In kupdate writeback, only expired inode (have been dirty for longer than
dirty_expire_interval) is supposed to be written back. However, kupdate
writeback will writeback non-expired inode left in b_io or b_more_io from
last wb_writeback. As a result, writeback will keep being triggered
unexpected when we keep dirtying pages even dirty memory is under
threshold and inode is not expired. To be more specific:
Assume dirty background threshold is > 1G and dirty_expire_centisecs is
> 60s. When we running fio -size=1G -invalidate=0 -ioengine=libaio
--time_based -runtime=60... (keep dirtying), the writeback will keep
being triggered as following:
wb_workfn
wb_do_writeback
wb_check_background_flush
/*
* Wb dirty background threshold starts at 0 if device was idle and
* grows up when bandwidth of wb is updated. So a background
* writeback is triggered.
*/
wb_over_bg_thresh
/*
* Dirtied inode will be written back and added to b_more_io list
* after slice used up (because we keep dirtying the inode).
*/
wb_writeback
Writeback is triggered per dirty_writeback_centisecs as following:
wb_workfn
wb_do_writeback
wb_check_old_data_flush
/*
* Write back inode left in b_io and b_more_io from last wb_writeback
* even the inode is non-expired and it will be added to b_more_io
* again as slice will be used up (because we keep dirtying the
* inode)
*/
wb_writeback
Fix this by moving non-expired inode to dirty list instead of more io
list for kupdate writeback in requeue_inode.
Test as following:
/* make it more easier to observe the issue */
echo 300000 > /proc/sys/vm/dirty_expire_centisecs
echo 100 > /proc/sys/vm/dirty_writeback_centisecs
/* create a idle device */
mkfs.ext4 -F /dev/vdb
mount /dev/vdb /bdi1/
/* run buffer write with fio */
fio -name test -filename=/bdi1/file -size=800M -ioengine=libaio -bs=4K \
-iodepth=1 -rw=write -direct=0 --time_based -runtime=60 -invalidate=0
Fio result before fix (run three tests):
1360MB/s
1329MB/s
1455MB/s
Fio result after fix (run three tests):
1737MB/s
1729MB/s
1789MB/s
Writeback for non-expired inode is gone as expeted. Observe this with trace
writeback_start and writeback_written as following:
echo 1 > /sys/kernel/debug/tracing/events/writeback/writeback_start/enab
echo 1 > /sys/kernel/debug/tracing/events/writeback/writeback_written/enable
cat /sys/kernel/tracing/trace_pipe
Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
---
fs/fs-writeback.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 5ab1aaf805f7..4e6166e07eaf 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1561,7 +1561,8 @@ static void inode_sleep_on_writeback(struct inode *inode)
* thread's back can have unexpected consequences.
*/
static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
- struct writeback_control *wbc)
+ struct writeback_control *wbc,
+ unsigned long dirtied_before)
{
if (inode->i_state & I_FREEING)
return;
@@ -1594,7 +1595,8 @@ static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
* We didn't write back all the pages. nfs_writepages()
* sometimes bales out without doing anything.
*/
- if (wbc->nr_to_write <= 0) {
+ if (wbc->nr_to_write <= 0 &&
+ !inode_dirtied_after(inode, dirtied_before)) {
/* Slice used up. Queue for next turn. */
requeue_io(inode, wb);
} else {
@@ -1862,6 +1864,11 @@ static long writeback_sb_inodes(struct super_block *sb,
unsigned long start_time = jiffies;
long write_chunk;
long total_wrote = 0; /* count both pages and inodes */
+ unsigned long dirtied_before = jiffies;
+
+ if (work->for_kupdate)
+ dirtied_before = jiffies -
+ msecs_to_jiffies(dirty_expire_interval * 10);
while (!list_empty(&wb->b_io)) {
struct inode *inode = wb_inode(wb->b_io.prev);
@@ -1967,7 +1974,7 @@ static long writeback_sb_inodes(struct super_block *sb,
spin_lock(&inode->i_lock);
if (!(inode->i_state & I_DIRTY_ALL))
total_wrote++;
- requeue_inode(inode, tmp_wb, &wbc);
+ requeue_inode(inode, tmp_wb, &wbc, dirtied_before);
inode_sync_complete(inode);
spin_unlock(&inode->i_lock);
--
2.30.0
