Date: Thu, 20 Nov 2008 08:58:29 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Bharata B Rao <bharata@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org,
       Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Dhaval Giani <dhaval@linux.vnet.ibm.com>
Subject: 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
Message-ID: <20081120075829.GA21785@elte.hu>
References: <20081120061854.GA4349@in.ibm.com>
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* 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.

	Ingo
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