On 06/17/2016 07:25 PM, 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 an 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 offlining are tmpfs/shmem pages. And those > references are under the user's control, so they are manageable. > > This patch introduces a private 16-bit memcg ID and switches swap and > cache shadow entries over to using that. This ID can then be recycled > after offlining when the CSS remains pinned only by objects that don't > specifically need it. > > This script demonstrates the problem by faulting one cache page in a > new cgroup and deleting it again: > > set -e > mkdir -p pages > for x in `seq 128000`; do > [ $((x % 1000)) -eq 0 ] && echo $x > mkdir /cgroup/foo > echo $$ >/cgroup/foo/cgroup.procs > echo trex >pages/$x > echo $$ >/cgroup/cgroup.procs > rmdir /cgroup/foo > done Perhaps you could send this script to the LTP project to have this as a regression test? > > When run on an unpatched kernel, we eventually run out of possible IDs > even though there are no visible cgroups: > > [root@ham ~]# ./cssidstress.sh > [...] > 65000 > mkdir: cannot create directory '/cgroup/foo': No space left on device > > After this patch, the IDs get released upon cgroup destruction and the > cache and css objects get released once memory reclaim kicks in. > > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
