Message-ID: <20260116145208.87445-1-frederic@kernel.org>
Date: Fri, 16 Jan 2026 15:51:53 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Frederic Weisbecker <frederic@...nel.org>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Christophe Leroy (CS GROUP)" <chleroy@...nel.org>,
	Kieran Bingham <kbingham@...nel.org>,
	Ben Segall <bsegall@...gle.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Ingo Molnar <mingo@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Xin Zhao <jackzxcui1989@....com>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	linuxppc-dev@...ts.ozlabs.org,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Uladzislau Rezki <urezki@...il.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	linux-pm@...r.kernel.org,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Nicholas Piggin <npiggin@...il.com>,
	linux-s390@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH 00/15] tick/sched: Refactor idle cputime accounting

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

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

HEAD: 6a3d814ef2f6142714bef862be36def5ca4c9d96
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: Consolidate get_cpu_[idle|iowait]_time_us()
      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       |  11 +-
 arch/s390/kernel/idle.c            |  13 +-
 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               |   4 -
 include/linux/vtime.h              |  20 ++-
 kernel/rcu/tree.c                  |   9 +-
 kernel/rcu/tree_stall.h            |   7 +-
 kernel/sched/cputime.c             | 302 +++++++++++++++++++++++++++++++------
 kernel/sched/idle.c                |  11 +-
 kernel/sched/sched.h               |   1 +
 kernel/time/tick-sched.c           | 203 +++++--------------------
 kernel/time/tick-sched.h           |  12 --
 kernel/time/timer_list.c           |   6 +-
 scripts/gdb/linux/timerlist.py     |   4 -
 22 files changed, 505 insertions(+), 364 deletions(-)

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