[PATCH 3/8 v2] xfs: prevent CIL push holdoff in log recovery

Dave Chinner
2019-09-06T02:08:19Z

From: Dave Chinner <dchinner@redhat.com>

generic/530 on a machine with enough ram and a non-preemptible
kernel can run the AGI processing phase of log recovery enitrely out
of cache. This means it never blocks on locks, never waits for IO
and runs entirely through the unlinked lists until it either
completes or blocks and hangs because it has run out of log space.

It runs out of log space because the background CIL push is
scheduled but never runs. queue_work() queues the CIL work on the
current CPU that is busy, and the workqueue code will not run it on
any other CPU. Hence if the unlinked list processing never yields
the CPU voluntarily, the push work is delayed indefinitely. This
results in the CIL aggregating changes until all the log space is
consumed.

When the log recoveyr processing evenutally blocks, the CIL flushes
but because the last iclog isn't submitted for IO because it isn't
full, the CIL flush never completes and nothing ever moves the log
head forwards, or indeed inserts anything into the tail of the log,
and hence nothing is able to get the log moving again and recovery
hangs.

There are several problems here, but the two obvious ones from
the trace are that:
a) log recovery does not yield the CPU for over 4 seconds,
b) binding CIL pushes to a single CPU is a really bad idea.

This patch addresses just these two aspects of the problem, and are
suitable for backporting to work around any issues in older kernels.
The more fundamental problem of preventing the CIL from consuming
more than 50% of the log without committing will take more invasive
and complex work, so will be done as followup work.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
---
fs/xfs/xfs_log_recover.c | 30 +++++++++++++++++++++---------
fs/xfs/xfs_super.c | 3 ++-
2 files changed, 23 insertions(+), 10 deletions(-)

V2: big comment update

diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
index f05c6c99c4f3..508319039dce 100644
--- a/fs/xfs/xfs_log_recover.c
+++ b/fs/xfs/xfs_log_recover.c
@@ -5024,16 +5024,27 @@ xlog_recover_process_one_iunlink(
}

/*
- * xlog_iunlink_recover
+ * Recover AGI unlinked lists
*
- * This is called during recovery to process any inodes which
- * we unlinked but not freed when the system crashed. These
- * inodes will be on the lists in the AGI blocks. What we do
- * here is scan all the AGIs and fully truncate and free any
- * inodes found on the lists. Each inode is removed from the
- * lists when it has been fully truncated and is freed. The
- * freeing of the inode and its removal from the list must be
- * atomic.
+ * This is called during recovery to process any inodes which we unlinked but
+ * not freed when the system crashed. These inodes will be on the lists in the
+ * AGI blocks. What we do here is scan all the AGIs and fully truncate and free
+ * any inodes found on the lists. Each inode is removed from the lists when it
+ * has been fully truncated and is freed. The freeing of the inode and its
+ * removal from the list must be atomic.
+ *
+ * If everything we touch in the agi processing loop is already in memory, this
+ * loop can hold the cpu for a long time. It runs without lock contention,
+ * memory allocation contention, the need wait for IO, etc, and so will run
+ * until we either run out of inodes to process, run low on memory or we run out
+ * of log space.
+ *
+ * This behaviour is bad for latency on single CPU and non-preemptible kernels,
+ * and can prevent other filesytem work (such as CIL pushes) from running. This
+ * can lead to deadlocks if the recovery process runs out of log reservation
+ * space. Hence we need to yield the CPU when there is other kernel work
+ * scheduled on this CPU to ensure other scheduled work can run without undue
+ * latency.
*/
STATIC void
xlog_recover_process_iunlinks(
@@ -5080,6 +5091,7 @@ xlog_recover_process_iunlinks(
while (agino != NULLAGINO) {
agino = xlog_recover_process_one_iunlink(mp,
agno, agino, bucket);
+ cond_resched();
}
}
xfs_buf_rele(agibp);
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index f9450235533c..391b4748cae3 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -818,7 +818,8 @@ xfs_init_mount_workqueues(
goto out_destroy_buf;

mp->m_cil_workqueue = alloc_workqueue("xfs-cil/%s",
- WQ_MEM_RECLAIM|WQ_FREEZABLE, 0, mp->m_fsname);
+ WQ_MEM_RECLAIM | WQ_FREEZABLE | WQ_UNBOUND,
+ 0, mp->m_fsname);
if (!mp->m_cil_workqueue)
goto out_destroy_unwritten;