Hung task detector versus NFS (TASK_KILLABLE)

Hello!

Back in 2008, you committed 316d9679f33caf7e683471647d1472bfe133d858 which changed softlockup.c (now moved to hung_task.c) to avoid logging a spew of soft lockup warnings when the Ethernet cable is unplugged with active NFS mounts.

Meanwhile, I've been seeing hung task warnings like this for years, so I wondered what the deal is. It seems there are VFS paths that can enter uninterruptible sleep as result of locks held in interruptible sleep.

For example, I can reproduce hung task warnings by firewalling NFS, then "cat a" twice: the second hangs in mutex_lock() from path_openat(), which then spews a hung task warning.

I write this because I would actually find it useful to see the original backtrace, even if it is interruptible, not just the collateral damage. Since the "skipping" of NFS is basically incomplete anyway, how big a deal is this "feature"?

Would anybody object if we just returned this to anything blocked?

The lines in question these days are here in kernel/hung_task.c:

		/* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
		if (t->state == TASK_UNINTERRUPTIBLE)
			check_hung_task(t, timeout);

It used to be t->state & TASK_UNINTERRUPTIBLE.

Simon-

Random backtrace spewing is always a misfeature for 99.99+% of the users for whom it is gibberish.

If you really need it yourself add a kprobe.

-Andi

Distributions all seem to ship with it on because apparently some people can read it. There was even discussion that the default 10 is not enough.

To emulate a hung task backtrace even when TASK_KILLABLE? That sounds like some hoop-jumping, but I don't know kprobes.

I'm just saying the current "NFS filter" is broken ("cat a" twice), but this really will make more noise for people (in cases where NFS is stuck for minutes), I guess I'll just sit in a corner with that line changed in my tree.

Simon-