Re: [PATCH] sched: Add SCHED_BGND (background) scheduling policy

[Ingo Molnar <mingo@elte.hu>]

* Peter Williams <pwil3058@bigpond.net.au> wrote:

> >>+static inline void inc_mutex_count(void)
> >>+{
> >>+	current->mutexes_held++;
> >>+}
> >>+
> >>+static inline void dec_mutex_count(void)
> >>+{
> >>+	current->mutexes_held--;
> >>+}
> >>+
> >
> >NACK! This whole patch is way too intrusive for such a relatively small 
> >gain.
> >
> >also, if something doesnt hold a mutex, it might still be unsafe to 
> >background it! For example if it holds a semaphore. Or an rwsem. Or any 
> >other kernel resource that has exclusion semantics.
> >
> >so unless this patch gets _much_ less complex and much less intrusive, 
> >we'll have to stay with SCHED_BATCH and nice +19.
> 
> This means being less strict but (as you imply) that may be not much 
> better than nice +19.  I'll have a look at it.

it's way too much pain for little gain.

> Of course, a comprehensive (as opposed to RT only) priority 
> inheritance mechanism would make the "safe/unsafe to background" 
> problem go away and make this patch very simple.  Any plans in that 
> direction?

that seems quite unlikely to happen. I think you are missing the biggest 
issue: for RT, if the priority inheritance mechanism does not extend to 
a given scheduling pattern it causes longer latencies, but no harm is 
done otherwise. But for SCHED_BGND we'd have to make sure _every_ place 
is priority-inversions safe - otherwise we risk a potential local DoS if 
a task with a critical resource is backgrounded! That's plain impossible 
to achieve.

	Ingo
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