[SRU][N:hwe-6.14][PATCH v2 0/1] enable MEMCG_V1 and CPUSETS_V1 on Noble HWE

[Krister Johansen]

BugLink: https://bugs.launchpad.net/bugs/2122368

[Impact]

Although v1 cgroups are deprecated in Noble, it was still possible for
users on 6.8 kernels to utilize them.  This was especially helpful in
the Noble migration process.  It allowed users to pick up the new OS and
then separately upgrade their remaining v1 cgroups applications.  This
unblocked the migration path for v1 cgroups users, because v1 support
could be provisionally enabled until the necessary support was available
in the applications that still lack v2 support.

Starting in 6.12, CONFIG_MEMCG_V1 and CONFIG_CPUSETS_V1 were added and
defaulted to false.  Noble 6.8 users that were unlucky enough to still
need these V1 cgroups found that they could no longer use them in the
6.14 kernel.

Some of the specific failures that were encountered include older JVMs
that fail to correctly handle missing controllers from /proc/cgroups.
If memory or cpuset are absent, the container limit detection is turned
off and the JVM uses the host's limits.  JVMs configured in containers
with specific memory usage percentages then end up consuming too much
memory and often crash.

Apache Yarn is still completing their v1 -> v2 migration, which leaves
some Hadoop use cases without proper support.

The request here is to enable these V1 controllers on Noble, but not
newer releases, for as long as the Noble HWE kernel train still has
kernels with upstream cgroup v1 support.  This gives users a little bit
longer to complete their migration while still using newer hardware, but
with the understanding that this really is the end of the line for v1
cgroups.

[Fix]

Re-enable the missing v1 controllers in the 6.14 Noble config.

In 6.8 there were 14 controllers.  In the current 6.14 config there are
also 14 controllers.  However, the difference is that the current 6.14
build the dmem controller was added, and the cpuset and memory
controllers were removed.

Diffing both the /proc/cgroups and configs between the 6.14 and 6.8
releases gives:

  -CPUSETS_V1 n
  -MEMCG_V1 n

These differences were also corroborated via source inspection.  Changes
in 6.12 moved these controllers to be guarded by ifdefs that default to
being disabled via make olddefconfig.

In order to ensure that 6.14 has the same v1 cgroup controllers enabled
as 6.8, enable both CONFIG_CPUSETS_V1 and CONFIG_MEMCG_V1 for Noble.

[Test]

Booted a kernel with this change and validated that the missing v1
memcgs were present again.

Before:

   $ grep memory /proc/cgroups 
   $ grep cpuset /proc/cgroups 
   
 with v1 cgroups enabled:
   
   $ mount | grep cgroup | grep memory
   $ mount | grep cgroup | grep cpuset
   
   $ ls /sys/fs/cgroup | grep memory
   $ ls /sys/fs/cgroup | grep cpuset

After:

   $ grep memory /proc/cgroups 
   memory	0	88	1
   $ grep cpuset /proc/cgroups 
   cpuset	0	88	1
   
 with v1 cgroups enabled:
   
   $ mount | grep cgroup | grep memory
   cgroup on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,memory)
   $ mount | grep cgroup | grep cpuset
   cgroup on /sys/fs/cgroup/cpuset type cgroup (rw,nosuid,nodev,noexec,relatime,cpuset)
   
   $ ls /sys/fs/cgroup | grep memory
   memory
   $ ls /sys/fs/cgroup | grep cpuset
   cpuset

A config diff of the previous build versus a build cranked from these
patches:

 CPUSETS_V1 n -> y
 MEMCG_V1 n -> y

[Where problems can occur]

Since these changes re-introduce code that was disabled via ifdef,
there's a possible increase in the binary size.  After comparing the
results from an identical build with these config flags disabled, the
difference in compressed artifact size for an x86 vmlinuz is an increase
of 16k.

The difference in uncompressed memory usage after boot is an increase of
40k, broken down as 21k code, 19k rwdata, 12k rodata, 8k init, -28k
bss, and 8k reserved.

The primary remaining risk is around future breakage of these interfaces
since they are no longer part of the default configuration.  If this is
not part of upstream's test matrix, then there is additional potential
breakage possible. However, the author has no knowledge of actual v1
cgroups breakage at the time this patch is being submitted.

Krister Johansen (1):
  UBUNTU: [Config] enable MEMCG_V1 and CPUSETS_V1 on Noble HWE

 debian.hwe-6.14/config/annotations | 6 ++++++
 1 file changed, 6 insertions(+)

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2.43.0