Message-ID: <CABdmKX2386gYbF5BXzbJ3awybF+edGAfGgFguhAbppejYMKGZA@mail.gmail.com>
Date: Wed, 10 Sep 2025 15:10:29 -0700
From: "T.J. Mercier" <tjmercier@...gle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Shakeel Butt <shakeel.butt@...ux.dev>, Lei Liu <liulei.rjpt@...o.com>, 
	Michal Hocko <mhocko@...e.com>, David Rientjes <rientjes@...gle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Kemeng Shi <shikemeng@...weicloud.com>, 
	Kairui Song <kasong@...cent.com>, Nhat Pham <nphamcs@...il.com>, Baoquan He <bhe@...hat.com>, 
	Barry Song <baohua@...nel.org>, Chris Li <chrisl@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Muchun Song <muchun.song@...ux.dev>, David Hildenbrand <david@...hat.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>, Brendan Jackman <jackmanb@...gle.com>, 
	Zi Yan <ziy@...dia.com>, "Peter Zijlstra (Intel)" <peterz@...radead.org>, Chen Yu <yu.c.chen@...el.com>, 
	Hao Jia <jiahao1@...iang.com>, "Kirill A. Shutemov" <kas@...nel.org>, 
	Usama Arif <usamaarif642@...il.com>, Oleg Nesterov <oleg@...hat.com>, 
	Christian Brauner <brauner@...nel.org>, Mateusz Guzik <mjguzik@...il.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Andrii Nakryiko <andrii@...nel.org>, 
	Al Viro <viro@...iv.linux.org.uk>, Fushuai Wang <wangfushuai@...du.com>, 
	"open list:MEMORY MANAGEMENT - OOM KILLER" <linux-mm@...ck.org>, open list <linux-kernel@...r.kernel.org>, 
	"open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)" <cgroups@...r.kernel.org>
Subject: Re: [PATCH v0 0/2] mm: swap: Gather swap entries and batch async release

On Wed, Sep 10, 2025 at 1:41 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Wed, Sep 10, 2025 at 1:10 PM Shakeel Butt <shakeel.butt@...ux.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@...ux.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?

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