| Message ID | 20190906020813.GN1119@dread.disaster.area (mailing list archive) |
|---|---|
| State | Accepted |
| Headers | show |
| Series | None | expand |
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; >
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;