Re: [PATCH v2] man/man7/sched.7: Mention autogroup disabled behavior

[Alejandro Colomar]

Hi Phil,

On Thu, Jan 16, 2025 at 02:37:47PM +0000, Phil Auld wrote:
> The autogroup feature can be contolled at runtime when
> built into the kernel. Disabling it in this case still
> creates autogroups and still shows the autogroup membership
> for the task in /proc.  The scheduler code will just not
> use the the autogroup task group.  This can be confusing
> to users. Add a sentence to this effect to sched.7 to
> point this out.
> 
> The kernel code shows how this is used. The
> sched_autogroup_enabled toggle is only used in one place.
> 
> kernel/sched/autogroup.h:
> 
> static inline struct task_group *
> autogroup_task_group(struct task_struct *p, struct task_group *tg)
> {
>         extern unsigned int sysctl_sched_autogroup_enabled;
>         int enabled = READ_ONCE(sysctl_sched_autogroup_enabled);
> 
>         if (enabled && task_wants_autogroup(p, tg))
>                 return p->signal->autogroup->tg;
> 
>         return tg;
> }
> 
> task_wants_autogroup() is in kernel/sched/autogroup.c:
> 
> bool task_wants_autogroup(struct task_struct *p, struct task_group *tg)
> {
>         if (tg != &root_task_group)
>                 return false;
>     ...
> 
>         return true;
> }
> 
> One can see that any group set other than root also bypasses the use of
> the autogroup.
> 
> All of the machinery around the creation of the autogroup is not
> effected by the toggle.
> 
> From userspace:
> 0
> /autogroup-112 nice 0
> 
> Note, systemd based system these days is not really using autogroups at all
> anyway because any task in a non-root cgroup bypasses the autogroup as
> well.
> 
> Signed-off-by: Phil Auld <pauld@xxxxxxxxxx>
> Cc: Alejandro Colomar <alx@xxxxxxxxxx>
> Cc: <linux-man@xxxxxxxxxxxxxxx>

Thanks!  I've applied the patch.
<https://www.alejandro-colomar.es/src/alx/linux/man-pages/man-pages.git/commit/?h=contrib&id=ca69daf45f94fda061f796efcc4f24ca76d8e380>


Have a lovely day!
Alex

> ---
>  man/man7/sched.7 | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/man/man7/sched.7 b/man/man7/sched.7
> index 71f098e48..f0a708cd7 100644
> --- a/man/man7/sched.7
> +++ b/man/man7/sched.7
> @@ -724,6 +724,8 @@ in the group terminates.
>  .P
>  When autogrouping is enabled, all of the members of an autogroup
>  are placed in the same kernel scheduler "task group".
> +When disabled the group creation happens as above, and autogroup membership
> +is still visible in /proc, but the autogroups are not used.
>  The CFS scheduler employs an algorithm that equalizes the
>  distribution of CPU cycles across task groups.
>  The benefits of this for interactive desktop performance
> -- 
> 2.47.1
> 
> 

-- 
<https://www.alejandro-colomar.es/>

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