hung_task: add task->flags, blocked by coredump to log

Resending this patch as I haven't received feedback on my initial
submission https://lore.kernel.org/all/[email protected]/

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".

[[email protected]: fix printk control string]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Oxana Kharitonova <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Ben Segall <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Dietmar Eggemann <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Peter Zijlstra (Intel) <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Valentin Schneider <[email protected]>
Cc: Vincent Guittot <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Oxana Kharitonova

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);

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%04x 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);