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Message-ID: <a4037857-b161-4536-9a2e-2cf4c168736d@linux.ibm.com>
Date: Tue, 20 Jan 2026 18:12:08 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Frederic Weisbecker <frederic@...nel.org>,
LKML <linux-kernel@...r.kernel.org>
Cc: "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: Re: [PATCH 00/15] tick/sched: Refactor idle cputime accounting
Hi Frederic.
On 1/16/26 8:27 PM, Frederic Weisbecker wrote:
> I forgot to mention I haven't yet tested CONFIG_VIRT_CPU_ACCOUNTING_NATIVE
> (s390 and powerpc).
>
> Thanks.
tl;dr
I ran this on powerNV(Non virtualized) with 144 CPUs with below config. (default ones)
Patch *breaks* the cpu idle stats most of the time. idle values are wrong.
Detailed info:
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++
In config i have this:
CONFIG_TICK_CPU_ACCOUNTING=y
# CONFIG_VIRT_CPU_ACCOUNTING_NATIVE is not set
# CONFIG_VIRT_CPU_ACCOUNTING_GEN is not set
# CONFIG_IRQ_TIME_ACCOUNTING is not set
# CONFIG_BSD_PROCESS_ACCT is not set
+++++++++
When system is fully idle, i see this.
06:44:26 AM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle
06:44:27 AM all 0.01 0.00 0.01 0.00 57.20 0.00 0.00 0.00 0.00 42.79
06:44:28 AM all 0.02 0.00 0.03 0.00 55.73 0.00 0.00 0.00 0.00 44.22
06:44:29 AM all 0.01 0.00 0.00 0.00 56.23 0.00 0.00 0.00 0.00 43.77
- Seeing 50%+ in irq time, which is clearly wrong.
+++++++++
When running stress-ng --cpu=72 (expectation is 50% idle time)
06:48:12 AM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle
06:48:13 AM all 49.98 0.00 0.01 0.00 15.81 0.00 0.00 0.00 0.00 34.20
06:48:14 AM all 49.93 0.00 0.00 0.00 15.15 0.00 0.00 0.00 0.00 34.91
06:48:15 AM all 49.99 0.00 0.01 0.00 15.29 0.00 0.00 0.00 0.00 34.72
- Wrong values again. 50% is expected idle time.
+++++++++
system is idle again.
06:48:46 AM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle
06:48:47 AM all 0.00 0.00 0.00 0.00 63.93 0.00 0.00 0.00 0.00 36.07
06:48:48 AM all 0.02 0.00 0.00 0.00 63.78 0.01 0.00 0.00 0.00 36.18
06:48:49 AM all 0.00 0.00 0.00 0.00 63.77 0.00 0.00 0.00 0.00 36.23
- Wrong values again. irq increased further.
+++++++++
I have seen the below warnings too.
WARNING: kernel/time/tick-sched.c:1353 at tick_nohz_idle_exit
[ T0] WARNING: kernel/time/tick-sched.c:1353 at tick_nohz_idle_exit+0x148/0x150, CPU#4: swapper/4/0
[ T0] Modules linked in: vmx_crypto gf128mul
[ T0] CPU: 4 UID: 0 PID: 0 Comm: swapper/4 Tainted: G W 6.19.0-rc5-00683-gbe7e8f3d5116 #61 PREEMPT(full)
[ T0] Tainted: [W]=WARN
[ T0] Hardware name: 0000000000000000 POWER9 0x4e1202 opal:v7.1 PowerNV
[ T0] NIP [c0000000002c8210] tick_nohz_idle_exit+0x148/0x150
[ T0] LR [c00000000022f10c] do_idle+0x1dc/0x328
WARNING: kernel/time/tick-sched.c:1274 at tick_nohz_get_sleep_length
T0] NIP [c0000000002c7fc0] tick_nohz_get_sleep_length+0x108/0x110
[ T0] LR [c000000000ca1548] menu_select+0x3c0/0x7b4
[ T0] Call Trace:
[ T0] [c000000003197e10] [c000000003197e50] 0xc000000003197e50 (unreliable)
[ T0] [c000000003197e50] [c000000000ca1548] menu_select+0x3c0/0x7b4
[ T0] [c000000003197ed0] [c000000000c9f120] cpuidle_select+0x34/0x48
[ T0] [c000000003197ef0] [c00000000022f184] do_idle+0x254/0x328
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++
I went back to baseline to confirm the original behaviour.
(d613f96096e4) Merge timers/vdso into tip/master
07:02:17 AM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle
07:02:18 AM all 0.01 0.00 0.01 0.01 1.19 0.00 0.00 0.00 0.00 98.77
07:02:19 AM all 0.01 0.00 0.01 0.00 0.84 0.00 0.00 0.00 0.00 99.14
07:02:20 AM all 0.00 0.00 0.01 0.00 0.99 0.00 0.00 0.00 0.00 99.00
07:02:21 AM all 0.01 0.00 0.00 0.00 0.83 0.00 0.00 0.00 0.00 99.16
Which is the working as expected.
PS: Initial data. I haven't gone through the series yet.
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