linux-kernel - Re: [PATCH 2/2] sched,time: call __acct_update

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Date:	Sun, 31 Jan 2016 03:52:25 +0100
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, mingo@...nel.org, peterz@...radead.org,
	luto@...capital.net, Clark Williams <williams@...hat.com>
Subject: Re: [PATCH 2/2] sched,time: call __acct_update_integrals once a
 jiffy

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