linux-kernel - Hung task detector versus NFS (TASK

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Message-ID: <20160307235944.GD15986@hostway.ca>
Date:	Mon, 7 Mar 2016 15:59:44 -0800
From:	Simon Kirby <sim@...tway.ca>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org
Subject: 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-