Re: [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>]

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.

-- 
regards,
Dhaval