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

[Phil Auld <pauld@redhat.com>]

On Sun, Feb 02, 2025 at 02:02:21PM +0100 Alejandro Colomar wrote:
> 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@redhat.com>
> > Cc: Alejandro Colomar <alx@kernel.org>
> > Cc: <linux-man@vger.kernel.org>
> 
> 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!

Thanks!

You too :)


Cheers,
Phil


> 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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