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

Darrick J. Wong
2019-09-06T03:01:50Z

On Fri, Sep 06, 2019 at 12:08:13PM +1000, Dave Chinner wrote:
> 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.

I agree that this is a much better comment for the function. Thanks for
writing this. :)

--D

> */
> 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;
>