Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android

[Suren Baghdasaryan <surenb@google.com>]

From: Sultan Alsawaf <sultan@kerneltoast.com>
Date: Tue, May 7, 2019 at 9:53 AM
To: Suren Baghdasaryan
Cc: Christian Brauner, Greg Kroah-Hartman, open list:ANDROID DRIVERS,
Daniel Colascione, Todd Kjos, Kees Cook, Peter Zijlstra, Martijn
Coenen, LKML, Tim Murray, Michal Hocko, linux-mm, Arve Hjønnevåg, Ingo
Molnar, Steven Rostedt, Oleg Nesterov, Joel Fernandes, Andy
Lutomirski, kernel-team

> On Tue, May 07, 2019 at 09:28:47AM -0700, Suren Baghdasaryan wrote:
> > Hi Sultan,
> > Looks like you are posting this patch for devices that do not use
> > userspace LMKD solution due to them using older kernels or due to
> > their vendors sticking to in-kernel solution. If so, I see couple
> > logistical issues with this patch. I don't see it being adopted in
> > upstream kernel 5.x since it re-implements a deprecated mechanism even
> > though vendors still use it. Vendors on the other hand, will not adopt
> > it until you show evidence that it works way better than what
> > lowmemorykilled driver does now. You would have to provide measurable
> > data and explain your tests before they would consider spending time
> > on this.
>
> Yes, this is mostly for the devices already produced that are forced to suffer
> with poor memory management. I can't even convince vendors to fix kernel
> memory leaks, so there's no way I'd be able to convince them of trying this
> patch, especially since it seems like you're having trouble convincing vendors
> to stop using lowmemorykiller in the first place. And thankfully, convincing
> vendors isn't my job :)
>
> > On the implementation side I'm not convinced at all that this would
> > work better on all devices and in all circumstances. We had cases when
> > a new mechanism would show very good results until one usecase
> > completely broke it. Bulk killing of processes that you are doing in
> > your patch was a very good example of such a decision which later on
> > we had to rethink. That's why baking these policies into kernel is
> > very problematic. Another problem I see with the implementation that
> > it ties process killing with the reclaim scan depth. It's very similar
> > to how vmpressure works and vmpressure in my experience is very
> > unpredictable.
>
> Could you elaborate a bit on why bulk killing isn't good?

Yes. Several months ago we got reports that LMKD got very aggressive
killing more processes in situations which did not require that many
kills to recover from memory pressure. After investigation we tracked
it to the bulk killing which would kill too many processes during a
memory usage spike. When killing gradually the kills would be avoided,
so we had to get back to a more balanced approach. The conceptual
issue with bulk killing is that you can't cancel it when device
recovers quicker than you expected.

> > > > I linked in the previous message, Google made a rather large set of
> > > > modifications to the supposedly-defunct lowmemorykiller.c not one month ago.
> > > > What's going on?
> >
> > If you look into that commit, it adds ability to report kill stats. If
> > that was a change in how that driver works it would be rejected.
>
> Fair, though it was quite strange seeing something that was supposedly totally
> abandoned receiving a large chunk of code for reporting stats.
>
> Thanks,
> Sultan