Re: [For Stable] mm: memcontrol: fix excessive complexity in memory.stat reporting

[Greg KH <greg@kroah.com>]

On Mon, Apr 01, 2019 at 01:34:14PM -0700, Vaibhav Rustagi wrote:
> In linux stable kernel (tested on 4.14), reading memory.stat in case
> of tens of thousands of ghost cgroups pinned by lingering page cache
> takes up to 100 ms ~ 700 ms to complete the reading.
> 
> Repro steps (tested on 4.14 kernel):
> 
> $ cat /tmp/make_zombies
> 
> mkdir /tmp/fs
> mount -t tmpfs nodev /tmp/fs
> for i in {1..10000}; do
>    mkdir /sys/fs/cgroup/memory/z$i
>    (echo $BASHPID >> /sys/fs/cgroup/memory/z$i/cgroup.procs && echo $i
> > /tmp/fs/$i)
>  done
> 
> # establish baseline
> $ perf stat -r3 cat /sys/fs/cgroup/memory/memory.stat > /dev/null
> 0.011642670 seconds time elapsed
> 
> $ bash /tmp/make_zombies
> $ perf stat -r3 cat /sys/fs/cgroup/memory/memory.stat > /dev/null
> 0.134939281 seconds time elapsed
> 
> $ rmdir /sys/fs/cgroup/memory/z*
> $ perf stat -r3 cat /sys/fs/cgroup/memory/memory.stat > /dev/null
> 0.135323145 seconds time elapsed
> # even after rmdir we have zombies, so still slow.
> 
> The fix is already present in linux master (since 4.16) by following commits:
> 
> c9019e9bf42e66d028d70d2da6206cad4dd9250d mm: memcontrol: eliminate raw
> access to stat and event counters
> 284542656e22c43fdada8c8cc0ca9ede8453eed7  mm: memcontrol: implement
> lruvec stat functions on top of each other
> a983b5ebee57209c99f68c8327072f25e0e6e3da  mm: memcontrol: fix
> excessive complexity in memory.stat reporting
> c3cc39118c3610eb6ab4711bc624af7fc48a35fe  mm: memcontrol: fix
> NR_WRITEBACK leak in memcg and system stats
> e27be240df53f1a20c659168e722b5d9f16cc7f4  mm: memcg: make sure
> memory.events is uptodate when waking pollers
> 
> I would like to request cherry-picking the above commits to
> linux-stable branch - 4.14.

Please resend and cc: all of the people on those patches so we can get
their opinion on if this is stable kernel material or not.

thanks,

greg k-h