Subject: [PATCH 00/15 v2] tick/sched: Refactor idle cputime accounting

From: Frederic Weisbecker <[email-masked]>
Date: Fri, 6 Feb 2026 15:22:30 +0100

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 v1:

- Fix deadlock involving double seq count lock on idle

- Fix build breakage on powerpc

- Fix build breakage on s390 (Heiko)

- Fix broken sysfs s390 idle time file (Heiko)

- Convert most ktime usage here into u64 (Peterz)

- Add missing (or too implicit) <linux/sched/clock.h> (Peterz)

- Fix whole idle time acccounting breakage due to missing TS_FLAG_ set
  on idle entry (Shrikanth Hegde)

git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
	timers/core-v2

HEAD: 21458b98c80a0567d48131240317b7b73ba34c3c
Thanks,
	Frederic
---

Frederic Weisbecker (15):
      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
      cpufreq: ondemand: Simplify idle cputime granularity test
      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       |  14 +-
 arch/s390/kernel/idle.c            |  19 ++-
 arch/s390/kernel/vtime.c           |  57 ++++++-
 drivers/cpufreq/cpufreq.c          |  29 +---
 drivers/cpufreq/cpufreq_governor.c |   6 +-
 drivers/cpufreq/cpufreq_ondemand.c |   7 +-
 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               |   6 +-
 include/linux/vtime.h              |  22 ++-
 kernel/rcu/tree.c                  |   9 +-
 kernel/rcu/tree_stall.h            |   7 +-
 kernel/sched/cputime.c             | 304 +++++++++++++++++++++++++++++++------
 kernel/sched/idle.c                |  13 +-
 kernel/sched/sched.h               |   1 +
 kernel/time/hrtimer.c              |   2 +-
 kernel/time/tick-internal.h        |   2 -
 kernel/time/tick-sched.c           | 210 ++++++-------------------
 kernel/time/tick-sched.h           |  12 --
 kernel/time/timer.c                |   2 +-
 kernel/time/timer_list.c           |   6 +-
 scripts/gdb/linux/timerlist.py     |   4 -
 25 files changed, 525 insertions(+), 374 deletions(-)