[PATCH 2/4] writeback: name the bdi in the slow writeback report

[Breno Leitao]

The slow writeback report names the waiting task and how long it has
waited, but not what it is waiting on. On a machine with several
filesystems there is no way to tell which device is slow.

Pass the bdi to wb_wait_for_completion() and print its name on a second
line, together with the number of work items still outstanding. A sync
issues one work item per cgroup wb with dirty inodes, so the count also
shows whether a single cgroup is holding up the others.

wb_wait_for_completion() took the bdi until commit 5b9cce4c7eb0
("writeback: Generalize and expose wb_completion") made it usable for
the memcg foreign flushes. memcg tracks those by bdi id only and passes
NULL, which bdi_dev_name() prints as "(unknown)".

The first line does not change, so anything matching on it keeps
working. A sync on a throttled null_blk device now reports:

  INFO: The task sync:180 has been waiting for writeback completion for more than 28 seconds.
        bdi 252:0, work items left: 1

Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 fs/fs-writeback.c           | 20 +++++++++++++-------
 include/linux/backing-dev.h |  3 ++-
 mm/memcontrol.c             |  2 +-
 3 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 01a00d7542b371..3dee9d65eaf721 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -196,16 +196,20 @@ static void wb_queue_work(struct bdi_writeback *wb,
 	spin_unlock_irq(&wb->work_lock);
 }
 
-static bool wb_wait_for_completion_cb(struct wb_completion *done)
+static bool wb_wait_for_completion_cb(struct wb_completion *done,
+				      struct backing_dev_info *bdi)
 {
 	unsigned long timeout = sysctl_hung_task_timeout_secs;
 	unsigned long waited_secs = (jiffies - done->wait_start) / HZ;
 
 	done->progress_stamp = jiffies;
-	if (timeout && (waited_secs > timeout))
+	if (timeout && (waited_secs > timeout)) {
 		pr_info("INFO: The task %s:%d has been waiting for writeback "
 			"completion for more than %lu seconds.\n",
 			current->comm, current->pid, waited_secs);
+		pr_info("      bdi %s, work items left: %d\n",
+			bdi_dev_name(bdi), atomic_read(&done->cnt));
+	}
 
 	return !atomic_read(&done->cnt);
 }
@@ -213,6 +217,7 @@ static bool wb_wait_for_completion_cb(struct wb_completion *done)
 /**
  * wb_wait_for_completion - wait for completion of bdi_writeback_works
  * @done: target wb_completion
+ * @bdi: bdi work items were issued to, NULL for the memcg foreign flushes
  *
  * Wait for one or more work items issued to @bdi with their ->done field
  * set to @done, which should have been initialized with
@@ -220,11 +225,12 @@ static bool wb_wait_for_completion_cb(struct wb_completion *done)
  * are completed.  Work items which are waited upon aren't freed
  * automatically on completion.
  */
-void wb_wait_for_completion(struct wb_completion *done)
+void wb_wait_for_completion(struct wb_completion *done,
+			    struct backing_dev_info *bdi)
 {
 	done->wait_start = jiffies;
 	atomic_dec(&done->cnt);		/* put down the initial count */
-	wait_event(*done->waitq, wb_wait_for_completion_cb(done));
+	wait_event(*done->waitq, wb_wait_for_completion_cb(done, bdi));
 }
 
 #ifdef CONFIG_CGROUP_WRITEBACK
@@ -1164,7 +1170,7 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
 		last_wb = wb;
 
 		rcu_read_unlock();
-		wb_wait_for_completion(&fallback_work_done);
+		wb_wait_for_completion(&fallback_work_done, bdi);
 		goto restart;
 	}
 	rcu_read_unlock();
@@ -2972,7 +2978,7 @@ static void __writeback_inodes_sb_nr(struct super_block *sb, unsigned long nr,
 	WARN_ON(!rwsem_is_locked(&sb->s_umount));
 
 	bdi_split_work_to_wbs(sb->s_bdi, &work, skip_if_busy);
-	wb_wait_for_completion(&done);
+	wb_wait_for_completion(&done, bdi);
 }
 
 /**
@@ -3069,7 +3075,7 @@ void sync_inodes_sb(struct super_block *sb)
 	/* protect against inode wb switch, see inode_switch_wbs_work_fn() */
 	bdi_down_write_wb_switch_rwsem(bdi);
 	bdi_split_work_to_wbs(bdi, &work, false);
-	wb_wait_for_completion(&done);
+	wb_wait_for_completion(&done, bdi);
 	bdi_up_write_wb_switch_rwsem(bdi);
 
 	wait_sb_inodes(sb);
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index c2284466e7aa66..808d939d2ca7f9 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -39,7 +39,8 @@ struct backing_dev_info *bdi_alloc(int node_id);
 void wb_start_background_writeback(struct bdi_writeback *wb);
 void wb_workfn(struct work_struct *work);
 
-void wb_wait_for_completion(struct wb_completion *done);
+void wb_wait_for_completion(struct wb_completion *done,
+			    struct backing_dev_info *bdi);
 
 extern spinlock_t bdi_lock;
 extern struct list_head bdi_list;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ac6c43ea8c5a6d..df462d279a655e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4525,7 +4525,7 @@ static void mem_cgroup_css_free(struct cgroup_subsys_state *css)
 
 #ifdef CONFIG_CGROUP_WRITEBACK
 	for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++)
-		wb_wait_for_completion(&memcg->cgwb_frn[i].done);
+		wb_wait_for_completion(&memcg->cgwb_frn[i].done, NULL);
 #endif
 	if (cgroup_subsys_on_dfl(memory_cgrp_subsys) && !cgroup_memory_nosocket)
 		static_branch_dec(&memcg_sockets_enabled_key);

-- 
2.53.0-Meta