Subject: Re: [PATCH 14/15] sched/cputime: Handle idle irqtime gracefully

From: Shrikanth Hegde <[email-masked]>
Date: Tue, 3 Mar 2026 16:41:18 +0530

Hi Frederic,

On 2/6/26 7:52 PM, Frederic Weisbecker wrote:
> The dyntick-idle cputime accounting always assumes that IRQ time
> accounting is enabled and consequently stops elapsing the idle time
> during dyntick-idle IRQs.
> 
> This doesn't mix up well with disabled IRQ time accounting because then
> idle IRQs become a cputime blind-spot. Also this feature is disabled
> on most configurations and the overhead of pausing dyntick-idle
> accounting while in idle IRQs could then be avoided.
> 
> Fix the situation with conditionally pausing dyntick-idle accounting
> during idle IRQs only if neither native vtime (which does IRQ time
> accounting) nor generic IRQ time accounting are enabled.
> 
> Also make sure that the accumulated IRQ time is not accidentally
> substracted from later accounting.
> 
> Signed-off-by: Frederic Weisbecker <[email-masked]>
> ---
>   kernel/sched/cputime.c | 24 +++++++++++++++++++++---
>   kernel/sched/sched.h   |  1 +
>   2 files changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index d588a4a50e57..92fa2f037b6e 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -46,7 +46,8 @@ static void irqtime_account_delta(struct irqtime *irqtime, u64 delta,
>   	u64_stats_update_begin(&irqtime->sync);
>   	cpustat[idx] += delta;
>   	irqtime->total += delta;
> -	irqtime->tick_delta += delta;
> +	if (!irqtime->idle_dyntick)
> +		irqtime->tick_delta += delta;

Wouldn't kcpustat_idle_dyntick achieve the same thing?

>   	u64_stats_update_end(&irqtime->sync);
>   }
>   
> @@ -81,6 +82,16 @@ void irqtime_account_irq(struct task_struct *curr, unsigned int offset)
>   		irqtime_account_delta(irqtime, delta, CPUTIME_SOFTIRQ);
>   }
>   
> +static inline void irqtime_dyntick_start(void)
> +{
> +	__this_cpu_write(cpu_irqtime.idle_dyntick, true);
> +}
> +
> +static inline void irqtime_dyntick_stop(void)
> +{
> +	__this_cpu_write(cpu_irqtime.idle_dyntick, false);
> +}
> +
>   static u64 irqtime_tick_accounted(u64 maxtime)
>   {
>   	struct irqtime *irqtime = this_cpu_ptr(&cpu_irqtime);
> @@ -94,6 +105,9 @@ static u64 irqtime_tick_accounted(u64 maxtime)
>   
>   #else /* !CONFIG_IRQ_TIME_ACCOUNTING: */
>   
> +static inline void irqtime_dyntick_start(void) { }
> +static inline void irqtime_dyntick_stop(void) { }
> +
>   static u64 irqtime_tick_accounted(u64 dummy)
>   {
>   	return 0;
> @@ -444,6 +458,7 @@ void kcpustat_dyntick_stop(u64 now)
>   		WARN_ON_ONCE(!kc->idle_dyntick);
>   		kcpustat_idle_stop(kc, now);
>   		kc->idle_dyntick = false;
> +		irqtime_dyntick_stop();
>   		vtime_dyntick_stop();
>   		steal_account_process_time(ULONG_MAX);
>   	}
> @@ -455,6 +470,7 @@ void kcpustat_dyntick_start(u64 now)
>   
>   	if (!vtime_generic_enabled_this_cpu()) {
>   		vtime_dyntick_start();
> +		irqtime_dyntick_start();
>   		kc->idle_dyntick = true;
>   		kcpustat_idle_start(kc, now);
>   	}
> @@ -464,7 +480,8 @@ void kcpustat_irq_enter(u64 now)
>   {
>   	struct kernel_cpustat *kc = kcpustat_this_cpu;
>   
> -	if (!vtime_generic_enabled_this_cpu())
> +	if (!vtime_generic_enabled_this_cpu() &&
> +	    (irqtime_enabled() || vtime_accounting_enabled_this_cpu()))
>   		kcpustat_idle_stop(kc, now);
>   }
Scenario: context_tracking is not enabled(since nohz_full or isolcpus not specified) and
irqtime/native is not enabled. ( config is CONFIG_VIRT_CPU_ACCOUNTING_GEN + IRQ_TIME=n)

cpu goes into tickless mode. Gets irqs, but kcpustat_irq_enter/exit is nop.
Then the time it spent in irq is still accounted for idle time, during kcpustat_dyntick_stop?

Who is going to account the irq time in this case?

>   
> @@ -472,7 +489,8 @@ void kcpustat_irq_exit(u64 now)
>   {
>   	struct kernel_cpustat *kc = kcpustat_this_cpu;
>   
> -	if (!vtime_generic_enabled_this_cpu())
> +	if (!vtime_generic_enabled_this_cpu() &&
> +	    (irqtime_enabled() || vtime_accounting_enabled_this_cpu()))
>   		kcpustat_idle_start(kc, now);
>   }
>   
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index d30cca6870f5..cf677ff12b10 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -3307,6 +3307,7 @@ static inline void sched_core_tick(struct rq *rq) { }
>   #ifdef CONFIG_IRQ_TIME_ACCOUNTING
>   
>   struct irqtime {
> +	bool			idle_dyntick;
>   	u64			total;
>   	u64			tick_delta;
>   	u64			irq_start_time;