Offline memory cgroups forbids creation new bdi_writebacks.
Each try wastes cpu cycles and increases contention around cgwb_lock.
For example each O_DIRECT read calls filemap_write_and_wait_range()
if inode has cached pages which tries to switch from dying writeback.
This patch switches inode writeback to closest online parent cgroup.
Fixes: e8a7abf5a5bd ("writeback: disassociate inodes from dying bdi_writebacks")
Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
---
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 542b02d170f8..3af44591a106 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -505,7 +505,7 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
/* find and pin the new wb */
rcu_read_lock();
memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
- if (memcg_css)
+ if (memcg_css && (memcg_css->flags & CSS_ONLINE))
isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
rcu_read_unlock();
if (!isw->new_wb)
@@ -579,9 +579,16 @@ void wbc_attach_and_unlock_inode(struct writeback_control *wbc,
/*
* A dying wb indicates that the memcg-blkcg mapping has changed
* and a new wb is already serving the memcg. Switch immediately.
+ * If memory cgroup is offline switch to closest online parent.
*/
- if (unlikely(wb_dying(wbc->wb)))
- inode_switch_wbs(inode, wbc->wb_id);
+ if (unlikely(wb_dying(wbc->wb))) {
+ struct cgroup_subsys_state *memcg_css = wbc->wb->memcg_css;
+
+ while (!(memcg_css->flags & CSS_ONLINE))
+ memcg_css = memcg_css->parent;
+
+ inode_switch_wbs(inode, memcg_css->id);
+ }
}
EXPORT_SYMBOL_GPL(wbc_attach_and_unlock_inode);
