On 1/31/22 13:25, Michal Hocko wrote:
On Mon 31-01-22 10:15:45, Roman Gushchin wrote:
On Mon, Jan 31, 2022 at 11:53:19AM -0500, Johannes Weiner wrote:
On Mon, Jan 31, 2022 at 10:38:51AM +0100, Michal Hocko wrote:
On Sat 29-01-22 15:53:15, Waiman Long wrote:
It was found that a number of offlined memcgs were not freed because
they were pinned by some charged pages that were present. Even "echo
1 > /proc/sys/vm/drop_caches" wasn't able to free those pages. These
offlined but not freed memcgs tend to increase in number over time with
the side effect that percpu memory consumption as shown in /proc/meminfo
also increases over time.
In order to find out more information about those pages that pin
offlined memcgs, the page_owner feature is extended to dump memory
cgroup information especially whether the cgroup is offlined or not.
It is not really clear to me how this is supposed to be used. Are you
really dumping all the pages in the system to find out offline memcgs?
That looks rather clumsy to me. I am not against adding memcg
information to the page owner output. That can be useful in other
contexts.
We've sometimes done exactly that in production, but with drgn
scripts. It's not very common, so it doesn't need to be very efficient
either. Typically, we'd encounter a host with an unusual number of
dying cgroups, ssh in and poke around with drgn to figure out what
kind of objects are still pinning the cgroups in question.
This patch would make that process a little easier, I suppose.
Right. Over last few years I've spent enormous amount of time digging into
various aspects of this problem and in my experience the combination of drgn
for the inspection of the current state and bpf for following various decisions
on the reclaim path was the most useful combination.
I really appreciate an effort to put useful tools to track memcg references
into the kernel tree, however the page_owner infra has a limited usefulness
as it has to be enabled on the boot. But because it doesn't add any overhead,
I also don't think there any reasons to not add it.
Would it be feasible to add a debugfs interface to displa dead memcg
information?
Originally, I added some debug code to keep track of the list of memcg
that has been offlined but not yet freed. After some more testing, I
figured out that the memcg's were not freed because they were pinned by
references in the page structs. At this point, I realize the using the
existing page owner debugging tool will be useful to track this kind of
problem since it already have all the infrastructure to list where the
pages were allocated as well as various field in the page structures.
Of course, it is also possible to have a debugfs interface to list those
dead memcg information, displaying more information about the page that
pins the memcg will be hard without using the page owner tool. Keeping
track of the list of dead memcg's may also have some runtime overhead.
Cheers,
Longman
