Re: [PATCH] mm: memcontrol: fix cgroup creation failure after many small jobs

Johannes Weiner
2016-06-17T16:43:19Z

On Fri, Jun 17, 2016 at 12:06:55PM +0300, Vladimir Davydov wrote:
> On Wed, Jun 15, 2016 at 11:42:44PM -0400, Johannes Weiner wrote:
> > The memory controller has quite a bit of state that usually outlives
> > the cgroup and pins its CSS until said state disappears. At the same
> > time it imposes a 16-bit limit on the CSS ID space to economically
> > store IDs in the wild. Consequently, when we use cgroups to contain
> > frequent but small and short-lived jobs that leave behind some page
> > cache, we quickly run into the 64k limitations of outstanding CSSs.
> > Creating a new cgroup fails with -ENOSPC while there are only a few,
> > or even no user-visible cgroups in existence.
> >
> > Although pinning CSSs past cgroup removal is common, there are only
> > two instances that actually need a CSS ID after a cgroup is deleted:
> > cache shadow entries and swapout records.
> >
> > Cache shadow entries reference the ID weakly and can deal with the CSS
> > having disappeared when it's looked up later. They pose no hurdle.
> >
> > Swap-out records do need to pin the css to hierarchically attribute
> > swapins after the cgroup has been deleted; though the only pages that
> > remain swapped out after a process exits are tmpfs/shmem pages. Those
> > references are under the user's control and thus manageable.
> >
> > This patch introduces a private 16bit memcg ID and switches swap and
> > cache shadow entries over to using that. It then decouples the CSS
> > lifetime from the CSS ID lifetime, such that a CSS ID can be recycled
> > when the CSS is only pinned by common objects that don't need an ID.
>
> There's already id which is only used for online memory cgroups - it's
> kmemcg_id. May be, instead of introducing one more idr, we could name it
> generically and reuse it for shadow entries?

Good point. But it seems mem_cgroup_idr is more generic, it makes
sense to switch slab accounting over to that. I'll look into that, but
as a refactoring patch on top of this fix.

> Regarding swap entries, would it really make much difference if we used
> 4 bytes per swap page instead of 2? For a 100 GB swap it'd increase
> overhead from 50 MB up to 100 MB, which still doesn't seem too much IMO,
> so may be just use plain unrestricted css->id for swap entries?

Yes and no. I agree that the increased consumption wouldn't be too
crazy, but if we have to maintain a 16-bit ID anyway, we might as well
use it for swap too to save that space. I don't think tmpfs and shmem
pins past offlining will be common enough to significantly eat into
the ID space of online cgroups.