On 5/19/20 3:51 PM, Christian Brauner wrote:

On Tue, May 19, 2020 at 03:36:28PM +0200, Michael Kerrisk (man-pages) wrote:

quoted

On 5/18/20 7:55 PM, Christian Brauner wrote:

quoted

From: Christian Brauner <redacted>
+
+Spawning a process into a cgroup different from the parent's cgroup
+makes it possible for a service manager to directly spawn new
+services into dedicated cgroups. This allows eliminating accounting
+jitter which would be caused by the new process living in the
+parent's cgroup for a short amount of time before being
+moved into the target cgroup. This flag also allows the creation of
+frozen child process by spawning them into a frozen cgroup (see
+.BR cgroups (7)
+for a description of the freezer feature in version 2 cgroups).
+For threaded applications or even thread implementations which
+make use of cgroups to limit individual threads it is possible to
+establish a fixed cgroup layout before spawning each thread
+directly into its target cgroup.

Thanks for these use cases; that's great!

So, I did some fairly heavy editing, which resulted in the
following (the sum of the diffs is shown at the end of this
mail):

       CLONE_INTO_CGROUP (since Linux 5.7)
              By default, a child process is placed in the same version 2
              cgroup  as  its  parent.   The CLONE_INTO_CGROUP allows the

Not a native speaker, but is this missing a noun like "flag"?
"The CLONE_INTO_CGROUP {flag,feature} allows the [...]"?

Yes, "flag" was missing. Thanks.

quoted

              child process to  be  created  in  a  different  version  2
              cgroup.   (Note  that CLONE_INTO_CGROUP has effect only for
              version 2 cgroups.)

              In order to place the child process in a different  cgroup,
              the caller specifies CLONE_INTO_CGROUP in cl_args.flags and
              passes a file descriptor that refers to a version 2  cgroup
              in  the cl_args.cgroup field.  (This file descriptor can be
              obtained by opening a cgroup v2 directory file using either

Should this just be "opening a cgroup v2 directory" and not "directory
file"? Feels redundant.

Yes, better. Changed.
 

quoted

              the  O_RDONLY  or  the  O_PATH flag.)  Note that all of the
              usual restrictions (described in cgroups(7)) on  placing  a
              process into a version 2 cgroup apply.

              Spawning  a  process  into a cgroup different from the par‐
              ent's cgroup makes it possible for  a  service  manager  to
              directly  spawn  new services into dedicated cgroups.  This
              eliminates the accounting jitter that would  be  caused  if
              the  child  process was first created in the same cgroup as
              the parent and then moved  into  the  target  cgroup.   The

I forgot to mention that spawning directly into a target cgroup is also
more efficient than moving it after creation. The specific reason is
mentioned in the commit message, the write lock of the semaphore need
not be taken in contrast to when it is moved afterwards. That
implementation details is not that interesting but it might be
interesting to know that it provides performance benefits in general.

Thanks. I added this sentence:

    Furthermore, spawning the child process directly into a 
    target cgroup is significantly cheaper than moving the child 
    process into the target cgroup after it has been created.

quoted

Look okay to you?

Yep, looks great!

Good!

Thanks for the review.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/