sched/build_policy.c:52:
   kernel/sched/rt.c: In function 'print_rt_stats':
>> kernel/sched/rt.c:2941:17: error: implicit declaration of function 'touch_nmi_watchdog' [-Wimplicit-function-declaration]
    2941 |                 touch_nmi_watchdog();
         |                 ^~~~~~~~~~~~~~~~~~
>> kernel/sched/rt.c:2942:17: error: implicit declaration of function 'touch_all_softlockup_watchdogs' [-Wimplicit-function-declaration]
    2942 |                 touch_all_softlockup_watchdogs();
         |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


vim +/touch_nmi_watchdog +13940 kernel/sched/fair.c

 13933	
 13934	void print_cfs_stats(struct seq_file *m, int cpu)
 13935	{
 13936		struct cfs_rq *cfs_rq, *pos;
 13937	
 13938		rcu_read_lock();
 13939		for_each_leaf_cfs_rq_safe(cpu_rq(cpu), cfs_rq, pos) {
 13940			touch_nmi_watchdog();
 13941			touch_all_softlockup_watchdogs();
 13942			print_cfs_rq(m, cpu, cfs_rq);
 13943		}
 13944		rcu_read_unlock();
 13945	}
 13946	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH] BUG: sysrq: Reset the watchdog during the polling of sysrq-t processing tasks

[kernel test robot]

Hi cx19970,

kernel test robot noticed the following build errors:

[auto build test ERROR on tip/sched/core]
[also build test ERROR on linus/master v6.18-rc6 next-20251114]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/cx19970/BUG-sysrq-Reset-the-watchdog-during-the-polling-of-sysrq-t-processing-tasks/20251117-130012
base:   tip/sched/core
patch link:    https://lore.kernel.org/r/tencent_B7DDA306E3B6A8CDF3195E4C083C9BD37E07%40qq.com
patch subject: [PATCH] BUG: sysrq: Reset the watchdog during the polling of sysrq-t processing tasks
config: arm-allnoconfig (https://download.01.org/0day-ci/archive/20251117/202511171548.yY6PHme5-lkp@intel.com/config)
compiler: clang version 22.0.0git (https://github.com/llvm/llvm-project 0bba1e76581bad04e7d7f09f5115ae5e2989e0d9)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20251117/202511171548.yY6PHme5-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202511171548.yY6PHme5-lkp@intel.com/

All errors (new ones prefixed by >>):

>> kernel/sched/fair.c:13940:3: error: call to undeclared function 'touch_nmi_watchdog'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    13940 |                 touch_nmi_watchdog();
          |                 ^
>> kernel/sched/fair.c:13941:3: error: call to undeclared function 'touch_all_softlockup_watchdogs'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    13941 |                 touch_all_softlockup_watchdogs();
          |                 ^
   2 errors generated.
--
   In file included from kernel/sched/build_policy.c:52:
>> kernel/sched/rt.c:2941:3: error: call to undeclared function 'touch_nmi_watchdog'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    2941 |                 touch_nmi_watchdog();
         |                 ^
>> kernel/sched/rt.c:2942:3: error: call to undeclared function 'touch_all_softlockup_watchdogs'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    2942 |                 touch_all_softlockup_watchdogs();
         |                 ^
   2 errors generated.


vim +/touch_nmi_watchdog +13940 kernel/sched/fair.c

 13933	
 13934	void print_cfs_stats(struct seq_file *m, int cpu)
 13935	{
 13936		struct cfs_rq *cfs_rq, *pos;
 13937	
 13938		rcu_read_lock();
 13939		for_each_leaf_cfs_rq_safe(cpu_rq(cpu), cfs_rq, pos) {
 13940			touch_nmi_watchdog();
 13941			touch_all_softlockup_watchdogs();
 13942			print_cfs_rq(m, cpu, cfs_rq);
 13943		}
 13944		rcu_read_unlock();
 13945	}
 13946	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki