On Sat, 2016-01-30 at 21:36 +0100, Frederic Weisbecker wrote:
> On Sat, Jan 30, 2016 at 06:53:05PM +0100, Mike Galbraith wrote:
> > On Sat, 2016-01-30 at 15:20 +0100, Frederic Weisbecker wrote:
> > > On Fri, Jan 29, 2016 at 05:43:28PM -0500, Rik van Riel wrote:
> >
> > > > Run times for the microbenchmark:
> > > >
> > > > 4.4 3.8 seconds
> > > > 4.5-rc1 3.7 seconds
> > > > 4.5-rc1 + first patch 3.3 seconds
> > > > 4.5-rc1 + both patches 2.3 seconds
> > >
> > > Very nice improvement!
> >
> > Tasty indeed.
> >
> > When nohz_full CPUs are not isolated, ie are being used as generic
> > CPUs, get_nohz_timer_target() is a problem with things like tbench.
>
> So by isolated CPU you mean those part of isolcpus= boot option,
> right?
Yes, isolated in the scheduler sense, either via isolcpus= or cpusets.
If the CPU is part of a scheduler domain, it is by definition part of
the generic work crew.
> > tbench 8 with Rik's patches applied:
> > nohz_full=empty
> > Throughput 3204.69 MB/sec 1.000
> > nohz_full=1-3,5-7
> > Throughput 1354.99 MB/sec .422 1.000
> > nohz_full=1-3,5-7 + club below
> > Throughput 2762.22 MB/sec .861 2.038
> >
> > With Rik's patches and a club, tbench becomes nearly acceptable.
> > ---
> > include/linux/tick.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > --- a/include/linux/tick.h
> > +++ b/include/linux/tick.h
> > @@ -184,7 +184,7 @@ static inline const struct cpumask *hous
> > static inline bool is_housekeeping_cpu(int cpu)
> > {
> > #ifdef CONFIG_NO_HZ_FULL
> > - if (tick_nohz_full_enabled())
> > + if (tick_nohz_full_enabled() && runqueue_is_isolated(cpu))
> > return cpumask_test_cpu(cpu, housekeeping_mask);
>
> This makes me confused. How forcing timers to CPUs in isolcpus is making
> better results?
It doesn't, it's shutting get_nohz_timer_target() down for those
nohz_full CPUs that are NOT currently isolated, are thus generic CPUs
with the capability to _become_ elite solo artists on demand.
-Mike
