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

From: Frederic Weisbecker <[email-masked]>
Date: Wed, 11 Feb 2026 18:06:33 +0100

Le Wed, Feb 11, 2026 at 07:13:45PM +0530, Shrikanth Hegde a écrit :
> Hi Frederic,
> Gave this series a spin on the same system as v1.
> 
> On 2/6/26 7:52 PM, Frederic Weisbecker wrote:
> > 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
> 
> idle and runtime utilization with mpstat while running stress-ng looks
> correct now.
> 
> However, when running hackbench I am noticing the below data. hackbench shows
> severe regressions.
> 
> base: tip/master at 9c61ebbdb587a3950072700ab74a9310afe3ad73.
> (nit: patch 7 is already part of tip. so skipped applying it)
> +-----------------------------------------------+-------+---------+-----------+
> | Test                                          | base  | +series | % Diff    |
> +-----------------------------------------------+-------+---------+-----------+
> | HackBench Process 10 groups                   |  2.23 |  3.05   |   -36.77%  |
> | HackBench Process 20 groups                   |  4.17 |  5.82   |   -39.57%  |
> | HackBench Process 30 groups                   |  6.04 |  8.49   |   -40.56%  |
> | HackBench Process 40 groups                   |  7.90 | 11.10   |   -40.51%  |
> | HackBench thread 10                           |  2.44 |  3.36   |   -37.70%  |
> | HackBench thread 20                           |  4.57 |  6.35   |   -38.95%  |
> | HackBench Process(Pipe) 10                    |  1.76 |  2.29   |   -30.11%  |
> | HackBench Process(Pipe) 20                    |  3.49 |  4.76   |   -36.39%  |
> | HackBench Process(Pipe) 30                    |  5.21 |  7.13   |   -36.85%  |
> | HackBench Process(Pipe) 40                    |  6.89 |  9.31   |   -35.12%  |
> | HackBench thread(Pipe) 10                     |  1.91 |  2.50   |   -30.89%  |
> | HackBench thread(Pipe) 20                     |  3.74 |  5.16   |   -37.97%  |
> +-----------------------------------------------+-------+---------+-----------+
> 
> I have these in .config and I don't have nohz_full or isolated cpus.
> 
> CONFIG_TICK_ONESHOT=y
> CONFIG_NO_HZ_COMMON=y
> # CONFIG_HZ_PERIODIC is not set
> # CONFIG_NO_HZ_IDLE is not set
> CONFIG_NO_HZ_FULL=y
> 
> # CPU/Task time and stats accounting
> #
> CONFIG_VIRT_CPU_ACCOUNTING=y
> CONFIG_VIRT_CPU_ACCOUNTING_GEN=y
> CONFIG_IRQ_TIME_ACCOUNTING=y
> CONFIG_HAVE_SCHED_AVG_IRQ=y
> 
> I did a git bisect and below is what it says.
> 
> git bisect start
> # status: waiting for both good and bad commits
> # bad: [6821315886a3b5267ea31d29dba26fd34647fbbc] sched/cputime: Handle dyntick-idle steal time correctly
> git bisect bad 6821315886a3b5267ea31d29dba26fd34647fbbc
> # status: waiting for good commit(s), bad commit known
> # good: [9c61ebbdb587a3950072700ab74a9310afe3ad73] Merge branch into tip/master: 'x86/sev'
> git bisect good 9c61ebbdb587a3950072700ab74a9310afe3ad73
> # good: [dc8bb3c84d162f7d9aa6becf9f8392474f92655a] tick/sched: Remove nohz disabled special case in cputime fetch
> git bisect good dc8bb3c84d162f7d9aa6becf9f8392474f92655a
> # good: [5070a778a581cd668f5d717f85fb22b078d8c20c] tick/sched: Account tickless idle cputime only when tick is stopped
> git bisect good 5070a778a581cd668f5d717f85fb22b078d8c20c
> # bad: [1e0ccc25a9a74b188b239c4de716fde279adbf8e] sched/cputime: Provide get_cpu_[idle|iowait]_time_us() off-case
> git bisect bad 1e0ccc25a9a74b188b239c4de716fde279adbf8e
> # bad: [ee7c735b76071000d401869fc2883c451ee3fa61] tick/sched: Consolidate idle time fetching APIs
> git bisect bad ee7c735b76071000d401869fc2883c451ee3fa61
> # first bad commit: [ee7c735b76071000d401869fc2883c451ee3fa61] tick/sched:
> Consolidate idle time fetching APIs

I see. Can you try this? (or fetch timers/core-v3 from my tree)
Perhaps that mistake had some impact on cpufreq.

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 057fdc00dbc6..08550a6d9469 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -524,7 +524,7 @@ static u64 get_cpu_sleep_time_us(int cpu, enum cpu_usage_stat idx,
 	do_div(res, NSEC_PER_USEC);
 
 	if (last_update_time)
-		*last_update_time = res;
+		*last_update_time = ktime_to_us(now);
 
 	return res;
 }