Subject: Re: [PATCH 05/15] s390/time: Prepare to stop elapsing in dynticks-idle

From: Heiko Carstens <[email-masked]>
Date: Wed, 21 Jan 2026 13:17:48 +0100

On Fri, Jan 16, 2026 at 03:51:58PM +0100, Frederic Weisbecker wrote:
> diff --git a/arch/s390/kernel/idle.c b/arch/s390/kernel/idle.c
> index 39cb8d0ae348..54bb932184dd 100644
> --- a/arch/s390/kernel/idle.c
> +++ b/arch/s390/kernel/idle.c
> @@ -35,6 +35,12 @@ void account_idle_time_irq(void)
>  			this_cpu_add(mt_cycles[i], cycles_new[i] - idle->mt_cycles_enter[i]);
>  	}
>  
> +	WRITE_ONCE(idle->idle_count, READ_ONCE(idle->idle_count) + 1);
> +
> +	/* Dyntick idle time accounted by nohz/scheduler */
> +	if (idle->idle_dyntick)
> +		return;
> +
>  	idle_time = lc->int_clock - idle->clock_idle_enter;
>  
>  	lc->steal_timer += idle->clock_idle_enter - lc->last_update_clock;
> @@ -45,7 +51,6 @@ void account_idle_time_irq(void)
>  
>  	/* Account time spent with enabled wait psw loaded as idle time. */
>  	WRITE_ONCE(idle->idle_time, READ_ONCE(idle->idle_time) + idle_time);
> -	WRITE_ONCE(idle->idle_count, READ_ONCE(idle->idle_count) + 1);
>  	account_idle_time(cputime_to_nsecs(idle_time));
>  }

This breaks idle time reporting (aka enabled wait psw time) via the per-cpu
sysfs files (see show_idle_time()). That is: the second WRITE_ONCE() should
also go above the early return statement; but of course this leads to other
dependencies...

Not sure what to do with this. I thought about removing those sysfs files
already in the past, since they are of very limited use; and most likely
nothing in user space would miss them.

Anyway, you need to integrate the trivial patch below, so everything compiles
for s390. It also _seems_ to work.

Guess I need to spend some more time on accounting and see what it would take
to convert to VIRT_CPU_ACCOUNTING_GEN, while keeping the current precision and
functionality.

diff --git a/arch/s390/include/asm/idle.h b/arch/s390/include/asm/idle.h
index 2770c4f761e1..285b3da318d6 100644
--- a/arch/s390/include/asm/idle.h
+++ b/arch/s390/include/asm/idle.h
@@ -8,6 +8,7 @@
 #ifndef _S390_IDLE_H
 #define _S390_IDLE_H
 
+#include <linux/percpu-defs.h>
 #include <linux/types.h>
 #include <linux/device.h>
 
@@ -20,6 +21,8 @@ struct s390_idle_data {
 	unsigned long	mt_cycles_enter[8];
 };
 
+DECLARE_PER_CPU(struct s390_idle_data, s390_idle);
+
 extern struct device_attribute dev_attr_idle_count;
 extern struct device_attribute dev_attr_idle_time_us;
 
diff --git a/arch/s390/kernel/idle.c b/arch/s390/kernel/idle.c
index 54bb932184dd..e3fe64e7adbe 100644
--- a/arch/s390/kernel/idle.c
+++ b/arch/s390/kernel/idle.c
@@ -19,7 +19,7 @@
 #include <asm/smp.h>
 #include "entry.h"
 
-static DEFINE_PER_CPU(struct s390_idle_data, s390_idle);
+DEFINE_PER_CPU(struct s390_idle_data, s390_idle);
 
 void account_idle_time_irq(void)
 {