linux-kernel - Re: [PATCH v2] cgroup/cpuset: Remove cpus_allowed/mems_allowed setup in cpuset_init

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Message-ID: <4c6847ba-4c8d-9776-a065-684a8b95130b@redhat.com>
Date:   Tue, 26 Apr 2022 22:34:21 -0400
From:   Waiman Long <longman@...hat.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH v2] cgroup/cpuset: Remove cpus_allowed/mems_allowed setup
 in cpuset_init_smp()

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:
>>>>    1) start_kernel -> cpuset_init()
>>>>    2) start_kernel -> cgroup_init() -> cpuset_bind()
>>>>    3) 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 = 0
>>> I 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