On 4/26/22 21:06, Feng Tang wrote:
On Tue, Apr 26, 2022 at 10:58:21PM +0800, Waiman Long wrote:
On 4/25/22 23:23, Feng Tang wrote:
Hi Waiman,
On Mon, Apr 25, 2022 at 11:55:05AM -0400, Waiman Long wrote:
There are 3 places where the cpu and node masks of the top cpuset can be initialized in the order they are executed:
- start_kernel -> cpuset_init()
- start_kernel -> cgroup_init() -> cpuset_bind()
- kernel_init_freeable() -> do_basic_setup() -> cpuset_init_smp()
The first cpuset_init() function just sets all the bits in the masks. The last one executed is cpuset_init_smp() which sets up cpu and node masks suitable for v1, but not v2. cpuset_bind() does the right setup for both v1 and v2.
For systems with cgroup v2 setup, cpuset_bind() is called once. For systems with cgroup v1 setup, cpuset_bind() is called twice. It is first called before cpuset_init_smp() in cgroup v2 mode. Then it is called again when cgroup v1 filesystem is mounted in v1 mode after cpuset_init_smp().
[ 2.609781] cpuset_bind() called - v2 = 1 [ 3.079473] cpuset_init_smp() called [ 7.103710] cpuset_bind() called - v2 = 0I run some test, on a server with centOS, this did happen that cpuset_bind() is called twice, first as v2 during kernel boot, and then as v1 post-boot.
However on a QEMU running with a basic debian rootfs image, the second call of cpuset_bind() didn't happen.
The first time cpuset_bind() is called in cgroup_init(), the kernel doesn't know if userspace is going to mount v1 or v2 cgroup. By default, it is assumed to be v2. However, if userspace mounts the cgroup v1 filesystem for cpuset, cpuset_bind() will be run at this point by rebind_subsystem() to set up cgroup v1 environment and cpus_allowed/mems_allowed will be correctly set at this point. Mounting the cgroup v2 filesystem, however, does not cause rebind_subsystem() to run and hence cpuset_bind() is not called again.
Is the QEMU setup not mounting any cgroup filesystem at all? If so, does it matter whether v1 or v2 setup is used?
When I got the cpuset binding error report, I tried first on qemu to reproduce and failed (due to there was no memory hotplug), then I reproduced it on a real server. For both system, I used "cgroup_no_v1=all" cmdline parameter to test cgroup-v2, could this be the reason? (TBH, this is the first time I use cgroup-v2).
Here is the info dump:
# mount | grep cgroup tmpfs on /sys/fs/cgroup type tmpfs (ro,nosuid,nodev,noexec,mode=755) cgroup on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,xattr,release_agent=/lib/systemd/systemd-cgroups-agent,name=systemd)
#cat /proc/filesystems | grep cgroup nodev cgroup nodev cgroup2
Thanks, Feng
For cgroup v2, cpus_allowed should be set to cpu_possible_mask and mems_allowed to node_possible_map as is done in the first invocation of cpuset_bind(). That is the correct behavior.
Cheers, Longman