From: Frederic Weisbecker <frederic@kernel•org>
To: LKML <linux-kernel@vger•kernel.org>
Cc: Frederic Weisbecker <frederic@kernel•org>,
"Rafael J . Wysocki" <rafael@kernel•org>,
Boqun Feng <boqun.feng@gmail•com>,
Thomas Gleixner <tglx@linutronix•de>,
Steven Rostedt <rostedt@goodmis•org>,
"Christophe Leroy (CS GROUP)" <chleroy@kernel•org>,
Kieran Bingham <kbingham@kernel•org>,
Ben Segall <bsegall@google•com>,
Michael Ellerman <mpe@ellerman•id.au>,
Ingo Molnar <mingo@redhat•com>,
Vincent Guittot <vincent.guittot@linaro•org>,
Juri Lelli <juri.lelli@redhat•com>,
Neeraj Upadhyay <neeraj.upadhyay@kernel•org>,
Xin Zhao <jackzxcui1989@163•com>,
Madhavan Srinivasan <maddy@linux•ibm.com>,
Mel Gorman <mgorman@suse•de>,
Valentin Schneider <vschneid@redhat•com>,
Christian Borntraeger <borntraeger@linux•ibm.com>,
Jan Kiszka <jan.kiszka@siemens•com>,
linuxppc-dev@lists•ozlabs.org,
"Paul E . McKenney" <paulmck@kernel•org>,
Viresh Kumar <viresh.kumar@linaro•org>,
Anna-Maria Behnsen <anna-maria@linutronix•de>,
Uladzislau Rezki <urezki@gmail•com>,
Dietmar Eggemann <dietmar.eggemann@arm•com>,
Heiko Carstens <hca@linux•ibm.com>,
linux-pm@vger•kernel.org,
Alexander Gordeev <agordeev@linux•ibm.com>,
Sven Schnelle <svens@linux•ibm.com>,
Vasily Gorbik <gor@linux•ibm.com>,
Joel Fernandes <joelagnelf@nvidia•com>,
Nicholas Piggin <npiggin@gmail•com>,
linux-s390@vger•kernel.org, Peter Zijlstra <peterz@infradead•org>
Subject: [PATCH 00/15] tick/sched: Refactor idle cputime accounting
Date: Fri, 16 Jan 2026 15:51:53 +0100 [thread overview]
Message-ID: <20260116145208.87445-1-frederic@kernel.org> (raw)
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(-)
next reply other threads:[~2026-01-16 14:52 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-01-16 14:51 Frederic Weisbecker [this message]
2026-01-16 14:51 ` [PATCH 01/15] sched/idle: Handle offlining first in idle loop Frederic Weisbecker
2026-01-19 12:53 ` Peter Zijlstra
2026-01-19 21:04 ` Frederic Weisbecker
2026-01-20 4:26 ` K Prateek Nayak
2026-01-20 14:52 ` Frederic Weisbecker
2026-01-16 14:51 ` [PATCH 02/15] sched/cputime: Remove superfluous and error prone kcpustat_field() parameter Frederic Weisbecker
2026-01-16 14:51 ` [PATCH 03/15] sched/cputime: Correctly support generic vtime idle time Frederic Weisbecker
2026-01-19 13:02 ` Peter Zijlstra
2026-01-19 21:35 ` Frederic Weisbecker
2026-01-16 14:51 ` [PATCH 04/15] powerpc/time: Prepare to stop elapsing in dynticks-idle Frederic Weisbecker
2026-02-25 17:53 ` Christophe Leroy (CS GROUP)
2026-01-16 14:51 ` [PATCH 05/15] s390/time: " Frederic Weisbecker
2026-01-21 12:17 ` Heiko Carstens
2026-01-21 18:04 ` Frederic Weisbecker
2026-01-22 14:40 ` Heiko Carstens
2026-01-27 14:45 ` Frederic Weisbecker
2026-01-16 14:51 ` [PATCH 06/15] tick/sched: Unify idle cputime accounting Frederic Weisbecker
2026-01-19 14:26 ` Peter Zijlstra
2026-01-19 22:00 ` Frederic Weisbecker
2026-01-16 14:52 ` [PATCH 07/15] cpufreq: ondemand: Simplify idle cputime granularity test Frederic Weisbecker
2026-01-19 5:37 ` Viresh Kumar
2026-01-19 12:30 ` Rafael J. Wysocki
2026-01-19 22:06 ` Frederic Weisbecker
2026-01-20 12:32 ` Rafael J. Wysocki
2026-01-20 14:28 ` Frederic Weisbecker
2026-01-16 14:52 ` [PATCH 08/15] tick/sched: Remove nohz disabled special case in cputime fetch Frederic Weisbecker
2026-01-16 14:52 ` [PATCH 09/15] tick/sched: Move dyntick-idle cputime accounting to cputime code Frederic Weisbecker
2026-01-19 14:35 ` Peter Zijlstra
2026-01-19 22:08 ` Frederic Weisbecker
2026-01-16 14:52 ` [PATCH 10/15] tick/sched: Remove unused fields Frederic Weisbecker
2026-01-16 14:52 ` [PATCH 11/15] tick/sched: Account tickless idle cputime only when tick is stopped Frederic Weisbecker
2026-01-16 14:52 ` [PATCH 12/15] tick/sched: Consolidate idle time fetching APIs Frederic Weisbecker
2026-01-16 14:52 ` [PATCH 13/15] sched/cputime: Consolidate get_cpu_[idle|iowait]_time_us() Frederic Weisbecker
2026-01-16 14:52 ` [PATCH 14/15] sched/cputime: Handle idle irqtime gracefully Frederic Weisbecker
2026-01-16 14:52 ` [PATCH 15/15] sched/cputime: Handle dyntick-idle steal time correctly Frederic Weisbecker
2026-01-16 14:57 ` [PATCH 00/15] tick/sched: Refactor idle cputime accounting Frederic Weisbecker
2026-01-20 12:42 ` Shrikanth Hegde
2026-01-21 16:55 ` Frederic Weisbecker
2026-01-19 14:53 ` Peter Zijlstra
2026-01-19 22:12 ` Frederic Weisbecker
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