Re: [RFC 8/8] Do not reclaim the whole CPU bandwidth

[Luca Abeni]

On 01/14/2016 08:59 PM, Peter Zijlstra wrote:
> On Thu, Jan 14, 2016 at 04:24:53PM +0100, Luca Abeni wrote:
> > Original GRUB tends to reclaim 100% of the CPU time... And this allows a
> > "CPU hog" (i.e., a busy loop) to starve non-deadline tasks.
> > To address this issue, allow the scheduler to reclaim only a specified
> > fraction of CPU time.
> > NOTE: the fraction of CPU time that cannot be reclaimed is currently
> > hardcoded as (1 << 20) / 10 -> 90%, but it must be made configurable!
>
> So the alternative is an explicit SCHED_OTHER server which is
> configurable.
Yes, I have thought about something similar (actually, this is the strategy
I implemented in my first CBS/GRUB scheduler. With the "old" 2.4 scheduler,
this was easier :).
But I think the solution I implemented in this patch is much simpler (it
just requires a very simple modification to grub_reclaim()) and is more
elegant from the theoretical point of view.


> That would maybe fit in nicely with the DL based FIFO/RR servers from
> this other pending project.
Yes, this reminds me about the half-finished patch for RT throttling using
SCHED_DEADLINE... But that patch needs much more work IMHO.


				Thanks,
					Luca
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -524,6 +524,10 @@ struct dl_rq {
> >   	 * and decreased when a task blocks
> >   	 */
> >   	s64 running_bw;
> > +	/* This is the amount of utilization that GRUB can not
> > +         * reclaim (per runqueue)
> > +         */
> > +	s64 unusable_bw;
>
>
> Wrong comment style and whitespace challenged.