Re: [PATCH 3/4] s390/idle: Provide arch specific kcpustat_field_idle()/kcpustat_field_iowait()

["Uwe Kleine-König" <u.kleine-koenig@baylibre.com>]

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Hello,

On Wed, May 13, 2026 at 04:01:28PM +0200, Heiko Carstens wrote:
> The former s390 specific arch_cpu_idle_time() implementation was
> removed, since its implementation was racy and reported idle time
> could go backwards [1].
> 
> However this removal was not necessary, since independently of the s390
> architecture specific races there exists the iowait counter update race,
> which can also lead to reported idle time going backwards [2].
> 
> With Frederic Weisbecker's recent cpu idle time accounting refactoring
> kernel_cpustat got a sequence counter. Use this to implement s390 specific
> variants of kcpustat_field_idle() and kcpustat_field_iowait(). This is
> logically a revert of [1] and moves cpu idle time accounting back into s390
> architecture code, which is also more precise than the dyntick idle time
> accounting by nohz/scheduler.
> 
> For comparing cross cpu time stamps it is necessary to use the stcke
> instead of the stckf instruction in irq entry path. Furthermore this
> open-codes a sequence lock in assembler and C code, which is required to
> update the irq entry time stamp to the per cpu idle_data structure in a
> race free manner.
> 
> [1] commit be76ea614460 ("s390/idle: remove arch_cpu_idle_time() and corresponding code")
> [2] commit ead70b752373 ("timers/nohz: Add a comment about broken iowait counter update race")
> 
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

This patch became 670e057744e0cc8047bf96d15d18c46e16ae2e93 and is
included in next-20260616.

s390 allmodconfig fails to build with:

ERROR: modpost: "arch_kcpustat_field_idle" [drivers/leds/trigger/ledtrig-activity.ko] undefined!
ERROR: modpost: "arch_kcpustat_field_iowait" [drivers/leds/trigger/ledtrig-activity.ko] undefined!

Best regards
Uwe

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