Hello,

The cgroup v2 freezer controller is useful for freezing background
applications so they don't contend with foreground tasks. However, this
may disrupt any internal monitoring that the application is performing,
as it may not be aware that it was frozen.

To illustrate, an application might implement a watchdog thread to
monitor a high-priority task by periodically checking its state to
ensure progress. The challenge is that the task only advances when the
application is running, but watchdog timers are set relative to system
time, not app time. If the app is frozen and misses the expected
deadline, the watchdog, unaware of this pause, may kill a healthy
process.

This series tracks the time that each cgroup spends "freezing" and
exposes it via cgroup.stat.local. Include several basic selftests to
demonstrate the expected behavior of this interface, including that:
  1. Freeze time will increase while a cgroup is freezing, regardless of
     whether it is frozen or not.
  2. Each cgroup's freeze time is independent from the other cgroups in
     its hierarchy.

Thanks,
Tiffany

Signed-off-by: Tiffany Yang <ynaffit@google.com>
---
v3: https://lore.kernel.org/all/20250805032940.3587891-4-ynaffit@google.com/
v2: https://lore.kernel.org/lkml/20250714050008.2167786-2-ynaffit@google.com/
v1: https://lore.kernel.org/lkml/20250603224304.3198729-3-ynaffit@google.com/

Cc: John Stultz <jstultz@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Anna-Maria Behnsen <anna-maria@linutronix.de>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Pavel Machek <pavel@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Tiffany Yang <ynaffit@google.com>

Tiffany Yang (2):
  cgroup: cgroup.stat.local time accounting
  cgroup: selftests: Add tests for freezer time

 Documentation/admin-guide/cgroup-v2.rst       |  18 +
 include/linux/cgroup-defs.h                   |  17 +
 kernel/cgroup/cgroup.c                        |  28 +
 kernel/cgroup/freezer.c                       |  16 +-
 tools/testing/selftests/cgroup/test_freezer.c | 663 ++++++++++++++++++
 5 files changed, 738 insertions(+), 4 deletions(-)

-- 
2.51.0.rc2.233.g662b1ed5c5-goog

On Thu, Aug 21, 2025 at 06:37:51PM -0700, Tiffany Yang wrote:
> Hello,
> 
> The cgroup v2 freezer controller is useful for freezing background
> applications so they don't contend with foreground tasks. However, this
> may disrupt any internal monitoring that the application is performing,
> as it may not be aware that it was frozen.
> 
> To illustrate, an application might implement a watchdog thread to
> monitor a high-priority task by periodically checking its state to
> ensure progress. The challenge is that the task only advances when the
> application is running, but watchdog timers are set relative to system
> time, not app time. If the app is frozen and misses the expected
> deadline, the watchdog, unaware of this pause, may kill a healthy
> process.
> 
> This series tracks the time that each cgroup spends "freezing" and
> exposes it via cgroup.stat.local. Include several basic selftests to
> demonstrate the expected behavior of this interface, including that:
>   1. Freeze time will increase while a cgroup is freezing, regardless of
>      whether it is frozen or not.
>   2. Each cgroup's freeze time is independent from the other cgroups in
>      its hierarchy.
> 
> Thanks,
> Tiffany
> 
> Signed-off-by: Tiffany Yang <ynaffit@google.com>

Applied to cgroup/for-6.18. Let's address further issues incrementally.

Thanks.

-- 
tejun