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

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  Re: [RFC 8/8] Do not reclaim the whole CPU bandwidth luca abeni <luca.abeni@unitn.it> - 2016-01-26 14:00 +0100
    Re: [RFC 8/8] Do not reclaim the whole CPU bandwidth Peter Zijlstra <peterz@infradead.org> - 2016-01-27 15:50 +0100

#1317867 — Re: [RFC 8/8] Do not reclaim the whole CPU bandwidth

Hi Peter,

On Fri, 15 Jan 2016 09:50:04 +0100
Peter Zijlstra <peterz@infradead.org> wrote:
[...]
> > >>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.
> 
> It is certainly simpler, agreed.
> 
> The trouble is with interfaces. Once we expose them we're stuck with
> them. And from that POV I think an explicit SCHED_OTHER server (or a
> minimum budget for a slack time scheme) makes more sense.
I am trying to work on this.
Which kind of interface is better for this? Would adding something like
/proc/sys/kernel/sched_other_period_us
/proc/sys/kernel/sched_other_runtime_us
be ok?

If this is ok, I'll add these two procfs files, and store
(sched_other_runtime / sched_other_period) << 20 in the runqueue field
which represents the unreclaimable utilization (implementing
hierarchical SCHED_DEADLINE/CFS scheduling right now is too complex for
this patchset... But if the exported interface is ok, it can be
implemented later).

Is this approach acceptable? Or am I misunderstanding your comment?



			Thanks,
				Luca

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#1319035

On Tue, Jan 26, 2016 at 01:52:19PM +0100, luca abeni wrote:

> > The trouble is with interfaces. Once we expose them we're stuck with
> > them. And from that POV I think an explicit SCHED_OTHER server (or a
> > minimum budget for a slack time scheme) makes more sense.

> I am trying to work on this.
> Which kind of interface is better for this? Would adding something like
> /proc/sys/kernel/sched_other_period_us
> /proc/sys/kernel/sched_other_runtime_us
> be ok?
> 
> If this is ok, I'll add these two procfs files, and store
> (sched_other_runtime / sched_other_period) << 20 in the runqueue field
> which represents the unreclaimable utilization (implementing
> hierarchical SCHED_DEADLINE/CFS scheduling right now is too complex for
> this patchset... But if the exported interface is ok, it can be
> implemented later).
> 
> Is this approach acceptable? Or am I misunderstanding your comment?

No, I think that's fine.

Altough now you have me worrying about per root_domain settings and the
like. But I think we can do that with additional interfaces, if needed.

So yes, please go with that.

And agreed, a full CFS server is a bit outside scope for this patch-set.

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