[PATCH resend] hung_task: add task->flags, blocked by coredump to log

[Oxana Kharitonova <oxana@cloudflare.com>]

Resending this patch as I haven't received feedback on my initial 
submission https://lore.kernel.org/all/20241204182953.10854-1-oxana@cloudflare.com/

For the processes which are terminated abnormally the kernel can provide 
a coredump if enabled. When the coredump is performed, the process and 
all its threads are put into the D state 
(TASK_UNINTERRUPTIBLE | TASK_FREEZABLE). 

On the other hand, we have kernel thread khungtaskd which monitors the 
processes in the D state. If the task stuck in the D state more than 
kernel.hung_task_timeout_secs, the hung_task alert appears in the kernel 
log.

The higher memory usage of a process, the longer it takes to create 
coredump, the longer tasks are in the D state. We have hung_task alerts 
for the processes with memory usage above 10Gb. Although, our 
kernel.hung_task_timeout_secs is 10 sec when the default is 120 sec.

Adding additional information to the log that the task is blocked by 
coredump will help with monitoring. Another approach might be to 
completely filter out alerts for such tasks, but in that case we would 
lose transparency about what is putting pressure on some system 
resources, e.g. we saw an increase in I/O when coredump occurs due its 
writing to disk.

Additionally, it would be helpful to have task_struct->flags in the log 
from the function sched_show_task(). Currently it prints 
task_struct->thread_info->flags, this seems misleading as the line 
starts with "task:xxxx".

Signed-off-by: Oxana Kharitonova <oxana@cloudflare.com>
---
 kernel/hung_task.c  | 2 ++
 kernel/sched/core.c | 4 ++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index c18717189f32..953169893a95 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -147,6 +147,8 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 			print_tainted(), init_utsname()->release,
 			(int)strcspn(init_utsname()->version, " "),
 			init_utsname()->version);
+		if (t->flags & PF_POSTCOREDUMP)
+			pr_err("      Blocked by coredump.\n");
 		pr_err("\"echo 0 > /proc/sys/kernel/hung_task_timeout_secs\""
 			" disables this message.\n");
 		sched_show_task(t);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3e5a6bf587f9..77b6af12e146 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7701,9 +7701,9 @@ void sched_show_task(struct task_struct *p)
 	if (pid_alive(p))
 		ppid = task_pid_nr(rcu_dereference(p->real_parent));
 	rcu_read_unlock();
-	pr_cont(" stack:%-5lu pid:%-5d tgid:%-5d ppid:%-6d flags:0x%08lx\n",
+	pr_cont(" stack:%-5lu pid:%-5d tgid:%-5d ppid:%-6d task_flags:0x%08lx flags:0x%08lx\n",
 		free, task_pid_nr(p), task_tgid_nr(p),
-		ppid, read_task_thread_flags(p));
+		ppid, p->flags, read_task_thread_flags(p));
 
 	print_worker_info(KERN_INFO, p);
 	print_stop_info(KERN_INFO, p);
-- 
2.39.5