linux-kernel - Re: RFC: documentation of the autogroup feature

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Message-ID: <CAKgNAkiLafr7KJNAo+_kwKQB7ueO-oontNL_FQrGtqitmJpEDg@mail.gmail.com>
Date:   Tue, 29 Nov 2016 10:10:22 +0100
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Mike Galbraith <efault@....de>
Cc:     Michael Kerrisk <mtk.manpages@...il.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...nel.org>,
        linux-man <linux-man@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: RFC: documentation of the autogroup feature

[Resending because of bounces from the lists. (Somehow my mailer
messed up the MIME labeling)]

Hi Mike,

On 11/28/2016 02:46 AM, Mike Galbraith wrote:
> On Sun, 2016-11-27 at 22:13 +0100, Michael Kerrisk (man-pages) wrote:
>
>> Here's my attempt to define the root task group:
>>
>>        *  If autogrouping is disabled, then all processes in the root CPU
>>           cgroup form a scheduling group (sometimes called the "root task
>>           group").
>>
>> Can you improve on this?

The below is helpful, but...

> A task group is a set of percpu runqueues.

The explanation needs really to be in terms of what user-space
understands and sees. "Runqueues" are a kernel scheduler implementation
detail.

> The root task group is the
> top level set in a hierarchy of such sets when group scheduling is
> enabled, or the only set when group scheduling is not enabled.  The
> autogroup hierarchy has a depth of one, ie all autogroups are peers
> who's common parent is the root task group.

Let's try and go further. How's this:

       When scheduling non-real-time  processes  (i.e.,  those  scheduled
       under  the SCHED_OTHER, SCHED_BATCH, and SCHED_IDLE policies), the
       CFS scheduler employs a technique known as "group scheduling",  if
       the  kernel was configured with the CONFIG_FAIR_GROUP_SCHED option
       (which is typical).

       Under group scheduling, threads are scheduled  in  "task  groups".
       Task  groups  have  a  hierarchical relationship, rooted under the
       initial task group on the system, known as the "root task  group".
       Task groups are formed in the following circumstances:

       *  All of the threads in a CPU cgroup form a task group.  The par‐
          ent of this task group is the task group of  the  corresponding
          parent cgroup.

       *  If  autogrouping  is  enabled, then all of the threads that are
          (implicitly) placed in an autogroup (i.e., the same session, as
          created by setsid(2)) form a task group.  Each new autogroup is
          thus a separate task group.  The root task group is the  parent
          of all such autogroups.

       *  If  autogrouping  is enabled, then the root task group consists
          of all processes in the root CPU cgroup that were not otherwise
          implicitly placed into a new autogroup.

       *  If  autogrouping is disabled, then the root task group consists
          of all processes in the root CPU cgroup.

       *  If group scheduling was disabled (i.e., the kernel was  config‐
          ured  without  CONFIG_FAIR_GROUP_SCHED),  then  all of the pro‐
          cesses on the system are notionally placed  in  a  single  task
          group.

       [To be followed by a discussion of the nice value and task groups]

?

Cheers,

Michael


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