Hi Alejandro,
On Thu, Jan 16, 2025 at 02:06:26PM +0100 Alejandro Colomar wrote:
> Hi Phil,
>
> > Subject: sched: Mention autogroup disabled behavior
>
> Please use the pathname of the modified file as a prefix:
>
> man/man7/sched.7: Mention autogroup disabled behavior
ack
>
> On Thu, Jan 16, 2025 at 12:46:54PM +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.
>
> Would you mind showing (in the commit message) a shell session that
> demonstrates this?
This is actually part of the problem. It's very hard to see this
from userspace. I can show a shell session that shows that autogroup
is disabled and that my task has an autogroup in /proc but determining
that the autogroup is not being used not so much. (I may be missing
something obvious but I could not find it).
I had to look at the kernel code:
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;
}
bool task_wants_autogroup(struct task_struct *p, struct task_group *tg)
{
if (tg != &root_task_group)
return false;
...
}
The former being called from sched_group_fork() and sched_get_task_group().
I suppose looking at /proc/pid/cgroup and seeing it report not "0::/"
is part of it since it then won't be in root task group.
To some extent any systemd based system these days is not really
using autogroup at all anyway.
I can put some of the above in there or just something like:
# cat /proc/sys/kernel/sched_autogroup_enabled
0
# cat /proc/$$/autogroup
/autogroup-112 nice 0
Thoughts?
Cheers,
Phil
>
> > This can be confusing
> > to users. Add a sentence to this effect to sched.7 to
> > point this out.
> >
> > Signed-off-by: Phil Auld <pauld@xxxxxxxxxx>
> > To: Alejandro Colomar <alx@xxxxxxxxxx>
> > Cc: <linux-man@xxxxxxxxxxxxxxx>
> > Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>
>
> Thanks!
>
> >
> > ---
> > 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
>
> s/disabled/&,/
>
> Also, please use semantic newlines. See man-pages(7):
>
> $ MANWIDTH=72 man man-pages | sed -n '/Use semantic newlines/,/^$/p'
> Use semantic newlines
> In the source of a manual page, new sentences should be started on
> new lines, long sentences should be split into lines at clause
> breaks (commas, semicolons, colons, and so on), and long clauses
> should be split at phrase boundaries. This convention, sometimes
> known as "semantic newlines", makes it easier to see the effect of
> patches, which often operate at the level of individual sentences,
> clauses, or phrases.
>
>
> Have a lovely day!
> Alex
>
> > +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
> > --
> > 2.47.0
> >
>
> --
> <https://www.alejandro-colomar.es/>
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