Re: cgroups(7): documenting cgroups v2 thread mode

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

On 01/09/2018 10:10 PM, Tejun Heo wrote:
> Hello,
> 
> On Tue, Jan 02, 2018 at 07:24:01PM +0100, Michael Kerrisk (man-pages) wrote:
>>        2. We write the string "threaded" to each of the domain invalid
>>           cgroups  under  y,  in  order  to  convert  them to the type
>>           threaded.  As a consequence of this step, all threads  under
>>           the  threaded  root  now  have  the  type  threaded  and the
>>           threaded subtree is now fully usable.   The  requirement  to
>>           write  "threaded"  to each of these cgroups is somewhat cum‐
>>           bersome, but allows for possible future  extensions  to  the
>>           thread-mode model.
>>
>>           ┌─────────────────────────────────────────────────────┐
>>           │FIXME                                                │
>>           ├─────────────────────────────────────────────────────┤
>>           │Re  the preceding paragraphs... Are there other rea‐ │
>>           │sosn  for  the  (cumbersome)  requirement  to  write │
>>           │'threaded'  to  each of the cgroup.type files in the │
>>           │threaded subtrees? Tejun Heo mentioned  the  follow‐ │
>>           │ing:                                                 │
>>           │                                                     │
>>           │    Consistency w/ the cgroups right under the root  │
>>           │    cgroup.  Because they can be both domains and    │
>>           │    threadroots, we can't switch the children over   │
>>           │    to thread mode automatically.  Doing that for    │
>>           │    cgroups further down in the hierarchy would be   │
>>           │    really inconsistent.                             │
>>           │                                                     │
>>           │But,  it's  not  clear  to  me  how  "Doing that for │
>>           │cgroups further  down  in  the  hierarchy  would  be │
>>           │really inconsistent", since in the current implemen‐ │
>>           │tation, those same thread groups  are  converted  to │
>>           │"domain invalid" type.  What am I missing?           │
>>           └─────────────────────────────────────────────────────┘
> 
> Yeah, I was confused with an earlier varient where we were marking
> threaded domains instead of threaded roots.  It's mostly about future
> extensibility (especially as Waiman was proposing related changes
> there) and not doing things automatically / recursively if possible.
> 
> Looks good to me.

One more thing. I added the following sentence to the text:

       The  cgroup.threads file is writable only for the cgroups inside a
       threaded subtree.

Can you confirm that that is correct, please.

Cheers,

Michael



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