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SubjectRe: [PATCH] sched: Don't allow priority switch to realtime when the task doesn't belong to init_task_group and when CONFIG_RT_GROUP_SCHED isn't set

* Dhaval Giani <dhaval@linux.vnet.ibm.com> wrote:

> On Thu, Nov 20, 2008 at 08:58:29AM +0100, Ingo Molnar wrote:
> >
> > * Bharata B Rao <bharata@linux.vnet.ibm.com> wrote:
> >
> > > Applies on 2.6.28-rc5.
> > >
> > > With CONFIG_RT_GROUP_SCHED not set, don't allow a task's priority
> > > switch to realtime if the task isn't part of init_task_group.
> > >
> > > A task belonging to a fair group could use
> > > sched_setscheduler/sched_setparam to become a realtime task. If such
> > > a task belongs to one of the child groups of init_task_group and if
> > > CONFIG_RT_GROUP_SCHED is not set, then it ends up getting queued in
> > > init_task_group's runqueue. So we have a situation where, a task
> > > belongs to one group (child) but ends in the runqueue of another
> > > group (init_task_group). This does not look correct.
> > >
> > > Fix this by failing such priority change requests in
> > > sched_setscheduler() and sched_setparam().
> > >
> > > Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> > > ---
> > > kernel/sched.c | 7 +++++++
> > > 1 file changed, 7 insertions(+)
> > >
> > > --- a/kernel/sched.c
> > > +++ b/kernel/sched.c
> > > @@ -5206,6 +5206,13 @@ recheck:
> > > if (rt_bandwidth_enabled() && rt_policy(policy) &&
> > > task_group(p)->rt_bandwidth.rt_runtime == 0)
> > > return -EPERM;
> > > +#elif defined(CONFIG_FAIR_GROUP_SCHED)
> > > + /*
> > > + * If the task doesn't belong to init_task_group, don't
> > > + * allow priority switch to realtime. (!CONFIG_RT_GROUP_SCHED)
> > > + */
> > > + if (rt_policy(policy) && (task_group(p) != &init_task_group))
> > > + return -EPERM;
> > > #endif
> > >
> > > retval = security_task_setscheduler(p, policy, param);
> >
> > hm, another option would be, instead of denying something (which
> > denial might not even be noticed by the app) that the app clearly has
> > enough privilege to request - to just act upon it and move the task to
> > the init_task_group?
> >
> > the app cannot expect fair scheduling for this task anyway. And if we
> > want to forbid tasks from doing so - do not give them privilege to go
> > to RT priorities.
> >
>
> I am wondering what would the right action then be if the task drops
> back to CFS.

yeah. If the integration artifacts around the edges get too awkward,
then the best would be to consolidate fair-group and rt-group into the
same group-sched config option and _eliminate_ such artifacts at their
root. rt-group was started as a separate option mostly because it was
new and experimental code - that splitup is not cast into stone.

Ingo


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