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

Darrick J. Wong
2019-09-06T00:15:56Z

On Fri, Sep 06, 2019 at 10:05:48AM +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>
> ---
> fs/xfs/xfs_log_recover.c | 1 +
> fs/xfs/xfs_super.c | 3 ++-
> 2 files changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
> index f05c6c99c4f3..c9665455431e 100644
> --- a/fs/xfs/xfs_log_recover.c
> +++ b/fs/xfs/xfs_log_recover.c
> @@ -5080,6 +5080,7 @@ xlog_recover_process_iunlinks(
> while (agino != NULLAGINO) {
> agino = xlog_recover_process_one_iunlink(mp,
> agno, agino, bucket);
> + cond_resched();

<urk> Now I wish I'd asked for a comment explaining why we
cond_resched()....

/* Don't let other workqueues (including the CIL ones) starve. */

(Dunno if you want to respin or if I'll just end up fixing it on the way
in...)

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> }
> }
> 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;
>
> --
> 2.23.0.rc1
>