Re: 3.17-rc6: bcache_gc: BUG: soft lockup - CPU#2 stuck for 23s!

Kent Overstreet
2014-11-01T20:42:48Z

On Sun, Sep 28, 2014 at 05:25:37PM -0700, Eric Wheeler wrote:
> Hello Kent, Ross, all:
>
> We're getting bcache_gc backtraces and soft lockups; the system continues to
> be responsive and eventually recovers. We are running 3.17-rc6. (This
> appears to be a continuation of the thread from 2014-09-15)
>
> Please see the following two backtraces. The first shows up in
> btree_gc_count_keys(), the other is triggered somehow by rcu_sched. We will
> test with -rc7 this week, though I didn't see any bcache commits in rc7.
>
> The server is quite busy:
> dd in userspace from dm-thinp snapshots to another server
> two DRBD verify's active backed by dm-thinp volumes
> note that, dd fills up the buffers so this could be operating with few
> pages free. (Though we have min-mem set to 256MB.)
>
> I see we are hitting functions like bch_ptr_bad() and bch_extent_bad().
> Could that indicate a cache corruption on our volume?

No - those are the normal "check the validity of medata" functions.

> I'm happy to test patches if you have any suggestions or tests that I should
> run it through.

I think it might just be a missing cond_resched()... there's a check during
garbage collection for need_resched() but it appears we might not actually be
calling schedule() then.

Try this patch:

commit a64afc92e17e709bdd1618edd04bc608f6a44c55
Author: Kent Overstreet <kmo@daterainc.com>
Date: Sat Nov 1 13:44:13 2014 -0700

bcache: Add a cond_resched() call to gc

Change-Id: Id4f18c533b80ddb40df94ed0bb5e2a236a4bc325

diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index 00cde40db5..218f21ac02 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -1741,6 +1741,7 @@ static void bch_btree_gc(struct cache_set *c)
do {
ret = btree_root(gc_root, c, &op, &writes, &stats);
closure_sync(&writes);
+ cond_resched();

if (ret && ret != -EAGAIN)
pr_warn("gc failed!");