[dm-devel] [RFC PATCH 4/4] dm mpath: eliminate use of spinlock in IO fast-paths
Mike Snitzer
snitzer at redhat.com
Thu Mar 31 20:04:26 UTC 2016
The primary motivation of this commit is to improve the scalability of
DM multipath on large NUMA systems where m->lock spinlock contention has
been proven to be a serious bottleneck on really fast storage.
The ability to atomically read a pointer, using lockless_dereference(),
is leveraged in this commit. But all pointer writes are still protected
by the m->lock spinlock (which is fine since these all now occur in the
slow-path).
The following functions no longer require the m->lock spinlock in their
fast-path: multipath_busy(), __multipath_map(), and do_end_io()
And choose_pgpath() is modified to _not_ update m->current_pgpath unless
it also switches the path-group. This is done to avoid needing to take
the m->lock everytime __multipath_map() calls choose_pgpath().
But m->current_pgpath will be reset if it is failed via fail_path().
Suggested-by: Jeff Moyer <jmoyer at redhat.com>
Signed-off-by: Mike Snitzer <snitzer at redhat.com>
---
drivers/md/dm-mpath.c | 170 +++++++++++++++++++++++++++-----------------------
1 file changed, 93 insertions(+), 77 deletions(-)
diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index 54daf96..52baf8a 100644
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -305,9 +305,21 @@ static int __pg_init_all_paths(struct multipath *m)
return atomic_read(&m->pg_init_in_progress);
}
-static void __switch_pg(struct multipath *m, struct pgpath *pgpath)
+static int pg_init_all_paths(struct multipath *m)
{
- m->current_pg = pgpath->pg;
+ int r;
+ unsigned long flags;
+
+ spin_lock_irqsave(&m->lock, flags);
+ r = __pg_init_all_paths(m);
+ spin_unlock_irqrestore(&m->lock, flags);
+
+ return r;
+}
+
+static void __switch_pg(struct multipath *m, struct priority_group *pg)
+{
+ m->current_pg = pg;
/* Must we initialise the PG first, and queue I/O till it's ready? */
if (m->hw_handler_name) {
@@ -321,26 +333,36 @@ static void __switch_pg(struct multipath *m, struct pgpath *pgpath)
atomic_set(&m->pg_init_count, 0);
}
-static int __choose_path_in_pg(struct multipath *m, struct priority_group *pg,
- size_t nr_bytes)
+static struct pgpath *choose_path_in_pg(struct multipath *m,
+ struct priority_group *pg,
+ size_t nr_bytes)
{
+ unsigned long flags;
struct dm_path *path;
+ struct pgpath *pgpath;
path = pg->ps.type->select_path(&pg->ps, nr_bytes);
if (!path)
- return -ENXIO;
+ return ERR_PTR(-ENXIO);
- m->current_pgpath = path_to_pgpath(path);
+ pgpath = path_to_pgpath(path);
- if (m->current_pg != pg)
- __switch_pg(m, m->current_pgpath);
+ if (unlikely(lockless_dereference(m->current_pg) != pg)) {
+ /* Only update current_pgpath if pg changed */
+ spin_lock_irqsave(&m->lock, flags);
+ m->current_pgpath = pgpath;
+ __switch_pg(m, pg);
+ spin_unlock_irqrestore(&m->lock, flags);
+ }
- return 0;
+ return pgpath;
}
-static void __choose_pgpath(struct multipath *m, size_t nr_bytes)
+static struct pgpath *choose_pgpath(struct multipath *m, size_t nr_bytes)
{
+ unsigned long flags;
struct priority_group *pg;
+ struct pgpath *pgpath;
bool bypassed = true;
if (!atomic_read(&m->nr_valid_paths)) {
@@ -349,16 +371,28 @@ static void __choose_pgpath(struct multipath *m, size_t nr_bytes)
}
/* Were we instructed to switch PG? */
- if (m->next_pg) {
+ if (lockless_dereference(m->next_pg)) {
+ spin_lock_irqsave(&m->lock, flags);
pg = m->next_pg;
+ if (!pg) {
+ spin_unlock_irqrestore(&m->lock, flags);
+ goto check_current_pg;
+ }
m->next_pg = NULL;
- if (!__choose_path_in_pg(m, pg, nr_bytes))
- return;
+ spin_unlock_irqrestore(&m->lock, flags);
+ pgpath = choose_path_in_pg(m, pg, nr_bytes);
+ if (!IS_ERR_OR_NULL(pgpath))
+ return pgpath;
}
/* Don't change PG until it has no remaining paths */
- if (m->current_pg && !__choose_path_in_pg(m, m->current_pg, nr_bytes))
- return;
+check_current_pg:
+ pg = lockless_dereference(m->current_pg);
+ if (pg) {
+ pgpath = choose_path_in_pg(m, pg, nr_bytes);
+ if (!IS_ERR_OR_NULL(pgpath))
+ return pgpath;
+ }
/*
* Loop through priority groups until we find a valid path.
@@ -370,31 +404,34 @@ static void __choose_pgpath(struct multipath *m, size_t nr_bytes)
list_for_each_entry(pg, &m->priority_groups, list) {
if (pg->bypassed == bypassed)
continue;
- if (!__choose_path_in_pg(m, pg, nr_bytes)) {
+ pgpath = choose_path_in_pg(m, pg, nr_bytes);
+ if (!IS_ERR_OR_NULL(pgpath)) {
if (!bypassed)
set_bit(MPATHF_PG_INIT_DELAY_RETRY, &m->flags);
- return;
+ return pgpath;
}
}
} while (bypassed--);
failed:
+ spin_lock_irqsave(&m->lock, flags);
m->current_pgpath = NULL;
m->current_pg = NULL;
+ spin_unlock_irqrestore(&m->lock, flags);
+
+ return NULL;
}
/*
* Check whether bios must be queued in the device-mapper core rather
* than here in the target.
*
- * m->lock must be held on entry.
- *
* If m->queue_if_no_path and m->saved_queue_if_no_path hold the
* same value then we are not between multipath_presuspend()
* and multipath_resume() calls and we have no need to check
* for the DMF_NOFLUSH_SUSPENDING flag.
*/
-static int __must_push_back(struct multipath *m)
+static int must_push_back(struct multipath *m)
{
return (test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags) ||
((test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags) !=
@@ -416,36 +453,31 @@ static int __multipath_map(struct dm_target *ti, struct request *clone,
struct block_device *bdev;
struct dm_mpath_io *mpio;
- spin_lock_irq(&m->lock);
-
/* Do we need to select a new pgpath? */
- if (!m->current_pgpath || !test_bit(MPATHF_QUEUE_IO, &m->flags))
- __choose_pgpath(m, nr_bytes);
-
- pgpath = m->current_pgpath;
+ pgpath = lockless_dereference(m->current_pgpath);
+ if (!pgpath || !test_bit(MPATHF_QUEUE_IO, &m->flags))
+ pgpath = choose_pgpath(m, nr_bytes);
if (!pgpath) {
- if (!__must_push_back(m))
+ if (!must_push_back(m))
r = -EIO; /* Failed */
- goto out_unlock;
+ return r;
} else if (test_bit(MPATHF_QUEUE_IO, &m->flags) ||
test_bit(MPATHF_PG_INIT_REQUIRED, &m->flags)) {
- __pg_init_all_paths(m);
- goto out_unlock;
+ pg_init_all_paths(m);
+ return r;
}
mpio = set_mpio(m, map_context);
if (!mpio)
/* ENOMEM, requeue */
- goto out_unlock;
+ return r;
mpio->pgpath = pgpath;
mpio->nr_bytes = nr_bytes;
bdev = pgpath->path.dev->bdev;
- spin_unlock_irq(&m->lock);
-
if (clone) {
/*
* Old request-based interface: allocated clone is passed in.
@@ -477,11 +509,6 @@ static int __multipath_map(struct dm_target *ti, struct request *clone,
&pgpath->path,
nr_bytes);
return DM_MAPIO_REMAPPED;
-
-out_unlock:
- spin_unlock_irq(&m->lock);
-
- return r;
}
static int multipath_map(struct dm_target *ti, struct request *clone,
@@ -1308,7 +1335,6 @@ static int do_end_io(struct multipath *m, struct request *clone,
* clone bios for it and resubmit it later.
*/
int r = DM_ENDIO_REQUEUE;
- unsigned long flags;
if (!error && !clone->errors)
return 0; /* I/O complete */
@@ -1319,17 +1345,15 @@ static int do_end_io(struct multipath *m, struct request *clone,
if (mpio->pgpath)
fail_path(mpio->pgpath);
- spin_lock_irqsave(&m->lock, flags);
if (!atomic_read(&m->nr_valid_paths)) {
if (!test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags)) {
- if (!__must_push_back(m))
+ if (!must_push_back(m))
r = -EIO;
} else {
if (error == -EBADE)
r = error;
}
}
- spin_unlock_irqrestore(&m->lock, flags);
return r;
}
@@ -1586,18 +1610,17 @@ static int multipath_prepare_ioctl(struct dm_target *ti,
struct block_device **bdev, fmode_t *mode)
{
struct multipath *m = ti->private;
- unsigned long flags;
+ struct pgpath *current_pgpath;
int r;
- spin_lock_irqsave(&m->lock, flags);
-
- if (!m->current_pgpath)
- __choose_pgpath(m, 0);
+ current_pgpath = lockless_dereference(m->current_pgpath);
+ if (!current_pgpath)
+ current_pgpath = choose_pgpath(m, 0);
- if (m->current_pgpath) {
+ if (current_pgpath) {
if (!test_bit(MPATHF_QUEUE_IO, &m->flags)) {
- *bdev = m->current_pgpath->path.dev->bdev;
- *mode = m->current_pgpath->path.dev->mode;
+ *bdev = current_pgpath->path.dev->bdev;
+ *mode = current_pgpath->path.dev->mode;
r = 0;
} else {
/* pg_init has not started or completed */
@@ -1611,17 +1634,13 @@ static int multipath_prepare_ioctl(struct dm_target *ti,
r = -EIO;
}
- spin_unlock_irqrestore(&m->lock, flags);
-
if (r == -ENOTCONN) {
- spin_lock_irqsave(&m->lock, flags);
- if (!m->current_pg) {
+ if (!lockless_dereference(m->current_pg)) {
/* Path status changed, redo selection */
- __choose_pgpath(m, 0);
+ (void) choose_pgpath(m, 0);
}
if (test_bit(MPATHF_PG_INIT_REQUIRED, &m->flags))
- __pg_init_all_paths(m);
- spin_unlock_irqrestore(&m->lock, flags);
+ pg_init_all_paths(m);
dm_table_run_md_queue_async(m->ti->table);
}
@@ -1672,39 +1691,37 @@ static int multipath_busy(struct dm_target *ti)
{
bool busy = false, has_active = false;
struct multipath *m = ti->private;
- struct priority_group *pg;
+ struct priority_group *pg, *next_pg;
struct pgpath *pgpath;
- unsigned long flags;
-
- spin_lock_irqsave(&m->lock, flags);
/* pg_init in progress or no paths available */
if (atomic_read(&m->pg_init_in_progress) ||
- (!atomic_read(&m->nr_valid_paths) && test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags))) {
- busy = true;
- goto out;
- }
+ (!atomic_read(&m->nr_valid_paths) && test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags)))
+ return true;
+
/* Guess which priority_group will be used at next mapping time */
- if (unlikely(!m->current_pgpath && m->next_pg))
- pg = m->next_pg;
- else if (likely(m->current_pg))
- pg = m->current_pg;
- else
+ pg = lockless_dereference(m->current_pg);
+ next_pg = lockless_dereference(m->next_pg);
+ if (unlikely(!lockless_dereference(m->current_pgpath) && next_pg))
+ pg = next_pg;
+
+ if (!pg) {
/*
* We don't know which pg will be used at next mapping time.
- * We don't call __choose_pgpath() here to avoid to trigger
+ * We don't call choose_pgpath() here to avoid to trigger
* pg_init just by busy checking.
* So we don't know whether underlying devices we will be using
* at next mapping time are busy or not. Just try mapping.
*/
- goto out;
+ return busy;
+ }
/*
* If there is one non-busy active path at least, the path selector
* will be able to select it. So we consider such a pg as not busy.
*/
busy = true;
- list_for_each_entry(pgpath, &pg->pgpaths, list)
+ list_for_each_entry(pgpath, &pg->pgpaths, list) {
if (pgpath->is_active) {
has_active = true;
if (!pgpath_busy(pgpath)) {
@@ -1712,17 +1729,16 @@ static int multipath_busy(struct dm_target *ti)
break;
}
}
+ }
- if (!has_active)
+ if (!has_active) {
/*
* No active path in this pg, so this pg won't be used and
* the current_pg will be changed at next mapping time.
* We need to try mapping to determine it.
*/
busy = false;
-
-out:
- spin_unlock_irqrestore(&m->lock, flags);
+ }
return busy;
}
--
2.6.4 (Apple Git-63)
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