[PATCH 12/37 V2] xfs: Do background CIL flushes via a workqueue

Dave Chinner
2012-04-23T07:54:39Z

On Mon, Apr 23, 2012 at 03:58:42PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> Doing background CIL flushes adds significant latency to whatever
> async transaction that triggers it. To avoid blocking async
> transactions on things like waiting for log buffer IO to complete,
> move the CIL push off into a workqueue. By moving the push work
> into a workqueue, we remove all the latency that the commit adds
> from the foreground transaction commit path. This also means that
> single threaded workloads won't do the CIL push procssing, leaving
> them more CPU to do more async transactions.
>
> To do this, we need to keep track of the sequence number we have
> pushed work for. This avoids having many transaction commits
> attempting to schedule work for the same sequence, and ensures that
> we only ever have one push (background or forced) in progress at a
> time. It also means that we don't need to take the CIL lock in write
> mode to check for potential background push races, which reduces
> lock contention.
>
> To avoid potential issues with "smart" IO schedulers, don't use the
> workqueue for log force triggered flushes. Instead, do them directly
> so that the log IO is done directly by the process issuing the log
> force and so doesn't get stuck on IO elevator queue idling
> incorrectly delaying the log IO from the workqueue.

I think I understand what is triggering this ASSERT:

ASSERT(push_seq > 0 && push_seq <= ctx->sequence);

as the condition that is triggering is push_seq = 0.

Basically, the situation where the CIL is empty when a push is
executed causes the xc_push_seq to be set to zero so that a
subsequent push of that sequence number can be flushed. i.e. if
there is force when tehre is no dirty items, the sequence number is
not incremented as tehre is no commit, and hence if have subsequent
modifications on that sequence number we need to be able to force it
again. Resetting the push_seq is how that is acheived. If we get a
push with a zero sequence number, the code actually handles that
just fine by ignoring it. So, effectively, the assert for push_seq >
no longer valid so it should just be removed.

It does, however, point out one thing - that we can be queuing
multiple works when we don't need to, so we also need to check for
whether we need to push based on the current sequence number in the
background push check. the updated patch below addresses both these
issues.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com

xfs: Do background CIL flushes via a workqueue

From: Dave Chinner <dchinner@redhat.com>

Doing background CIL flushes adds significant latency to whatever
async transaction that triggers it. To avoid blocking async
transactions on things like waiting for log buffer IO to complete,
move the CIL push off into a workqueue. By moving the push work
into a workqueue, we remove all the latency that the commit adds
from the foreground transaction commit path. This also means that
single threaded workloads won't do the CIL push procssing, leaving
them more CPU to do more async transactions.

To do this, we need to keep track of the sequence number we have
pushed work for. This avoids having many transaction commits
attempting to schedule work for the same sequence, and ensures that
we only ever have one push (background or forced) in progress at a
time. It also means that we don't need to take the CIL lock in write
mode to check for potential background push races, which reduces
lock contention.

To avoid potential issues with "smart" IO schedulers, don't use the
workqueue for log force triggered flushes. Instead, do them directly
so that the log IO is done directly by the process issuing the log
force and so doesn't get stuck on IO elevator queue idling
incorrectly delaying the log IO from the workqueue.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>

---
fs/xfs/xfs_log_cil.c | 244 ++++++++++++++++++++++++++++++-------------------
fs/xfs/xfs_log_priv.h | 2 +
fs/xfs/xfs_mount.h | 1 +
fs/xfs/xfs_super.c | 7 ++
4 files changed, 160 insertions(+), 94 deletions(-)

diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
index d4fadbe..5fc7ec5 100644
--- a/fs/xfs/xfs_log_cil.c
+++ b/fs/xfs/xfs_log_cil.c
@@ -32,58 +32,6 @@
#include "xfs_discard.h"

/*
- * Perform initial CIL structure initialisation.
- */
-int
-xlog_cil_init(
- struct log *log)
-{
- struct xfs_cil *cil;
- struct xfs_cil_ctx *ctx;
-
- cil = kmem_zalloc(sizeof(*cil), KM_SLEEP|KM_MAYFAIL);
- if (!cil)
- return ENOMEM;
-
- ctx = kmem_zalloc(sizeof(*ctx), KM_SLEEP|KM_MAYFAIL);
- if (!ctx) {
- kmem_free(cil);
- return ENOMEM;
- }
-
- INIT_LIST_HEAD(&cil->xc_cil);
- INIT_LIST_HEAD(&cil->xc_committing);
- spin_lock_init(&cil->xc_cil_lock);
- init_rwsem(&cil->xc_ctx_lock);
- init_waitqueue_head(&cil->xc_commit_wait);
-
- INIT_LIST_HEAD(&ctx->committing);
- INIT_LIST_HEAD(&ctx->busy_extents);
- ctx->sequence = 1;
- ctx->cil = cil;
- cil->xc_ctx = ctx;
- cil->xc_current_sequence = ctx->sequence;
-
- cil->xc_log = log;
- log->l_cilp = cil;
- return 0;
-}
-
-void
-xlog_cil_destroy(
- struct log *log)
-{
- if (log->l_cilp->xc_ctx) {
- if (log->l_cilp->xc_ctx->ticket)
- xfs_log_ticket_put(log->l_cilp->xc_ctx->ticket);
- kmem_free(log->l_cilp->xc_ctx);
- }
-
- ASSERT(list_empty(&log->l_cilp->xc_cil));
- kmem_free(log->l_cilp);
-}
-
-/*
* Allocate a new ticket. Failing to get a new ticket makes it really hard to
* recover, so we don't allow failure here. Also, we allocate in a context that
* we don't want to be issuing transactions from, so we need to tell the
@@ -426,8 +374,7 @@ xlog_cil_committed(
*/
STATIC int
xlog_cil_push(
- struct log *log,
- xfs_lsn_t push_seq)
+ struct log *log)
{
struct xfs_cil *cil = log->l_cilp;
struct xfs_log_vec *lv;
@@ -443,39 +390,36 @@ xlog_cil_push(
struct xfs_log_iovec lhdr;
struct xfs_log_vec lvhdr = { NULL };
xfs_lsn_t commit_lsn;
+ xfs_lsn_t push_seq;

if (!cil)
return 0;

- ASSERT(!push_seq || push_seq <= cil->xc_ctx->sequence);
-
new_ctx = kmem_zalloc(sizeof(*new_ctx), KM_SLEEP|KM_NOFS);
new_ctx->ticket = xlog_cil_ticket_alloc(log);

- /*
- * Lock out transaction commit, but don't block for background pushes
- * unless we are well over the CIL space limit. See the definition of
- * XLOG_CIL_HARD_SPACE_LIMIT() for the full explanation of the logic
- * used here.
- */
- if (!down_write_trylock(&cil->xc_ctx_lock)) {
- if (!push_seq &&
- cil->xc_ctx->space_used < XLOG_CIL_HARD_SPACE_LIMIT(log))
- goto out_free_ticket;
- down_write(&cil->xc_ctx_lock);
- }
+ down_write(&cil->xc_ctx_lock);
ctx = cil->xc_ctx;

- /* check if we've anything to push */
- if (list_empty(&cil->xc_cil))
- goto out_skip;
+ spin_lock(&cil->xc_cil_lock);
+ push_seq = cil->xc_push_seq;
+ ASSERT(push_seq <= ctx->sequence);

- /* check for spurious background flush */
- if (!push_seq && cil->xc_ctx->space_used < XLOG_CIL_SPACE_LIMIT(log))
+ /*
+ * Check if we've anything to push. If there is nothing, then we don't
+ * move on to a new sequence number and so we have to be able to push
+ * this sequence again later.
+ */
+ if (list_empty(&cil->xc_cil)) {
+ cil->xc_push_seq = 0;
+ spin_unlock(&cil->xc_cil_lock);
goto out_skip;
+ }
+ spin_unlock(&cil->xc_cil_lock);
+

/* check for a previously pushed seqeunce */
- if (push_seq && push_seq < cil->xc_ctx->sequence)
+ if (push_seq < cil->xc_ctx->sequence)
goto out_skip;

/*
@@ -629,7 +573,6 @@ restart:

out_skip:
up_write(&cil->xc_ctx_lock);
-out_free_ticket:
xfs_log_ticket_put(new_ctx->ticket);
kmem_free(new_ctx);
return 0;
@@ -641,6 +584,82 @@ out_abort:
return XFS_ERROR(EIO);
}

+static void
+xlog_cil_push_work(
+ struct work_struct *work)
+{
+ struct xfs_cil *cil = container_of(work, struct xfs_cil,
+ xc_push_work);
+ xlog_cil_push(cil->xc_log);
+}
+
+/*
+ * We need to push CIL every so often so we don't cache more than we can fit in
+ * the log. The limit really is that a checkpoint can't be more than half the
+ * log (the current checkpoint is not allowed to overwrite the previous
+ * checkpoint), but commit latency and memory usage limit this to a smaller
+ * size.
+ */
+static void
+xlog_cil_push_background(
+ struct log *log)
+{
+ struct xfs_cil *cil = log->l_cilp;
+
+ /*
+ * The cil won't be empty because we are called while holding the
+ * context lock so whatever we added to the CIL will still be there
+ */
+ ASSERT(!list_empty(&cil->xc_cil));
+
+ /*
+ * don't do a background push if we haven't used up all the
+ * space available yet.
+ */
+ if (cil->xc_ctx->space_used < XLOG_CIL_SPACE_LIMIT(log))
+ return;
+
+ spin_lock(&cil->xc_cil_lock);
+ if (cil->xc_push_seq < cil->xc_current_sequence) {
+ cil->xc_push_seq = cil->xc_current_sequence;
+ queue_work(log->l_mp->m_cil_workqueue, &cil->xc_push_work);
+ }
+ spin_unlock(&cil->xc_cil_lock);
+
+}
+
+static void
+xlog_cil_push_foreground(
+ struct log *log,
+ xfs_lsn_t push_seq)
+{
+ struct xfs_cil *cil = log->l_cilp;
+
+ if (!cil)
+ return;
+
+ ASSERT(push_seq && push_seq <= cil->xc_current_sequence);
+
+ /* start on any pending background push to minimise wait time on it */
+ flush_work(&cil->xc_push_work);
+
+ /*
+ * If the CIL is empty or we've already pushed the sequence then
+ * there's no work we need to do.
+ */
+ spin_lock(&cil->xc_cil_lock);
+ if (list_empty(&cil->xc_cil) || push_seq <= cil->xc_push_seq) {
+ spin_unlock(&cil->xc_cil_lock);
+ return;
+ }
+
+ cil->xc_push_seq = push_seq;
+ spin_unlock(&cil->xc_cil_lock);
+
+ /* do the push now */
+ xlog_cil_push(log);
+}
+
/*
* Commit a transaction with the given vector to the Committed Item List.
*
@@ -667,7 +686,6 @@ xfs_log_commit_cil(
{
struct log *log = mp->m_log;
int log_flags = 0;
- int push = 0;
struct xfs_log_vec *log_vector;

if (flags & XFS_TRANS_RELEASE_LOG_RES)
@@ -719,21 +737,9 @@ xfs_log_commit_cil(
*/
xfs_trans_free_items(tp, *commit_lsn, 0);

- /* check for background commit before unlock */
- if (log->l_cilp->xc_ctx->space_used > XLOG_CIL_SPACE_LIMIT(log))
- push = 1;
+ xlog_cil_push_background(log);

up_read(&log->l_cilp->xc_ctx_lock);
-
- /*
- * We need to push CIL every so often so we don't cache more than we
- * can fit in the log. The limit really is that a checkpoint can't be
- * more than half the log (the current checkpoint is not allowed to
- * overwrite the previous checkpoint), but commit latency and memory
- * usage limit this to a smaller size in most cases.
- */
- if (push)
- xlog_cil_push(log, 0);
return 0;
}

@@ -746,9 +752,6 @@ xfs_log_commit_cil(
*
* We return the current commit lsn to allow the callers to determine if a
* iclog flush is necessary following this call.
- *
- * XXX: Initially, just push the CIL unconditionally and return whatever
- * commit lsn is there. It'll be empty, so this is broken for now.
*/
xfs_lsn_t
xlog_cil_force_lsn(
@@ -766,8 +769,7 @@ xlog_cil_force_lsn(
* xlog_cil_push() handles racing pushes for the same sequence,
* so no need to deal with it here.
*/
- if (sequence == cil->xc_current_sequence)
- xlog_cil_push(log, sequence);
+ xlog_cil_push_foreground(log, sequence);

/*
* See if we can find a previous sequence still committing.
@@ -826,3 +828,57 @@ xfs_log_item_in_current_chkpt(
return false;
return true;
}
+
+/*
+ * Perform initial CIL structure initialisation.
+ */
+int
+xlog_cil_init(
+ struct log *log)
+{
+ struct xfs_cil *cil;
+ struct xfs_cil_ctx *ctx;
+
+ cil = kmem_zalloc(sizeof(*cil), KM_SLEEP|KM_MAYFAIL);
+ if (!cil)
+ return ENOMEM;
+
+ ctx = kmem_zalloc(sizeof(*ctx), KM_SLEEP|KM_MAYFAIL);
+ if (!ctx) {
+ kmem_free(cil);
+ return ENOMEM;
+ }
+
+ INIT_WORK(&cil->xc_push_work, xlog_cil_push_work);
+ INIT_LIST_HEAD(&cil->xc_cil);
+ INIT_LIST_HEAD(&cil->xc_committing);
+ spin_lock_init(&cil->xc_cil_lock);
+ init_rwsem(&cil->xc_ctx_lock);
+ init_waitqueue_head(&cil->xc_commit_wait);
+
+ INIT_LIST_HEAD(&ctx->committing);
+ INIT_LIST_HEAD(&ctx->busy_extents);
+ ctx->sequence = 1;
+ ctx->cil = cil;
+ cil->xc_ctx = ctx;
+ cil->xc_current_sequence = ctx->sequence;
+
+ cil->xc_log = log;
+ log->l_cilp = cil;
+ return 0;
+}
+
+void
+xlog_cil_destroy(
+ struct log *log)
+{
+ if (log->l_cilp->xc_ctx) {
+ if (log->l_cilp->xc_ctx->ticket)
+ xfs_log_ticket_put(log->l_cilp->xc_ctx->ticket);
+ kmem_free(log->l_cilp->xc_ctx);
+ }
+
+ ASSERT(list_empty(&log->l_cilp->xc_cil));
+ kmem_free(log->l_cilp);
+}
+
diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
index 2152900..735ff1e 100644
--- a/fs/xfs/xfs_log_priv.h
+++ b/fs/xfs/xfs_log_priv.h
@@ -417,6 +417,8 @@ struct xfs_cil {
struct list_head xc_committing;
wait_queue_head_t xc_commit_wait;
xfs_lsn_t xc_current_sequence;
+ struct work_struct xc_push_work;
+ xfs_lsn_t xc_push_seq;
};

/*
diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index dbd9d42..401ca2e 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -214,6 +214,7 @@ typedef struct xfs_mount {

struct workqueue_struct *m_data_workqueue;
struct workqueue_struct *m_unwritten_workqueue;
+ struct workqueue_struct *m_cil_workqueue;
} xfs_mount_t;

/*
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index eb19efb..21db0d5 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -773,8 +773,14 @@ xfs_init_mount_workqueues(
if (!mp->m_unwritten_workqueue)
goto out_destroy_data_iodone_queue;

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

+out_destroy_unwritten:
+ destroy_workqueue(mp->m_unwritten_workqueue);
out_destroy_data_iodone_queue:
destroy_workqueue(mp->m_data_workqueue);
out:
@@ -785,6 +791,7 @@ STATIC void
xfs_destroy_mount_workqueues(
struct xfs_mount *mp)
{
+ destroy_workqueue(mp->m_cil_workqueue);
destroy_workqueue(mp->m_data_workqueue);
destroy_workqueue(mp->m_unwritten_workqueue);
}