Re: [PATCH] staging, android: remove lowmemory killer from the tree

[Michal Hocko]

On Fri 24-02-17 13:19:46, peter enderborg wrote:
> On 02/23/2017 09:36 PM, Martijn Coenen wrote:
> > On Thu, Feb 23, 2017 at 9:24 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
> >> So, just for context, Android does have a userland LMK daemon (using
> >> the mempressure notifiers) as you mentioned, but unfortunately I'm
> >> unaware of any devices that ship with that implementation.
> > I've previously worked on enabling userspace lmkd for a previous
> > release, but ran into some issues there (see below).
> >
> >> This is reportedly because while the mempressure notifiers provide a
> >> the signal to userspace, the work the deamon then has to do to look up
> >> per process memory usage, in order to figure out who is best to kill
> >> at that point was too costly and resulted in poor device performance.
> > In particular, mempressure requires memory cgroups to function, and we
> > saw performance regressions due to the accounting done in mem cgroups.
> > At the time we didn't have enough time left to solve this before the
> > release, and we reverted back to kernel lmkd.
> >
> >> So for shipping Android devices, the LMK is still needed. However, its
> >> not critical for basic android development, as the system will
> >> function without it.
> > It will function, but it most likely will perform horribly (as the
> > page cache will be trashed to such a level that the system will be
> > unusable).
> >
> >> Additionally I believe most vendors heavily
> >> customize the LMK in their vendor tree, so the value of having it in
> >> staging might be relatively low.
> >>
> >> It would be great however to get a discussion going here on what the
> >> ulmkd needs from the kernel in order to efficiently determine who best
> >> to kill, and how we might best implement that.
> > The two main issues I think we need to address are:
> > 1) Getting the right granularity of events from the kernel; I once
> > tried to submit a patch upstream to address this:
> > https://lkml.org/lkml/2016/2/24/582
> > 2) Find out where exactly the memory cgroup overhead is coming from,
> > and how to reduce it or work around it to acceptable levels for
> > Android. This was also on 3.10, and maybe this has long been fixed or
> > improved in more recent kernel versions.
> >
> > I don't have cycles to work on this now, but I'm happy to talk to
> > whoever picks this up on the Android side.
> I sent some patches that is different approach. It still uses shrinkers
> but it has a kernel part that do the kill part better than the old one
> but it does it the android way. The future for this is get it triggered
> with other path's than slab shrinker. But we will not continue unless
> we get google-android to be part of it. Hocko objected heavy on
> the patches but seems not to see that we need something to
> do the job before we can disconnect from shrinker.

Yeah, I strongly believe that the chosen approach is completely wrong.
Both in abusing the shrinker interface and abusing oom_score_adj as the
only criterion for the oom victim selection.
-- 
Michal Hocko
SUSE Labs