Re: [PATCH RESEND v9 00/10] sched: consolidation of CPU capacity and usage

Morten Rasmussen
2015-02-24T11:29:31Z

On Tue, Feb 24, 2015 at 10:38:29AM +0000, Vincent Guittot wrote:
> On 23 February 2015 at 16:45, Morten Rasmussen <morten.rasmussen@arm.com> wrote:
> > On Fri, Feb 20, 2015 at 02:54:09PM +0000, Vincent Guittot wrote:
> >> On 20 February 2015 at 15:35, Morten Rasmussen <morten.rasmussen@arm.com> wrote:
> >> > On Fri, Feb 20, 2015 at 02:13:21PM +0000, Vincent Guittot wrote:
> >> >> On 20 February 2015 at 12:52, Morten Rasmussen <morten.rasmussen@arm.com> wrote:
> >> >> > On Fri, Feb 20, 2015 at 11:34:47AM +0000, Peter Zijlstra wrote:
> >> >> >> On Thu, Feb 19, 2015 at 12:49:40PM +0000, Morten Rasmussen wrote:
> >> >> >>
> >> >> >> > Also, it still not clear why patch 10 uses relative capacity reduction
> >> >> >> > instead of absolute capacity available to CFS tasks.
> >> >> >>
> >> >> >> As present in your asymmetric big and small systems? Yes it would be
> >> >> >> unfortunate to migrate a task to an idle small core when the big core is
> >> >> >> still faster, even if reduced by rt/irq work.
> >> >> >
> >> >> > Yes, exactly. I don't think it would cause any harm for symmetric cases
> >> >> > to use absolute capacity instead. Am I missing something?
> >> >>
> >> >> If absolute capacity is used, we will trig an active load balance from
> >> >> little to big core each time a little has got 1 task and a big core is
> >> >> idle whereas we only want to trig an active migration is the src_cpu's
> >> >> capacity that is available for the cfs task is significantly reduced
> >> >> by rt tasks.
> >> >>
> >> >> I can mix absolute and relative tests by 1st testing that the capacity
> >> >> of the src is reduced and then ensure that the dst_cpu has more
> >> >> absolute capacity than src_cpu
> >> >
> >> > If we use absolute capacity and check if the source cpu is fully
> >> > utilized, wouldn't that work? We want to migrate the task if it is
> >>
> >> we want to trig the migration before the cpu is fully utilized by
> >> rt/irq (which almost never occurs)
> >
> > I meant fully utilized by rt/irq and cfs tasks, sorry. Essentially,
> > get_cpu_usage() ~= capacity_of(). If get_cpu_usage() is signficantly
> > smaller than capacity_of() which is may be reduced by rt/irq
> > utilization, there are still spare cycles and it is not strictly
> > required to migrate tasks away using active LB. But, tasks would be
> > moved away if the tasks are being allowed less cpu time due to rt/irq
> > (get_cpu_usage() >= capacity_of()). Wouldn't that work? Or, do you want
> > to migrate tasks regardless of whether there are still spare cycles
> > available on the cpu doing rt/irq work?
>
> In fact, we can see perf improvement even if the cpu is not fully used
> by thread and interrupts because the task becomes significantly
> preempted by interruptions.

Unless the tasks are the consumers of those interrupts, then it would
harm performance to migrate them away :) I get your point though. Could
we have a short comment stating the intentions so we don't forget in a
couple of months?

>
> >
> > The advantage of comparing get_cpu_usage() with capacity_of() is that it
> > would work for migrating cpu-intensive tasks away from little cpu on
> > big.LITTLE as well. Then we don't need another almost identical check
> > for that purpose :)
>
> I understand your point but the patch becomes inefficient for part of
> the issue that it's trying to originally solve if we compare
> get_cpu_usage with capacity_of. So we will probably need to add few
> more tests for the issue you point out above

Right. If your goal is to avoid preemptions and not just make sure that
cpus aren't fully utilized then my proposal isn't sufficient. We will
have to add another condition to solve the big.LITTLE capacity thing
later. In fact we already have that somewhere deep down in the pile of
patches I posted some weeks ago.

> >> > currently being restricted by the available capacity (due to rt/irq
> >> > work, being a little cpu, or both) and if there is a destination cpu
> >> > with more absolute capacity available. No?
> >>
> >> yes, so the relative capacity (cpu_capacity vs cpu_capacity_orig)
> >> enables us to know if the cpu is significantly used by irq/rt so it's
> >> worth to do an active load balance of the task. Then the absolute
> >> comparison of cpu_capacity of src_cpu vs cpu_capacity of dst_cpu
> >> checks that the dst_cpu is a better choice
> >>
> >> something like :
> >> if ((check_cpu_capacity(src_rq, sd)) &&
> >> (capacity_of(src_cpu)*sd->imbalce_pct < capacity_of(dst_cpu)*100))
> >> return 1;
> >
> > It should solve the big.LITTLE issue. Though I would prefer
> > get_cpu_usage() ~= capacity_of() approach as it could even improve
> > performance on big.LITTLE.
>
> ok. IMHO, it's worth having a dedicated patch for this issue

Fine by me as long as we get the extra check you proposed above to fix
the big.LITTLE issue.

Morten