[PATCH 06/13] writeback: support > 1 flusher thread per bdi

From: Jens Axboe
Date: Wed Apr 08 2009 - 08:08:55 EST

Build on the bdi_writeback support by allowing registration of
more than 1 flusher thread. File systems can call bdi_add_flusher_task(bdi)
to add more flusher threads to the device. If they do so, they must also
provide a super_operations function to return the suitable bdi_writeback
struct from any given inode.

Signed-off-by: Jens Axboe <jens.axboe@xxxxxxxxxx>
---
fs/fs-writeback.c | 47 ++++++++++-
include/linux/backing-dev.h | 15 ++++
include/linux/fs.h | 3 +
mm/backing-dev.c | 185 +++++++++++++++++++++++++++++++++++--------
4 files changed, 211 insertions(+), 39 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 2ec8569..756714f 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -98,16 +98,30 @@ static void wb_start_writeback(struct bdi_writeback *wb, struct super_block *sb,
int bdi_start_writeback(struct backing_dev_info *bdi, struct super_block *sb,
long nr_pages)
{
+ struct bdi_writeback *wb;
+
/*
* This only happens the first time someone kicks this bdi, so put
* it out-of-line.
*/
- if (unlikely(!bdi->wb.task)) {
+ if (unlikely(list_empty_careful(&bdi->wb_list))) {
bdi_add_default_flusher_task(bdi);
return 1;
}

- wb_start_writeback(&bdi->wb, sb, nr_pages);
+ if (!bdi_wblist_needs_lock(bdi))
+ wb_start_writeback(&bdi->wb, sb, nr_pages);
+ else {
+ int idx;
+
+ idx = srcu_read_lock(&bdi->srcu);
+
+ list_for_each_entry_rcu(wb, &bdi->wb_list, list)
+ wb_start_writeback(wb, sb, nr_pages);
+
+ srcu_read_unlock(&bdi->srcu, idx);
+ }
+
return 0;
}

@@ -261,11 +275,18 @@ restart:
}

/*
- * We have only a single wb per bdi, so just return that.
+ * If the filesystem didn't provide a way to map an inode to a dedicated
+ * flusher thread, it doesn't support more than 1 thread. So we know it's
+ * the default thread, return that.
*/
static inline struct bdi_writeback *inode_get_wb(struct inode *inode)
{
- return &inode_to_bdi(inode)->wb;
+ const struct super_operations *sop = inode->i_sb->s_op;
+
+ if (!sop->inode_get_wb)
+ return &inode_to_bdi(inode)->wb;
+
+ return sop->inode_get_wb(inode);
}

/**
@@ -719,8 +740,24 @@ void generic_sync_bdi_inodes(struct super_block *sb,
struct writeback_control *wbc)
{
struct backing_dev_info *bdi = wbc->bdi;
+ struct bdi_writeback *wb;
+
+ /*
+ * Common case is just a single wb thread and that is embedded in
+ * the bdi, so it doesn't need locking
+ */
+ if (!bdi_wblist_needs_lock(bdi))
+ generic_sync_wb_inodes(&bdi->wb, sb, wbc);
+ else {
+ int idx;

- generic_sync_wb_inodes(&bdi->wb, sb, wbc);
+ idx = srcu_read_lock(&bdi->srcu);
+
+ list_for_each_entry_rcu(wb, &bdi->wb_list, list)
+ generic_sync_wb_inodes(wb, sb, wbc);
+
+ srcu_read_unlock(&bdi->srcu, idx);
+ }
}

/*
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index a0c70f1..c596bf6 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -13,6 +13,7 @@
#include <linux/proportions.h>
#include <linux/kernel.h>
#include <linux/fs.h>
+#include <linux/srcu.h>
#include <asm/atomic.h>

struct page;
@@ -25,6 +26,7 @@ struct dentry;
enum bdi_state {
BDI_pending, /* On its way to being activated */
BDI_wb_alloc, /* Default embedded wb allocated */
+ BDI_wblist_lock, /* bdi->wb_list now needs locking */
BDI_async_congested, /* The async (write) queue is getting full */
BDI_sync_congested, /* The sync queue is getting full */
BDI_unused, /* Available bits start here */
@@ -41,6 +43,8 @@ enum bdi_stat_item {
#define BDI_STAT_BATCH (8*(1+ilog2(nr_cpu_ids)))

struct bdi_writeback {
+ struct list_head list; /* hangs off the bdi */
+
struct backing_dev_info *bdi; /* our parent bdi */
unsigned int nr;

@@ -54,8 +58,11 @@ struct bdi_writeback {
struct super_block *sb;
};

+#define BDI_MAX_FLUSHERS 32
+
struct backing_dev_info {
struct rcu_head rcu_head;
+ struct srcu_struct srcu; /* for wb_list read side protection */
struct list_head bdi_list;
unsigned long ra_pages; /* max readahead in PAGE_CACHE_SIZE units */
unsigned long state; /* Always use atomic bitops on this */
@@ -74,6 +81,8 @@ struct backing_dev_info {
unsigned int max_ratio, max_prop_frac;

struct bdi_writeback wb; /* default writeback info for this bdi */
+ spinlock_t wb_lock; /* protects update side of wb_list */
+ struct list_head wb_list; /* the flusher threads hanging off this bdi */
unsigned long wb_active; /* bitmap of active tasks */
unsigned long wb_mask; /* number of registered tasks */

@@ -97,11 +106,17 @@ int bdi_start_writeback(struct backing_dev_info *bdi, struct super_block *sb,
int bdi_writeback_task(struct bdi_writeback *wb);
void bdi_writeback_all(struct super_block *sb, long nr_pages);
void bdi_add_default_flusher_task(struct backing_dev_info *bdi);
+void bdi_add_flusher_task(struct backing_dev_info *bdi);
int bdi_has_dirty_io(struct backing_dev_info *bdi);

extern spinlock_t bdi_lock;
extern struct list_head bdi_list;

+static inline int bdi_wblist_needs_lock(struct backing_dev_info *bdi)
+{
+ return test_bit(BDI_wblist_lock, &bdi->state);
+}
+
static inline int wb_has_dirty_io(struct bdi_writeback *wb)
{
return !list_empty(&wb->b_dirty) ||
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 99a9986..9e25cbb 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1477,11 +1477,14 @@ extern ssize_t vfs_readv(struct file *, const struct iovec __user *,
extern ssize_t vfs_writev(struct file *, const struct iovec __user *,
unsigned long, loff_t *);

+struct bdi_writeback;
+
struct super_operations {
struct inode *(*alloc_inode)(struct super_block *sb);
void (*destroy_inode)(struct inode *);

void (*dirty_inode) (struct inode *);
+ struct bdi_writeback *(*inode_get_wb) (struct inode *);
int (*write_inode) (struct inode *, int);
void (*drop_inode) (struct inode *);
void (*delete_inode) (struct inode *);
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 677a8c6..4ebcc49 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -225,24 +225,48 @@ static void bdi_flush_io(struct backing_dev_info *bdi)

static int wb_assign_nr(struct backing_dev_info *bdi, struct bdi_writeback *wb)
{
- set_bit(0, &bdi->wb_mask);
- wb->nr = 0;
+ unsigned long mask = BDI_MAX_FLUSHERS - 1;
+ unsigned int nr;
+
+ do {
+ if ((bdi->wb_mask & mask) == mask)
+ return 1;
+
+ nr = find_first_zero_bit(&bdi->wb_mask, BDI_MAX_FLUSHERS);
+ } while (test_and_set_bit(nr, &bdi->wb_mask));
+
+ wb->nr = nr;
return 0;
}

static void bdi_put_wb(struct backing_dev_info *bdi, struct bdi_writeback *wb)
{
clear_bit(wb->nr, &bdi->wb_mask);
- clear_bit(BDI_wb_alloc, &bdi->state);
+
+ if (wb == &bdi->wb)
+ clear_bit(BDI_wb_alloc, &bdi->state);
+ else
+ kfree(wb);
}

static struct bdi_writeback *bdi_new_wb(struct backing_dev_info *bdi)
{
struct bdi_writeback *wb;

- set_bit(BDI_wb_alloc, &bdi->state);
- wb = &bdi->wb;
- wb_assign_nr(bdi, wb);
+ if (!test_and_set_bit(BDI_wb_alloc, &bdi->state)) {
+ wb = &bdi->wb;
+ wb_assign_nr(bdi, wb);
+ } else {
+ wb = kmalloc(sizeof(struct bdi_writeback), GFP_KERNEL);
+ if (wb) {
+ bdi_wb_init(wb, bdi);
+ if (wb_assign_nr(bdi, wb)) {
+ kfree(wb);
+ wb = NULL;
+ }
+ }
+ }
+
return wb;
}

@@ -251,7 +275,22 @@ static int bdi_start_fn(void *ptr)
struct bdi_writeback *wb = ptr;
struct backing_dev_info *bdi = wb->bdi;
struct task_struct *tsk = current;
- int ret;
+ int was_empty, ret;
+
+ /*
+ * Add us to the active bdi_list. If we are adding threads beyond
+ * the default embedded bdi_writeback, then we need to start using
+ * proper locking. Check the list for empty first, then set the
+ * BDI_wblist_lock flag if there's > 1 entry on the list now
+ */
+ spin_lock(&bdi->wb_lock);
+
+ was_empty = list_empty(&bdi->wb_list);
+ list_add_tail_rcu(&wb->list, &bdi->wb_list);
+ if (!was_empty)
+ set_bit(BDI_wblist_lock, &bdi->state);
+
+ spin_unlock(&bdi->wb_lock);

tsk->flags |= PF_FLUSHER | PF_SWAPWRITE;
set_freezable();
@@ -269,13 +308,44 @@ static int bdi_start_fn(void *ptr)

ret = bdi_writeback_task(wb);

+ /*
+ * Remove us from the list
+ */
+ spin_lock(&bdi->wb_lock);
+ list_del_rcu(&wb->list);
+ spin_unlock(&bdi->wb_lock);
+
+ /*
+ * wait for rcu grace period to end, so we can free wb
+ */
+ synchronize_srcu(&bdi->srcu);
+
bdi_put_wb(bdi, wb);
return ret;
}

int bdi_has_dirty_io(struct backing_dev_info *bdi)
{
- return wb_has_dirty_io(&bdi->wb);
+ struct bdi_writeback *wb;
+ int ret = 0;
+
+ if (!bdi_wblist_needs_lock(bdi))
+ ret = wb_has_dirty_io(&bdi->wb);
+ else {
+ int idx;
+
+ idx = srcu_read_lock(&bdi->srcu);
+
+ list_for_each_entry_rcu(wb, &bdi->wb_list, list) {
+ ret = wb_has_dirty_io(wb);
+ if (ret)
+ break;
+ }
+
+ srcu_read_unlock(&bdi->srcu, idx);
+ }
+
+ return ret;
}

static int bdi_forker_task(void *ptr)
@@ -367,33 +437,73 @@ static void bdi_add_to_pending(struct rcu_head *head)
wake_up(&default_backing_dev_info.wb.wait);
}

-/*
- * Add a new flusher task that gets created for any bdi
- * that has dirty data pending writeout
- */
-void bdi_add_default_flusher_task(struct backing_dev_info *bdi)
+static int sched_wait(void *word)
+{
+ schedule();
+ return 0;
+}
+
+static void bdi_add_one_flusher_task(struct backing_dev_info *bdi,
+ int(*func)(struct backing_dev_info *))
{
if (!bdi_cap_writeback_dirty(bdi))
return;

/*
- * Someone already marked this pending for task creation
+ * Check with the helper whether to proceed adding a task. Will only
+ * abort if we two or more simultanous calls to
+ * bdi_add_default_flusher_task() occured, further additions will block
+ * waiting for previous additions to finish.
*/
- if (test_and_set_bit(BDI_pending, &bdi->state))
- return;
+ if (!func(bdi)) {
+ spin_lock_bh(&bdi_lock);
+ list_del_rcu(&bdi->bdi_list);
+ spin_unlock_bh(&bdi_lock);

- spin_lock_bh(&bdi_lock);
- list_del_rcu(&bdi->bdi_list);
- spin_unlock_bh(&bdi_lock);
+ /*
+ * We need to wait for the current grace period to end,
+ * in case others were browsing the bdi_list as well.
+ * So defer the adding and wakeup to after the RCU
+ * grace period has ended.
+ */
+ call_rcu(&bdi->rcu_head, bdi_add_to_pending);
+ }
+}

- /*
- * We need to wait for the current grace period to end,
- * in case others were browsing the bdi_list as well.
- * So defer the adding and wakeup to after the RCU
- * grace period has ended.
- */
- call_rcu(&bdi->rcu_head, bdi_add_to_pending);
+static int flusher_add_helper_block(struct backing_dev_info *bdi)
+{
+ wait_on_bit_lock(&bdi->state, BDI_pending, sched_wait,
+ TASK_UNINTERRUPTIBLE);
+ return 0;
+}
+
+static int flusher_add_helper_test(struct backing_dev_info *bdi)
+{
+ return test_and_set_bit(BDI_pending, &bdi->state);
+}
+
+/*
+ * Add the default flusher task that gets created for any bdi
+ * that has dirty data pending writeout
+ */
+void bdi_add_default_flusher_task(struct backing_dev_info *bdi)
+{
+ bdi_add_one_flusher_task(bdi, flusher_add_helper_test);
+}
+
+/**
+ * bdi_add_flusher_task - add one more flusher task to this @bdi
+ * @bdi: the bdi
+ *
+ * Add an additional flusher task to this @bdi. Will block waiting on
+ * previous additions, if any.
+ *
+ */
+void bdi_add_flusher_task(struct backing_dev_info *bdi)
+{
+ bdi_add_one_flusher_task(bdi, flusher_add_helper_block);
}
+EXPORT_SYMBOL(bdi_add_flusher_task);

int bdi_register(struct backing_dev_info *bdi, struct device *parent,
const char *fmt, ...)
@@ -454,17 +564,13 @@ int bdi_register_dev(struct backing_dev_info *bdi, dev_t dev)
}
EXPORT_SYMBOL(bdi_register_dev);

-static int sched_wait(void *word)
-{
- schedule();
- return 0;
-}
-
/*
* Remove bdi from global list and shutdown any threads we have running
*/
static void bdi_wb_shutdown(struct backing_dev_info *bdi)
{
+ struct bdi_writeback *wb;
+
if (!bdi_cap_writeback_dirty(bdi))
return;

@@ -488,9 +594,11 @@ static void bdi_wb_shutdown(struct backing_dev_info *bdi)
synchronize_rcu();

/*
- * Finally, kill the kernel thread
+ * Finally, kill the kernel threads. We don't need to be RCU
+ * safe anymore, since the bdi is gone from visibility.
*/
- kthread_stop(bdi->wb.task);
+ list_for_each_entry(wb, &bdi->wb_list, list)
+ kthread_stop(wb->task);
}

void bdi_unregister(struct backing_dev_info *bdi)
@@ -515,7 +623,9 @@ int bdi_init(struct backing_dev_info *bdi)
bdi->min_ratio = 0;
bdi->max_ratio = 100;
bdi->max_prop_frac = PROP_FRAC_BASE;
+ spin_lock_init(&bdi->wb_lock);
INIT_LIST_HEAD(&bdi->bdi_list);
+ INIT_LIST_HEAD(&bdi->wb_list);
bdi->wb_mask = bdi->wb_active = 0;

bdi_wb_init(&bdi->wb, bdi);
@@ -526,10 +636,15 @@ int bdi_init(struct backing_dev_info *bdi)
goto err;
}

+ err = init_srcu_struct(&bdi->srcu);
+ if (err)
+ goto err;
+
bdi->dirty_exceeded = 0;
err = prop_local_init_percpu(&bdi->completions);

if (err) {
+ cleanup_srcu_struct(&bdi->srcu);
err:
while (i--)
percpu_counter_destroy(&bdi->bdi_stat[i]);
@@ -547,6 +662,8 @@ void bdi_destroy(struct backing_dev_info *bdi)

bdi_unregister(bdi);

+ cleanup_srcu_struct(&bdi->srcu);
+
for (i = 0; i < NR_BDI_STAT_ITEMS; i++)
percpu_counter_destroy(&bdi->bdi_stat[i]);

--
1.6.2.2.446.gfbdc0

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