Subject: Re: [PATCH 00/15 v4] tick/sched: Refactor idle cputime accounting

From: Frederic Weisbecker <[email-masked]>
Date: Tue, 26 May 2026 12:42:16 +0200

Hi,

I don't see any further concern. What should we do with this? It could
either go through the scheduler tree or the timer tree.

Thanks.

Le Fri, May 08, 2026 at 03:16:32PM +0200, Frederic Weisbecker a écrit :
> Hi,
> 
> After the issue reported here:
> 
>         https://lore.kernel.org/all/[email-masked]/
> 
> It occurs that the idle cputime accounting is a big mess that
> accumulates within two concurrent statistics, each having their own
> shortcomings:
> 
> * The accounting for online CPUs which is based on the delta between
>   tick_nohz_start_idle() and tick_nohz_stop_idle().
> 
>   Pros:
>        - Works when the tick is off
> 
>        - Has nsecs granularity
> 
>   Cons:
>        - Account idle steal time but doesn't substract it from idle
>          cputime.
> 
>        - Assumes CONFIG_IRQ_TIME_ACCOUNTING by not accounting IRQs but
>          the IRQ time is simply ignored when
>          CONFIG_IRQ_TIME_ACCOUNTING=n
> 
>        - The windows between 1) idle task scheduling and the first call
>          to tick_nohz_start_idle() and 2) idle task between the last
>          tick_nohz_stop_idle() and the rest of the idle time are
>          blindspots wrt. cputime accounting (though mostly insignificant
>          amount)
> 
>        - Relies on private fields outside of kernel stats, with specific
>          accessors.
> 
> * The accounting for offline CPUs which is based on ticks and the
>   jiffies delta during which the tick was stopped.
> 
>   Pros:
>        - Handles steal time correctly
> 
>        - Handle CONFIG_IRQ_TIME_ACCOUNTING=y and
>          CONFIG_IRQ_TIME_ACCOUNTING=n correctly.
> 
>        - Handles the whole idle task
> 
>        - Accounts directly to kernel stats, without midlayer accumulator.
> 
>    Cons:
>        - Doesn't elapse when the tick is off, which doesn't make it
>          suitable for online CPUs.
> 
>        - Has TICK_NSEC granularity (jiffies)
> 
>        - Needs to track the dyntick-idle ticks that were accounted and
>          substract them from the total jiffies time spent while the tick
>          was stopped. This is an ugly workaround.
> 
> Having two different accounting for a single context is not the only
> problem: since those accountings are of different natures, it is
> possible to observe the global idle time going backward after a CPU goes
> offline, as reported by Xin Zhao.
> 
> Clean up the situation with introducing a hybrid approach that stays
> coherent, fixes the backward jumps and works for both online and offline
> CPUs:
> 
> * Tick based or native vtime accounting operate before the tick is
>   stopped and resumes once the tick is restarted.
> 
> * When the idle loop starts, switch to dynticks-idle accounting as is
>   done currently, except that the statistics accumulate directly to the
>   relevant kernel stat fields.
> 
> * Private dyntick cputime accounting fields are removed.
> 
> * Works on both online and offline case.
> 
> * Move most of the relevant code to the common sched/cputime subsystem
> 
> * Handle CONFIG_IRQ_TIME_ACCOUNTING=n correctly such that the
>   dynticks-idle accounting still elapses while on IRQs.
> 
> * Correctly substract idle steal cputime from idle time
> 
> Changes since v3 (among which a lot of relevant reviews from Sashiko):
> 
> - Add new tags
> 
> - Rebase on latest -rc1
> 
> - Add "tick/sched: Fix TOCTOU in nohz idle time fetch" (Sashiko)
> 
> - Fix buggy state refetch in kcpustat_cpu_fetch_vtime() (Sashiko)
> 
> - Fix build issue on powerpc (Christophe Leroy)
> 
> - Fix s390 lost steal time occuring on idle IRQs (call vtime_flush() on
>   vtime_account_hardirq() and vtime_account_softirq()) (Sashiko)
> 
> - Fix build issue on s390
> 
> - Fix uninitialized idle_sleeptime_seq (Sashiko)
> 
> - Fix irqtime being disabled or enabled in the middle of an idle IRQ
>   (Sashiko)
>   
> - Fix tick restart and then restop in the same idle loop (Sashiko)
> 
> - Fix "sched/cputime: Handle idle irqtime gracefully" changelog (Sashiko)
> 
> - Fix idle steal time substracted from the wrong index between idle and
>   iowait kcpustat. (Sashiko)
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> 	timers/core-v4
> 
> HEAD: e64ba052ce04e363ff76d3cb8bedc5f812188acb
> Thanks,
> 	Frederic
> ---
> 
> Frederic Weisbecker (15):
>       tick/sched: Fix TOCTOU in nohz idle time fetch
>       sched/idle: Handle offlining first in idle loop
>       sched/cputime: Remove superfluous and error prone kcpustat_field() parameter
>       sched/cputime: Correctly support generic vtime idle time
>       powerpc/time: Prepare to stop elapsing in dynticks-idle
>       s390/time: Prepare to stop elapsing in dynticks-idle
>       tick/sched: Unify idle cputime accounting
>       tick/sched: Remove nohz disabled special case in cputime fetch
>       tick/sched: Move dyntick-idle cputime accounting to cputime code
>       tick/sched: Remove unused fields
>       tick/sched: Account tickless idle cputime only when tick is stopped
>       tick/sched: Consolidate idle time fetching APIs
>       sched/cputime: Provide get_cpu_[idle|iowait]_time_us() off-case
>       sched/cputime: Handle idle irqtime gracefully
>       sched/cputime: Handle dyntick-idle steal time correctly
> 
>  arch/powerpc/kernel/time.c         |  41 +++++
>  arch/s390/include/asm/idle.h       |   2 +
>  arch/s390/kernel/idle.c            |   5 +-
>  arch/s390/kernel/vtime.c           |  75 ++++++++-
>  drivers/cpufreq/cpufreq.c          |  29 +---
>  drivers/cpufreq/cpufreq_governor.c |   6 +-
>  drivers/macintosh/rack-meter.c     |   2 +-
>  fs/proc/stat.c                     |  40 +----
>  fs/proc/uptime.c                   |   8 +-
>  include/linux/kernel_stat.h        |  76 +++++++--
>  include/linux/tick.h               |   4 -
>  include/linux/vtime.h              |  22 ++-
>  kernel/rcu/tree.c                  |   9 +-
>  kernel/rcu/tree_stall.h            |   7 +-
>  kernel/sched/core.c                |   6 +-
>  kernel/sched/cputime.c             | 308 +++++++++++++++++++++++++++++++------
>  kernel/sched/idle.c                |  13 +-
>  kernel/time/tick-sched.c           | 212 ++++++-------------------
>  kernel/time/tick-sched.h           |  12 --
>  kernel/time/timer_list.c           |   6 +-
>  scripts/gdb/linux/timerlist.py     |   4 -
>  21 files changed, 529 insertions(+), 358 deletions(-)

-- 
Frederic Weisbecker
SUSE Labs