Re: Fw: [lkp-developer] [sched,rcu] cf7a2dca60: [No primary change] +186% will-it-scale.time.involuntary_context_switches

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Dec 14, 2016 at 05:15:41PM +0100, Michal Hocko wrote:
> On Wed 14-12-16 03:06:09, Paul E. McKenney wrote:
> > On Wed, Dec 14, 2016 at 10:54:25AM +0100, Michal Hocko wrote:
> > > On Tue 13-12-16 07:14:08, Paul E. McKenney wrote:
> > > > Just FYI for the moment...
> > > > 
> > > > So even with the slowed-down checking, making cond_resched() do what
> > > > cond_resched_rcu_qs() does results in a smallish but quite measurable
> > > > degradation according to 0day.
> > > 
> > > So if I understand those results properly, the reason seems to be the
> > > increased involuntary context switches, right? Or am I misreading the
> > > data?
> > > I am looking at your "sched,rcu: Make cond_resched() provide RCU
> > > quiescent state" in linux-next and I am wondering whether rcu_all_qs has
> > > to be called unconditionally and not only when should_resched failed few
> > > times? I guess you have discussed that with Peter already but do not
> > > remember the outcome.
> > 
> > My first thought is to wait for the grace period to age further before
> > checking, the idea being to avoid increasing cond_resched() overhead
> > any further.  But if that doesn't work, then yes, I may have to look at
> > adding more checks to cond_resched().
> 
> This might be really naive but would something like the following work?
> The overhead should be pretty much negligible, I guess. Ideally the pcp
> variable could be set somewhere from check_cpu_stall() but I couldn't
> wrap my head around that code to see how exactly.

My concern (perhaps misplaced) with this approach is that there are
quite a few tight loops containing cond_resched().  So I would still
need to throttle the resulting grace-period acceleration to keep the
context switches down to a dull roar.

							Thanx, Paul

> --- 
> diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> index ac81e4063b40..1c005c5304a3 100644
> --- a/include/linux/rcutiny.h
> +++ b/include/linux/rcutiny.h
> @@ -243,6 +243,10 @@ static inline void rcu_all_qs(void)
>  	barrier(); /* Avoid RCU read-side critical sections leaking across. */
>  }
> 
> +static inline void cond_resched_rcu_check(void)
> +{
> +}
> +
>  /* RCUtree hotplug events */
>  #define rcutree_prepare_cpu      NULL
>  #define rcutree_online_cpu       NULL
> diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
> index 63a4e4cf40a5..176f6e386379 100644
> --- a/include/linux/rcutree.h
> +++ b/include/linux/rcutree.h
> @@ -110,6 +110,18 @@ extern int rcu_scheduler_active __read_mostly;
>  bool rcu_is_watching(void);
> 
>  void rcu_all_qs(void);
> +#ifndef CONFIG_PREEMPT
> +DECLARE_PER_CPU(int, rcu_needs_qs);
> +
> +static inline void cond_resched_rcu_check(void)
> +{
> +	/* Make sure we do not miss rcu_all_qs at least every now and then */
> +	if (this_cpu_inc_return(rcu_needs_qs) > 10) {
> +		this_cpu_write(rcu_needs_qs, 0);
> +		rcu_all_qs();
> +	}
> +}
> +#endif
> 
>  /* RCUtree hotplug events */
>  int rcutree_prepare_cpu(unsigned int cpu);
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 69a5611a7e7c..783c74ae9930 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -268,6 +268,9 @@ void rcu_bh_qs(void)
>  }
> 
>  static DEFINE_PER_CPU(int, rcu_sched_qs_mask);
> +#ifndef CONFIG_PREEMPT
> +DEFINE_PER_CPU(int, rcu_needs_qs);
> +#endif
> 
>  static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
>  	.dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 154fd689fe02..a58844be2ef1 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4905,6 +4905,8 @@ int __sched _cond_resched(void)
>  	if (should_resched(0)) {
>  		preempt_schedule_common();
>  		return 1;
> +	} else {
> +		cond_resched_rcu_check();
>  	}
>  	return 0;
>  }
> -- 
> Michal Hocko
> SUSE Labs
> 

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