[PATCH v3 3/3] writeback: use a per-sb counter to drain inode wb switches at umount

[Baokun Li <libaokun@linux.alibaba.com>]

Tracking in-flight inode wb switches with a single global counter
(isw_nr_in_flight) plus a synchronize_rcu() based wait in
cgroup_writeback_umount() forces every umount to take a global hit
whenever any other superblock on the system has wb switches in flight,
even if the superblock being unmounted has none of its own.

Replace the global synchronize_rcu()/flush_workqueue() pair with a
per-sb counter, s_isw_nr_in_flight, plus three small helpers:

  - cgroup_writeback_pin(sb)   - increment counter
  - cgroup_writeback_unpin(sb) - decrement and wake drainer if last
  - cgroup_writeback_drain(sb) - wait for counter to reach zero

The wiring is:

  - inode_prepare_wbs_switch() pins before checking SB_ACTIVE and
    grabbing the inode; failure paths unpin before returning.  A
    lockless SB_ACTIVE check at the top of the function lets us skip
    the atomic_inc/smp_mb dance once SB_ACTIVE has been cleared (it
    is monotonic and never set back).
  - process_inode_switch_wbs() unpins after the matching iput().
  - cgroup_writeback_umount() drains the per-sb counter via
    wait_var_event().

The smp_mb() pair between inode_prepare_wbs_switch() and
cgroup_writeback_umount() keeps the SB_ACTIVE / counter ordering:
either the umounter sees a non-zero counter and waits, or the
switcher sees SB_ACTIVE cleared and aborts before grabbing the
inode.

The global isw_nr_in_flight is left in place, since it is still used
to throttle in-flight switches via WB_FRN_MAX_IN_FLIGHT.

The rcu_read_lock() extension in inode_switch_wbs() and
cleanup_offline_cgwb() that the race fix added is no longer needed
and is reverted; the synchronize_rcu() that the race fix added to
cgroup_writeback_umount() is dropped as well.

The following numbers were measured on a 16 vCPU QEMU guest with 4
background superblocks each churning "create memcg -> write 1 MiB ->
rmdir memcg" to keep the global isw_nr_in_flight non-zero.  Latencies
are wall-clock around umount(8); only the target sb's umount is
measured.

Target sb runs its own cgwb churn:

                              p50      p95      p99      max
  global synchronize_rcu()   67.6 ms  88.3 ms  88.3 ms  96.8 ms
  per-sb counter (this)       7.9 ms  10.0 ms  10.0 ms  10.1 ms

Idle target umount latency under cross-sb cgwb-switch pressure:

                              p50      p95      p99      max
  global synchronize_rcu()   62.7 ms  95.4 ms 108.1 ms 108.6 ms
  per-sb counter (this)       5.3 ms   6.9 ms   7.4 ms   7.4 ms
  no-pressure baseline        4.9 ms   5.9 ms   6.3 ms   6.7 ms

8 concurrent umounts of idle sbs under the same pressure:

                              p50      p95      max
  global synchronize_rcu()   61.3 ms  99.5 ms 113.7 ms
  per-sb counter (this)       8.1 ms   9.1 ms   9.5 ms

In-kernel cgroup_writeback_umount() time across the same run
(bpftrace, ~340 calls covering all scenarios):

  global synchronize_rcu()    12371 ms total (~36 ms / call)
  per-sb counter (this)        1.37 ms total ( ~4 us / call)

Suggested-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/177910456953.488929.2169908940676707307.b4-review@b4
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
 fs/fs-writeback.c              | 87 ++++++++++++++++++----------------
 include/linux/fs/super_types.h |  8 ++++
 2 files changed, 54 insertions(+), 41 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 325a30cc35bf..32fec4b9094e 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -497,6 +497,23 @@ static bool inode_do_switch_wbs(struct inode *inode,
 	return switched;
 }
 
+static inline void cgroup_writeback_pin(struct super_block *sb)
+{
+	atomic_inc(&sb->s_isw_nr_in_flight);
+}
+
+static inline void cgroup_writeback_unpin(struct super_block *sb)
+{
+	if (atomic_dec_and_test(&sb->s_isw_nr_in_flight))
+		wake_up_var(&sb->s_isw_nr_in_flight);
+}
+
+static inline void cgroup_writeback_drain(struct super_block *sb)
+{
+	wait_var_event(&sb->s_isw_nr_in_flight,
+		       !atomic_read(&sb->s_isw_nr_in_flight));
+}
+
 static void process_inode_switch_wbs(struct bdi_writeback *new_wb,
 				     struct inode_switch_wbs_context *isw)
 {
@@ -554,8 +571,12 @@ static void process_inode_switch_wbs(struct bdi_writeback *new_wb,
 		wb_put_many(old_wb, nr_switched);
 	}
 
-	for (inodep = isw->inodes; *inodep; inodep++)
+	for (inodep = isw->inodes; *inodep; inodep++) {
+		struct super_block *sb = (*inodep)->i_sb;
+
 		iput(*inodep);
+		cgroup_writeback_unpin(sb);
+	}
 	wb_put(new_wb);
 	kfree(isw);
 	atomic_dec(&isw_nr_in_flight);
@@ -598,16 +619,19 @@ void inode_switch_wbs_work_fn(struct work_struct *work)
 static bool inode_prepare_wbs_switch(struct inode *inode,
 				     struct bdi_writeback *new_wb)
 {
+	/* Avoid the atomic_inc/smp_mb dance once SB_ACTIVE is gone. */
+	if (!(inode->i_sb->s_flags & SB_ACTIVE))
+		return false;
+
 	/*
-	 * Paired with smp_mb() in cgroup_writeback_umount().
-	 * isw_nr_in_flight must be increased before checking SB_ACTIVE and
-	 * grabbing an inode, otherwise isw_nr_in_flight can be observed as 0
-	 * in cgroup_writeback_umount() and the isw_wq will be not flushed.
+	 * Pairs with smp_mb() in cgroup_writeback_umount(): the umounter either
+	 * sees a non-zero counter and waits, or we see SB_ACTIVE clear below.
 	 */
+	cgroup_writeback_pin(inode->i_sb);
 	smp_mb();
 
 	if (IS_DAX(inode))
-		return false;
+		goto out_dec;
 
 	/* while holding I_WB_SWITCH, no one else can update the association */
 	spin_lock(&inode->i_lock);
@@ -615,13 +639,17 @@ static bool inode_prepare_wbs_switch(struct inode *inode,
 	    inode_state_read(inode) & (I_WB_SWITCH | I_FREEING | I_WILL_FREE) ||
 	    inode_to_wb(inode) == new_wb) {
 		spin_unlock(&inode->i_lock);
-		return false;
+		goto out_dec;
 	}
 	inode_state_set(inode, I_WB_SWITCH);
 	__iget(inode);
 	spin_unlock(&inode->i_lock);
 
 	return true;
+
+out_dec:
+	cgroup_writeback_unpin(inode->i_sb);
+	return false;
 }
 
 static void wb_queue_isw(struct bdi_writeback *wb,
@@ -673,6 +701,7 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 	memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
 	if (memcg_css && !css_tryget(memcg_css))
 		memcg_css = NULL;
+	rcu_read_unlock();
 	if (!memcg_css)
 		goto out_free;
 
@@ -688,11 +717,9 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 
 	trace_inode_switch_wbs_queue(inode->i_wb, new_wb, 1);
 	wb_queue_isw(new_wb, isw);
-	rcu_read_unlock();
 	return;
 
 out_free:
-	rcu_read_unlock();
 	atomic_dec(&isw_nr_in_flight);
 	if (new_wb)
 		wb_put(new_wb);
@@ -750,14 +777,6 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
 		new_wb = &wb->bdi->wb; /* wb_get() is noop for bdi's wb */
 
 	nr = 0;
-	/*
-	 * Paired with synchronize_rcu() in cgroup_writeback_umount().
-	 * Holding rcu_read_lock across the SB_ACTIVE check, the inode grab
-	 * and wb_queue_isw() ensures synchronize_rcu() cannot return until
-	 * the work is queued, so the subsequent flush_workqueue() will wait
-	 * for the switch.
-	 */
-	rcu_read_lock();
 	spin_lock(&wb->list_lock);
 	/*
 	 * In addition to the inodes that have completed writeback, also switch
@@ -775,7 +794,6 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
 
 	/* no attached inodes? bail out */
 	if (nr == 0) {
-		rcu_read_unlock();
 		atomic_dec(&isw_nr_in_flight);
 		wb_put(new_wb);
 		kfree(isw);
@@ -784,7 +802,6 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
 
 	trace_inode_switch_wbs_queue(wb, new_wb, nr);
 	wb_queue_isw(new_wb, isw);
-	rcu_read_unlock();
 
 	return restart;
 }
@@ -1217,39 +1234,27 @@ int cgroup_writeback_by_id(u64 bdi_id, int memcg_id,
 }
 
 /**
- * cgroup_writeback_umount - flush inode wb switches for umount
+ * cgroup_writeback_umount - wait for in-flight inode wb switches on @sb
  * @sb: target super_block
  *
- * This function is called when a super_block is about to be destroyed and
- * flushes in-flight inode wb switches.  An inode wb switch goes through
- * RCU and then workqueue, so the two need to be flushed in order to ensure
- * that all previously scheduled switches are finished.  As wb switches are
- * rare occurrences and synchronize_rcu() can take a while, perform
- * flushing iff wb switches are in flight.
+ * Wait until every inode wb switch that already passed the SB_ACTIVE
+ * check on this superblock has been completed by the worker.  Since
+ * SB_ACTIVE is cleared before this is called, no new switches can start
+ * for @sb, so s_isw_nr_in_flight will monotonically drop to zero.
  */
 void cgroup_writeback_umount(struct super_block *sb)
 {
-
 	if (!(sb->s_bdi->capabilities & BDI_CAP_WRITEBACK))
 		return;
 
 	/*
-	 * SB_ACTIVE should be reliably cleared before checking
-	 * isw_nr_in_flight, see generic_shutdown_super().
+	 * Pairs with smp_mb() in inode_prepare_wbs_switch(): we either observe
+	 * a non-zero counter and wait, or the switcher sees SB_ACTIVE clear
+	 * (cleared by generic_shutdown_super()) and bails before grabbing the
+	 * inode.
 	 */
 	smp_mb();
-
-	if (atomic_read(&isw_nr_in_flight)) {
-		/*
-		 * Paired with rcu_read_lock() in inode_switch_wbs() and
-		 * cleanup_offline_cgwb().  synchronize_rcu() waits for any
-		 * in-flight switcher that already passed the SB_ACTIVE check
-		 * to finish queueing its work, so flush_workqueue() below
-		 * will then drain it.
-		 */
-		synchronize_rcu();
-		flush_workqueue(isw_wq);
-	}
+	cgroup_writeback_drain(sb);
 }
 
 static int __init cgroup_writeback_init(void)
diff --git a/include/linux/fs/super_types.h b/include/linux/fs/super_types.h
index 383050e7fdf5..1ab4e2265129 100644
--- a/include/linux/fs/super_types.h
+++ b/include/linux/fs/super_types.h
@@ -274,6 +274,14 @@ struct super_block {
 
 	/* number of fserrors that are being sent to fsnotify/filesystems */
 	refcount_t				s_pending_errors;
+
+#ifdef CONFIG_CGROUP_WRITEBACK
+	/*
+	 * Number of in-flight inode wb switches for this sb.  Drained by
+	 * cgroup_writeback_umount() before tear-down.
+	 */
+	atomic_t				s_isw_nr_in_flight;
+#endif
 } __randomize_layout;
 
 /*
-- 
2.43.7