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

Frederic Weisbecker
2026-02-06T14:23:03Z

Hi,

After the issue reported here:

https://lore.kernel.org/all/20251210083135.3993562-1-jackzxcui1989@163.com/

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(-)