RE: [PATCH 1/2] nohz: Affine unpinned timers to housekeepers

[Jiang, Yunhong]

> -----Original Message-----
> From: linux-kernel-owner@xxxxxxxxxxxxxxx [mailto:linux-kernel-
> owner@xxxxxxxxxxxxxxx] On Behalf Of Frederic Weisbecker
> Sent: Tuesday, September 1, 2015 7:51 AM
> To: Ingo Molnar; Peter Zijlstra
> Cc: LKML; Vatika Harlalka; Chris Metcalf; Thomas Gleixner; Preeti U Murthy;
> Christoph Lameter; Frederic Weisbecker; Paul E . McKenney
> Subject: [PATCH 1/2] nohz: Affine unpinned timers to housekeepers
> 
> From: Vatika Harlalka <vatikaharlalka@xxxxxxxxx>
> 
> The problem addressed in this patch is about affining unpinned timers.
> Adaptive or Full Dynticks CPUs are currently disturbed by unnecessary
> jitter due to firing of such timers on them.
> 
> This patch will affine timers to online CPUs which are not full dynticks
> in NOHZ_FULL configured systems. It should not introduce overhead in
> nohz full off case due to static keys.
> 
> Reviewed-by: Preeti U Murthy <preeti@xxxxxxxxxxxxxxxxxx>
> Signed-off by: Vatika Harlalka <vatikaharlalka@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Chris Metcalf <cmetcalf@xxxxxxxxxx>
> Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> ---
>  include/linux/tick.h | 9 +++++++++
>  kernel/sched/core.c  | 7 +++++--
>  2 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index 48d901f..e312219 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -147,11 +147,20 @@ static inline void tick_nohz_full_add_cpus_to(struct
> cpumask *mask)
>  		cpumask_or(mask, mask, tick_nohz_full_mask);
>  }
> 
> +static inline int housekeeping_any_cpu(void)
> +{
> +	return cpumask_any_and(housekeeping_mask, cpu_online_mask);
> +}
> +
>  extern void tick_nohz_full_kick(void);
>  extern void tick_nohz_full_kick_cpu(int cpu);
>  extern void tick_nohz_full_kick_all(void);
>  extern void __tick_nohz_task_switch(void);
>  #else
> +static inline int housekeeping_any_cpu(void)
> +{
> +	return smp_processor_id();
> +}
>  static inline bool tick_nohz_full_enabled(void) { return false; }
>  static inline bool tick_nohz_full_cpu(int cpu) { return false; }
>  static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask) { }
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 8b864ec..0902e4d 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -623,18 +623,21 @@ int get_nohz_timer_target(void)
>  	int i, cpu = smp_processor_id();
>  	struct sched_domain *sd;
> 
> -	if (!idle_cpu(cpu))
> +	if (!idle_cpu(cpu) && is_housekeeping_cpu(cpu))
>  		return cpu;
> 
>  	rcu_read_lock();
>  	for_each_domain(cpu, sd) {
>  		for_each_cpu(i, sched_domain_span(sd)) {
> -			if (!idle_cpu(i)) {
> +			if (!idle_cpu(i) && is_housekeeping_cpu(cpu)) {

Hi, Frederic, sorry for a naive question. Per my understanding, the tick_nohz_full_mask is added to cpu_isolated_map in sched_init_smp(), and the cpu_isolated_map is excluded from sched_domain in init_sched_domains(), so why check here?

Thanks
--jyh
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