Re: [PATCH v0 0/2] mm: swap: Gather swap entries and batch async release

["T.J. Mercier" <tjmercier@google.com>]

On Wed, Sep 10, 2025 at 1:41 PM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Wed, Sep 10, 2025 at 1:10 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> > On Tue, Sep 09, 2025 at 12:48:02PM -0700, Suren Baghdasaryan wrote:
> > > On Tue, Sep 9, 2025 at 12:21 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > > >
> > > > On Tue, Sep 09, 2025 at 02:53:39PM +0800, Lei Liu wrote:
> > > > > 1. Problem Scenario
> > > > > On systems with ZRAM and swap enabled, simultaneous process exits create
> > > > > contention. The primary bottleneck occurs during swap entry release
> > > > > operations, causing exiting processes to monopolize CPU resources. This
> > > > > leads to scheduling delays for high-priority processes.
> > > > >
> > > > > 2. Android Use Case
> > > > > During camera launch, LMKD terminates background processes to free memory.
> > > >
> > > > How does LMKD trigger the kills? SIGKILL or cgroup.kill?
> > >
> > > SIGKILL
> > >
> > > >
> > > > > Exiting processes compete for CPU cycles, delaying the camera preview
> > > > > thread and causing visible stuttering - directly impacting user
> > > > > experience.
> > > >
> > > > Since the exit/kill is due to low memory situation, punting the memory
> > > > freeing to a low priority async mechanism will help in improving user
> > > > experience. Most probably the application (camera preview here) will get
> > > > into global reclaim and will compete for CPU with the async memory
> > > > freeing.
> > > >
> > > > What we really need is faster memory freeing and we should explore all
> > > > possible ways. As others suggested fix/improve the bottleneck in the
> > > > memory freeing path. In addition I think we should explore parallelizing
> > > > this as well.
> > > >
> > > > On Android, I suppose most of the memory is associated with single or
> > > > small set of processes and parallelizing memory freeing would be
> > > > challenging. BTW is LMKD using process_mrelease() to release the killed
> > > > process memory?
> > >
> > > Yes, LMKD has a reaper thread which wakes up and calls
> > > process_mrelease() after the main LMKD thread issued SIGKILL.
> > >
> >
> > Thanks Suren. I remember Android is planning to use Apps in cgroup. Is
> > that still the plan? I am actually looking into cgroup.kill, beside
> > sending SIGKILL, putting the processes of the target cgroup in the oom
> > reaper list. In addition, making oom reaper able to reap processes in
> > parallel. I am hoping that functionality to be useful to Android as
> > well.
>
> Yes, cgroups v2 with per-app hierarchy is already enabled on Android
> as of about a year or so ago. The first usecase was the freezer. TJ
> (CC'ing him here) also changed how ActivityManager Service (AMS) kills
> process groups to use cgroup.kill (think when you force-stop an app
> that's what will happen). LMKD has not been changed to use cgroup.kill
> but that might be worth doing now. TJ, WDYT?

Sounds like it's worth trying here [1].

One potential downside of cgroup.kill is that it requires taking the
cgroup_mutex, which is one of our most heavily contended locks.

We already have logic that waits for exits in libprocessgroup's
KillProcessGroup [2], but I don't think LMKD needs or wants that from
its main thread. I think we'll still want process_mrelease [3] from
LMKD's reaper thread.

[1] https://cs.android.com/android/platform/superproject/main/+/main:system/memory/lmkd/reaper.cpp;drc=88ca1a4963004011669da415bc421b846936071f;l=233
[2] https://cs.android.com/android/platform/superproject/main/+/main:system/core/libprocessgroup/processgroup.cpp;drc=61197364367c9e404c7da6900658f1b16c42d0da;l=537
[3] https://cs.android.com/android/platform/superproject/main/+/main:system/memory/lmkd/reaper.cpp;drc=88ca1a4963004011669da415bc421b846936071f;l=123

Shakeel could we not also invoke the oom reaper's help for regular
kill(SIGKILL)s?