linux-kernel - [PATCH 3.2 46/94] virtio_balloon: don't softlockup on huge balloon changes.

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Message-ID: <lsq.1398647482.719686308@decadent.org.uk>
Date:	Mon, 28 Apr 2014 02:11:22 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org,
	"Gopesh Kumar Chaudhary" <gopchaud@...ibm.com>,
	"Rusty Russell" <rusty@...tcorp.com.au>
Subject: [PATCH 3.2 46/94] virtio_balloon: don't softlockup on huge
 balloon changes.

3.2.58-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Rusty Russell <rusty@...tcorp.com.au>

commit 1f74ef0f2d7d692fcd615621e0e734c3e7771413 upstream.

When adding or removing 100G from a balloon:

    BUG: soft lockup - CPU#0 stuck for 22s! [vballoon:367]

We have a wait_event_interruptible(), but the condition is always true
(more ballooning to do) so we don't ever sleep.  We also have a
wait_event() for the host to ack, but that is also always true as QEMU
is synchronous for balloon operations.

Reported-by: Gopesh Kumar Chaudhary <gopchaud@...ibm.com>
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/virtio/virtio_balloon.c | 6 ++++++
 1 file changed, 6 insertions(+)

--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -271,6 +271,12 @@ static int balloon(void *_vballoon)
 		else if (diff < 0)
 			leak_balloon(vb, -diff);
 		update_balloon_size(vb);
+
+		/*
+		 * For large balloon changes, we could spend a lot of time
+		 * and always have work to do.  Be nice if preempt disabled.
+		 */
+		cond_resched();
 	}
 	return 0;
 }

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