[PATCH v2 2/6] writeback: collect stats of all wb of bdi in bdi_debug_stats_show

[Kemeng Shi]

/sys/kernel/debug/bdi/xxx/stats is supposed to show writeback information
of whole bdi, but only writeback information of bdi in root cgroup is
collected. So writeback information in non-root cgroup are missing now.
To be more specific, considering following case:

/* create writeback cgroup */
cd /sys/fs/cgroup
echo "+memory +io" > cgroup.subtree_control
mkdir group1
cd group1
echo $$ > cgroup.procs
/* do writeback in cgroup */
fio -name test -filename=/dev/vdb ...
/* get writeback info of bdi */
cat /sys/kernel/debug/bdi/xxx/stats
The cat result unexpectedly implies that there is no writeback on target
bdi.

Fix this by collecting stats of all wb in bdi instead of only wb in
root cgroup.

Following domain hierarchy is tested:
                global domain (320G)
                /                 \
        cgroup domain1(10G)     cgroup domain2(10G)
                |                 |
bdi            wb1               wb2

/* all writeback info of bdi is successfully collected */
cat stats
BdiWriteback:             2912 kB
BdiReclaimable:        1598464 kB
BdiDirtyThresh:      167479028 kB
DirtyThresh:         195038532 kB
BackgroundThresh:     32466728 kB
BdiDirtied:           19141696 kB
BdiWritten:           17543456 kB
BdiWriteBandwidth:     1136172 kBps
b_dirty:                     2
b_io:                        0
b_more_io:                   1
b_dirty_time:                0
bdi_list:                    1
state:                       1

Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
---
 mm/backing-dev.c | 100 +++++++++++++++++++++++++++++++++--------------
 1 file changed, 71 insertions(+), 29 deletions(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 70f02959f3bd..8daf950e6855 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -39,6 +39,19 @@ struct workqueue_struct *bdi_wq;
 #include <linux/debugfs.h>
 #include <linux/seq_file.h>
 
+struct wb_stats {
+	unsigned long nr_dirty;
+	unsigned long nr_io;
+	unsigned long nr_more_io;
+	unsigned long nr_dirty_time;
+	unsigned long nr_writeback;
+	unsigned long nr_reclaimable;
+	unsigned long nr_dirtied;
+	unsigned long nr_written;
+	unsigned long dirty_thresh;
+	unsigned long wb_thresh;
+};
+
 static struct dentry *bdi_debug_root;
 
 static void bdi_debug_init(void)
@@ -65,16 +78,54 @@ static struct backing_dev_info *lookup_bdi(struct seq_file *m)
 	return NULL;
 }
 
+static void collect_wb_stats(struct wb_stats *stats,
+			     struct bdi_writeback *wb)
+{
+	struct inode *inode;
+
+	spin_lock(&wb->list_lock);
+	list_for_each_entry(inode, &wb->b_dirty, i_io_list)
+		stats->nr_dirty++;
+	list_for_each_entry(inode, &wb->b_io, i_io_list)
+		stats->nr_io++;
+	list_for_each_entry(inode, &wb->b_more_io, i_io_list)
+		stats->nr_more_io++;
+	list_for_each_entry(inode, &wb->b_dirty_time, i_io_list)
+		if (inode->i_state & I_DIRTY_TIME)
+			stats->nr_dirty_time++;
+	spin_unlock(&wb->list_lock);
+
+	stats->nr_writeback += wb_stat(wb, WB_WRITEBACK);
+	stats->nr_reclaimable += wb_stat(wb, WB_RECLAIMABLE);
+	stats->nr_dirtied += wb_stat(wb, WB_DIRTIED);
+	stats->nr_written += wb_stat(wb, WB_WRITTEN);
+	stats->wb_thresh += wb_calc_thresh(wb, stats->dirty_thresh);
+}
+
+#ifdef CONFIG_CGROUP_WRITEBACK
+static void bdi_collect_stats(struct backing_dev_info *bdi,
+			      struct wb_stats *stats)
+{
+	struct bdi_writeback *wb;
+
+	list_for_each_entry_rcu(wb, &bdi->wb_list, bdi_node)
+		collect_wb_stats(stats, wb);
+}
+#else
+static void bdi_collect_stats(struct backing_dev_info *bdi,
+			      struct wb_stats *stats)
+{
+	collect_wb_stats(stats, &bdi->wb);
+}
+#endif
 
 static int bdi_debug_stats_show(struct seq_file *m, void *v)
 {
 	struct backing_dev_info *bdi;
-	struct bdi_writeback *wb;
 	unsigned long background_thresh;
 	unsigned long dirty_thresh;
-	unsigned long wb_thresh;
-	unsigned long nr_dirty, nr_io, nr_more_io, nr_dirty_time;
-	struct inode *inode;
+	struct wb_stats stats;
+	unsigned long tot_bw;
 
 	rcu_read_lock();
 	bdi = lookup_bdi(m);
@@ -83,22 +134,13 @@ static int bdi_debug_stats_show(struct seq_file *m, void *v)
 		return -EEXIST;
 	}
 
-	wb = &bdi->wb;
-	nr_dirty = nr_io = nr_more_io = nr_dirty_time = 0;
-	spin_lock(&wb->list_lock);
-	list_for_each_entry(inode, &wb->b_dirty, i_io_list)
-		nr_dirty++;
-	list_for_each_entry(inode, &wb->b_io, i_io_list)
-		nr_io++;
-	list_for_each_entry(inode, &wb->b_more_io, i_io_list)
-		nr_more_io++;
-	list_for_each_entry(inode, &wb->b_dirty_time, i_io_list)
-		if (inode->i_state & I_DIRTY_TIME)
-			nr_dirty_time++;
-	spin_unlock(&wb->list_lock);
-
 	global_dirty_limits(&background_thresh, &dirty_thresh);
-	wb_thresh = wb_calc_thresh(wb, dirty_thresh);
+
+	memset(&stats, 0, sizeof(stats));
+	stats.dirty_thresh = dirty_thresh;
+	bdi_collect_stats(bdi, &stats);
+
+	tot_bw = atomic_long_read(&bdi->tot_write_bandwidth);
 
 	seq_printf(m,
 		   "BdiWriteback:       %10lu kB\n"
@@ -115,18 +157,18 @@ static int bdi_debug_stats_show(struct seq_file *m, void *v)
 		   "b_dirty_time:       %10lu\n"
 		   "bdi_list:           %10u\n"
 		   "state:              %10lx\n",
-		   (unsigned long) K(wb_stat(wb, WB_WRITEBACK)),
-		   (unsigned long) K(wb_stat(wb, WB_RECLAIMABLE)),
-		   K(wb_thresh),
+		   K(stats.nr_writeback),
+		   K(stats.nr_reclaimable),
+		   K(stats.wb_thresh),
 		   K(dirty_thresh),
 		   K(background_thresh),
-		   (unsigned long) K(wb_stat(wb, WB_DIRTIED)),
-		   (unsigned long) K(wb_stat(wb, WB_WRITTEN)),
-		   (unsigned long) K(wb->write_bandwidth),
-		   nr_dirty,
-		   nr_io,
-		   nr_more_io,
-		   nr_dirty_time,
+		   K(stats.nr_dirtied),
+		   K(stats.nr_written),
+		   K(tot_bw),
+		   stats.nr_dirty,
+		   stats.nr_io,
+		   stats.nr_more_io,
+		   stats.nr_dirty_time,
 		   !list_empty(&bdi->bdi_list), bdi->wb.state);
 
 	rcu_read_unlock();
-- 
2.30.0