Re: [PATCH tip/core/rcu 11/17] rcu: Bind grace-period kthreads to non-NO_HZ_FULL CPUs

[Paul E. McKenney]

On Tue, Jul 08, 2014 at 05:24:00PM +0200, Frederic Weisbecker wrote:
> On Mon, Jul 07, 2014 at 03:38:15PM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> > 
> > Binding the grace-period kthreads to the timekeeping CPU resulted in
> > significant performance decreases for some workloads.  For more detail,
> > see:
> > 
> > https://lkml.org/lkml/2014/6/3/395 for benchmark numbers
> > 
> > https://lkml.org/lkml/2014/6/4/218 for CPU statistics
> > 
> > It turns out that it is necessary to bind the grace-period kthreads
> > to the timekeeping CPU only when all but CPU 0 is a nohz_full CPU
> > on the one hand or if CONFIG_NO_HZ_FULL_SYSIDLE=y on the other.
> > In other cases, it suffices to bind the grace-period kthreads to the
> > set of non-nohz_full CPUs.
> > 
> > This commit therefore creates a tick_nohz_not_full_mask that is the
> > complement of tick_nohz_full_mask, and then binds the grace-period
> > kthread to the set of CPUs indicated by this new mask, which covers
> > the CONFIG_NO_HZ_FULL_SYSIDLE=n case.  The CONFIG_NO_HZ_FULL_SYSIDLE=y
> > case still binds the grace-period kthreads to the timekeeping CPU.
> > This commit also includes the tick_nohz_full_enabled() check suggested
> > by Frederic Weisbecker.
> > 
> > Reported-by: Jet Chen <jet.chen@xxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > [ paulmck: Created housekeeping_affine() per fweisbec feedback. ]
> > ---
> >  include/linux/tick.h     | 19 +++++++++++++++++++
> >  kernel/rcu/tree_plugin.h | 14 +++++++++-----
> >  kernel/time/tick-sched.c | 10 ++++++++++
> >  3 files changed, 38 insertions(+), 5 deletions(-)
> > 
> > diff --git a/include/linux/tick.h b/include/linux/tick.h
> > index b84773cb9f4c..c39af3261351 100644
> > --- a/include/linux/tick.h
> > +++ b/include/linux/tick.h
> > @@ -12,6 +12,7 @@
> >  #include <linux/hrtimer.h>
> >  #include <linux/context_tracking_state.h>
> >  #include <linux/cpumask.h>
> > +#include <linux/sched.h>
> >  
> >  #ifdef CONFIG_GENERIC_CLOCKEVENTS
> >  
> > @@ -162,6 +163,7 @@ static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }
> >  #ifdef CONFIG_NO_HZ_FULL
> >  extern bool tick_nohz_full_running;
> >  extern cpumask_var_t tick_nohz_full_mask;
> > +extern cpumask_var_t tick_nohz_not_full_mask;
> 
> So I'm still puzzled by this mask.
> 
> How about creating a temporary cpumask on top of tick_nohz_full_mask
> from housekeeping_affine().
> 
> If you wonder about performance, this can be called once for good from
> rcu_spawn_gp_kthread() (that would be much better than checking that all
> the time from the kthread itself anyway).

I was figuring that a fair number of the kthreads might eventually
be using this, not just for the grace-period kthreads.  In addition,
my concern about once-for-good affinity is for the NO_HZ_FULL_SYSIDLE=y
case, where moving the grace-period kthreads prevents ever entering
full-system idle state.

Or am I missing some use case?

							Thanx, Paul

> In case you're out of time, I can handle that. Just checking your opinion.
> 
> Thanks.
> 
> >  
> >  static inline bool tick_nohz_full_enabled(void)
> >  {
> > @@ -194,6 +196,23 @@ static inline void tick_nohz_full_kick_all(void) { }
> >  static inline void __tick_nohz_task_switch(struct task_struct *tsk) { }
> >  #endif
> >  
> > +static inline bool is_housekeeping_cpu(int cpu)
> > +{
> > +#ifdef CONFIG_NO_HZ_FULL
> > +	if (tick_nohz_full_enabled())
> > +		return cpumask_test_cpu(cpu, tick_nohz_not_full_mask);
> > +#endif
> > +	return true;
> > +}
> > +
> > +static inline void housekeeping_affine(struct task_struct *t)
> > +{
> > +#ifdef CONFIG_NO_HZ_FULL
> > +	if (tick_nohz_full_enabled())
> > +		set_cpus_allowed_ptr(t, tick_nohz_not_full_mask);
> > +#endif
> > +}
> > +
> >  static inline void tick_nohz_full_check(void)
> >  {
> >  	if (tick_nohz_full_enabled())
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 719587af7b10..b39ba7239bd6 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -2846,12 +2846,16 @@ static bool rcu_nohz_full_cpu(struct rcu_state *rsp)
> >   */
> >  static void rcu_bind_gp_kthread(void)
> >  {
> > -#ifdef CONFIG_NO_HZ_FULL
> > -	int cpu = tick_do_timer_cpu;
> > +	int __maybe_unused cpu;
> >  
> > -	if (cpu < 0 || cpu >= nr_cpu_ids)
> > +	if (!tick_nohz_full_enabled())
> >  		return;
> > -	if (raw_smp_processor_id() != cpu)
> > +#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
> > +	cpu = tick_do_timer_cpu;
> > +	if (cpu >= 0 && cpu < nr_cpu_ids && raw_smp_processor_id() != cpu)
> >  		set_cpus_allowed_ptr(current, cpumask_of(cpu));
> > -#endif /* #ifdef CONFIG_NO_HZ_FULL */
> > +#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
> > +	if (!is_housekeeping_cpu(raw_smp_processor_id()))
> > +		housekeeping_affine(current);
> > +#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
> >  }
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 6558b7ac112d..e023134d63a1 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -154,6 +154,7 @@ static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
> >  
> >  #ifdef CONFIG_NO_HZ_FULL
> >  cpumask_var_t tick_nohz_full_mask;
> > +cpumask_var_t tick_nohz_not_full_mask;
> >  bool tick_nohz_full_running;
> >  
> >  static bool can_stop_full_tick(void)
> > @@ -281,6 +282,7 @@ static int __init tick_nohz_full_setup(char *str)
> >  	int cpu;
> >  
> >  	alloc_bootmem_cpumask_var(&tick_nohz_full_mask);
> > +	alloc_bootmem_cpumask_var(&tick_nohz_not_full_mask);
> >  	if (cpulist_parse(str, tick_nohz_full_mask) < 0) {
> >  		pr_warning("NOHZ: Incorrect nohz_full cpumask\n");
> >  		return 1;
> > @@ -291,6 +293,8 @@ static int __init tick_nohz_full_setup(char *str)
> >  		pr_warning("NO_HZ: Clearing %d from nohz_full range for timekeeping\n", cpu);
> >  		cpumask_clear_cpu(cpu, tick_nohz_full_mask);
> >  	}
> > +	cpumask_andnot(tick_nohz_not_full_mask,
> > +		       cpu_possible_mask, tick_nohz_full_mask);
> >  	tick_nohz_full_running = true;
> >  
> >  	return 1;
> > @@ -332,9 +336,15 @@ static int tick_nohz_init_all(void)
> >  		pr_err("NO_HZ: Can't allocate full dynticks cpumask\n");
> >  		return err;
> >  	}
> > +	if (!alloc_cpumask_var(&tick_nohz_not_full_mask, GFP_KERNEL)) {
> > +		pr_err("NO_HZ: Can't allocate not-full dynticks cpumask\n");
> > +		return err;
> > +	}
> >  	err = 0;
> >  	cpumask_setall(tick_nohz_full_mask);
> >  	cpumask_clear_cpu(smp_processor_id(), tick_nohz_full_mask);
> > +	cpumask_clear(tick_nohz_not_full_mask);
> > +	cpumask_set_cpu(smp_processor_id(), tick_nohz_not_full_mask);
> >  	tick_nohz_full_running = true;
> >  #endif
> >  	return err;
> > -- 
> > 1.8.1.5
> > 
> 

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