Re: [PATCH] memcg: deprecate memory.force_empty knob

[Greg Thelen <gthelen@google.com>]

On Tue, May 13 2014, Michal Hocko <mhocko@suse.cz> wrote:

> force_empty has been introduced primarily to drop memory before it gets
> reparented on the group removal. This alone doesn't sound fully
> justified because reparented pages which are not in use can be reclaimed
> also later when there is a memory pressure on the parent level.
>
> Mark the knob CFTYPE_INSANE which tells the cgroup core that it
> shouldn't create the knob with the experimental sane_behavior. Other
> users will get informed about the deprecation and asked to tell us more
> because I do not expect most users will use sane_behavior cgroups mode
> very soon.
> Anyway I expect that most users will be simply cgroup remove handlers
> which do that since ever without having any good reason for it.
>
> If somebody really cares because reparented pages, which would be
> dropped otherwise, push out more important ones then we should fix the
> reparenting code and put pages to the tail.

I should mention a case where I've needed to use memory.force_empty: to
synchronously flush stats from child to parent.  Without force_empty
memory.stat is temporarily inconsistent until async css_offline
reparents charges.  Here is an example on v3.14 showing that
parent/memory.stat contents are in-flux immediately after rmdir of
parent/child.

$ cat /test
#!/bin/bash

# Create parent and child.  Add some non-reclaimable anon rss to child,
# then move running task to parent.
mkdir p p/c
(echo $BASHPID > p/c/cgroup.procs && exec sleep 1d) &
pid=$!
sleep 1
echo $pid > p/cgroup.procs 

grep 'rss ' {p,p/c}/memory.stat
if [[ $1 == force ]]; then
  echo 1 > p/c/memory.force_empty
fi
rmdir p/c

echo 'For a small time the p/c memory has not been reparented to p.'
grep 'rss ' {p,p/c}/memory.stat

sleep 1
echo 'After waiting all memory has been reparented'
grep 'rss ' {p,p/c}/memory.stat

kill $pid
rmdir p


-- First, demonstrate that just rmdir, without memory.force_empty,
   temporarily hides reparented child memory stats.

$ /test
p/memory.stat:rss 0
p/memory.stat:total_rss 69632
p/c/memory.stat:rss 69632
p/c/memory.stat:total_rss 69632
For a small time the p/c memory has not been reparented to p.
p/memory.stat:rss 0
p/memory.stat:total_rss 0
grep: p/c/memory.stat: No such file or directory
After waiting all memory has been reparented
p/memory.stat:rss 69632
p/memory.stat:total_rss 69632
grep: p/c/memory.stat: No such file or directory
/test: Terminated              ( echo $BASHPID > p/c/cgroup.procs && exec sleep 1d )

-- Demonstrate that using memory.force_empty before rmdir, behaves more
   sensibly.  Stats for reparented child memory are not hidden.

$ /test force
p/memory.stat:rss 0
p/memory.stat:total_rss 69632
p/c/memory.stat:rss 69632
p/c/memory.stat:total_rss 69632
For a small time the p/c memory has not been reparented to p.
p/memory.stat:rss 69632
p/memory.stat:total_rss 69632
grep: p/c/memory.stat: No such file or directory
After waiting all memory has been reparented
p/memory.stat:rss 69632
p/memory.stat:total_rss 69632
grep: p/c/memory.stat: No such file or directory
/test: Terminated              ( echo $BASHPID > p/c/cgroup.procs && exec sleep 1d )