Hi Dennis,
Sorry for the later reply. Becase I cann't reproduce this problem by
local test,
and online environment is not allowed to operate, I am constructing
scenario
to reproduce it in recent days.
在 2019/5/14 2:30, Dennis Zhou 写道:
Hi Liguang,
On Fri, May 10, 2019 at 09:54:27AM +0800, 乱石 wrote:
Hi Tejun,
在 2019/5/10 0:48, Tejun Heo 写道:
Hi Tejun,
On Thu, May 09, 2019 at 04:03:53PM +0800, zhangliguang wrote:
There might have tons of files queued in the writeback, awaiting for
writing back. Unfortunately, the writeback's cgroup has been dead. In
this case, we reassociate the inode with another writeback cgroup, but
we possibly can't because the writeback associated with the dead cgroup
is the only valid one. In this case, the new writeback is allocated,
initialized and associated with the inode. It causes unnecessary high
system load and latency.
This fixes the issue by enforce moving the inode to root cgroup when the
previous binding cgroup becomes dead. With it, no more unnecessary
writebacks are created, populated and the system load decreased by about
6x in the online service we encounted:
Without the patch: about 30% system load
With the patch: about 5% system load
Can you please describe the scenario with more details? I'm having a
bit of hard time understanding the amount of cpu cycles being
consumed.
Thanks.
Our search line reported a problem, when containerA was removed,
containerB and containerC's system load were up to 30%.
We record the trace with 'perf record cycles:k -g -a', found that wb_init
was the hotspot function.
Function call:
generic_file_direct_write
filemap_write_and_wait_range
__filemap_fdatawrite_range
wbc_attach_fdatawrite_inode
inode_attach_wb
__inode_attach_wb
wb_get_create
wbc_attach_and_unlock_inode
if (unlikely(wb_dying(wbc->wb)))
inode_switch_wbs
wb_get_create
; Search bdi->cgwb_tree from memcg_css->id
; OR cgwb_create
kmalloc
wb_init // hot spot
; Insert to bdi->cgwb_tree, mmecg_css->id as key
We discussed it through, base on the analysis: When we running into the
issue, there is cgroups are being deleted. The inodes (files) that were
associated with these cgroups have to switch into another newly created
writeback. We think there are huge amount of inodes in the writeback list
that time. So we don't think there is anything abnormal. However, one
thing we possibly can do: enforce these inodes to BDI embedded wirteback
and we needn't create huge amount of writebacks in that case, to avoid
the high system load phenomenon. We expect correct wb (best candidate) is
picked up in next round.
Thanks,
Liguang
If I understand correctly, this is mostlikely caused by a file shared by
cgroup A and cgroup B. This means cgroup B is doing direct io against
the file owned by the dying cgroup A. In this case, the code tries to do
a wb switch. However, it fails to reallocate the wb as it's deleted and
for the original cgrouip A's memcg id.
I think the below may be a better solution. Could you please test it? If
it works, I'll spin a patch with a more involved description.
Thanks,
Dennis
---
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 36855c1f8daf..fb331ea2a626 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -577,7 +577,7 @@ 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 (unlikely(wb_dying(wbc->wb)))
+ if (unlikely(wb_dying(wbc->wb)) && !css_is_dying(wbc->wb->memcg_css))
inode_switch_wbs(inode, wbc->wb_id);
}
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 72e6d0c55cfa..685563ed9788 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -659,7 +659,7 @@ struct bdi_writeback *wb_get_create(struct backing_dev_info *bdi,
might_sleep_if(gfpflags_allow_blocking(gfp));
- if (!memcg_css->parent)
+ if (!memcg_css->parent || css_is_dying(memcg_css))
return &bdi->wb;
do {
