Re: [PATCH v4 1/4] sched: Define sched_clock_irqtime as static key

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 01, 2024 at 11:17:47AM +0800, Yafang Shao wrote:
> Since CPU time accounting is a performance-critical path, let's define
> sched_clock_irqtime as a static key to minimize potential overhead.
> 
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> ---
>  kernel/sched/cputime.c | 16 +++++++---------
>  kernel/sched/sched.h   |  1 +
>  2 files changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 0bed0fa1acd9..d0b6ea737d04 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -7,6 +7,8 @@
>   #include <asm/cputime.h>
>  #endif
>  
> +DEFINE_STATIC_KEY_FALSE(sched_clock_irqtime);
> +
>  #ifdef CONFIG_IRQ_TIME_ACCOUNTING
>  
>  /*
> @@ -22,16 +24,14 @@
>   */
>  DEFINE_PER_CPU(struct irqtime, cpu_irqtime);
>  
> -static int sched_clock_irqtime;
> -
>  void enable_sched_clock_irqtime(void)
>  {
> -	sched_clock_irqtime = 1;
> +	static_branch_enable(&sched_clock_irqtime);
>  }
>  
>  void disable_sched_clock_irqtime(void)
>  {
> -	sched_clock_irqtime = 0;
> +	static_branch_disable(&sched_clock_irqtime);
>  }
>  
>  static void irqtime_account_delta(struct irqtime *irqtime, u64 delta,
> @@ -57,7 +57,7 @@ void irqtime_account_irq(struct task_struct *curr, unsigned int offset)
>  	s64 delta;
>  	int cpu;
>  
> -	if (!sched_clock_irqtime)
> +	if (!static_branch_likely(&sched_clock_irqtime))
>  		return;
>  
>  	cpu = smp_processor_id();
> @@ -90,8 +90,6 @@ static u64 irqtime_tick_accounted(u64 maxtime)
>  
>  #else /* CONFIG_IRQ_TIME_ACCOUNTING */
>  
> -#define sched_clock_irqtime	(0)
> -
>  static u64 irqtime_tick_accounted(u64 dummy)
>  {
>  	return 0;

This makes no sense... in the IRQ_TIME_ACCOUNTING=n case you shouldn't
be using the static key.

> @@ -478,7 +476,7 @@ void account_process_tick(struct task_struct *p, int user_tick)
>  	if (vtime_accounting_enabled_this_cpu())
>  		return;
>  
> -	if (sched_clock_irqtime) {
> +	if (static_branch_likely(&sched_clock_irqtime)) {
>  		irqtime_account_process_tick(p, user_tick, 1);
>  		return;
>  	}
> @@ -507,7 +505,7 @@ void account_idle_ticks(unsigned long ticks)
>  {
>  	u64 cputime, steal;
>  
> -	if (sched_clock_irqtime) {
> +	if (static_branch_likely(&sched_clock_irqtime)) {
>  		irqtime_account_idle_ticks(ticks);
>  		return;
>  	}
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 081519ffab46..038ce65d6635 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -3179,6 +3179,7 @@ struct irqtime {
>  };
>  
>  DECLARE_PER_CPU(struct irqtime, cpu_irqtime);
> +DECLARE_STATIC_KEY_FALSE(sched_clock_irqtime);
>  
>  /*
>   * Returns the irqtime minus the softirq time computed by ksoftirqd.
> -- 
> 2.43.5
>