On 02/24/2017 01:28 PM, Michal Hocko wrote:

On Fri 24-02-17 13:19:46, peter enderborg wrote:

quoted

On 02/23/2017 09:36 PM, Martijn Coenen wrote:

quoted

On Thu, Feb 23, 2017 at 9:24 PM, John Stultz [off-list ref] wrote:

quoted

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).

quoted

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.

quoted

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).

quoted

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.

No one is arguing that shrinker is not problematic. And would be great if it is removed from lmk.
The oom_score_adj is the way user-space tells the kernel what the user-space has as prio. And android
is using that very much. It's a core part. I have never seen it be used on other
linux system so what is the intended usage of oom_score_adj? Is this really abusing?

I think I can help out with removing  shrinker from lmk. Not using oom_score_adj is harder and
has a bigger impact on android, except the trivial solution by adding replacement
oom_user_prio and use that within android and kernel.


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