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

Shrikanth Hegde
2026-02-11T13:44:40Z

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/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

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 did a perf diff between the two (collected perf record -a for hackbench 60 process 10000 loops)

perf diff base series:
# Baseline Delta Abs Shared Object Symbol
# ........ ......... ........................... ................................................
#
+5.43% [kernel.kallsyms] [k] __update_freelist_slow
0.00% +4.55% [kernel.kallsyms] [k] _raw_spin_lock
+3.35% [kernel.kallsyms] [k] __memcg_slab_free_hook
0.55% +2.58% [kernel.kallsyms] [k] sock_wfree
+2.51% [kernel.kallsyms] [k] __account_obj_stock
2.29% -2.29% [kernel.kallsyms] [k] _raw_write_lock_irq
+2.25% [kernel.kallsyms] [k] _copy_from_iter
+1.96% [kernel.kallsyms] [k] fdget_pos
+1.87% [kernel.kallsyms] [k] _copy_to_iter
+1.69% [kernel.kallsyms] [k] sock_def_readable
2.32% -1.68% [kernel.kallsyms] [k] mod_memcg_lruvec_state
0.82% +1.67% [kernel.kallsyms] [k] skb_set_owner_w
0.08% +1.65% [kernel.kallsyms] [k] vfs_read
0.42% +1.57% [kernel.kallsyms] [k] kmem_cache_alloc_node_noprof
1.53% -1.53% [kernel.kallsyms] [k] kmem_cache_alloc_lru_noprof
1.56% -1.41% [kernel.kallsyms] [k] simple_copy_to_iter
0.27% +1.32% [kernel.kallsyms] [k] kfree
0.01% +1.25% [kernel.kallsyms] [k] __slab_free
0.19% +1.24% [kernel.kallsyms] [k] kmem_cache_free
1.23% -1.23% [kernel.kallsyms] [k] __pcs_replace_full_main
0.35% +1.21% [kernel.kallsyms] [k] __skb_datagram_iter
0.21% +1.13% [kernel.kallsyms] [k] sock_alloc_send_pskb
+1.09% [kernel.kallsyms] [k] mutex_lock
+0.98% [kernel.kallsyms].head.text [k] 0x0000000000013004

I haven't gone through the series yet. trying to go through meanwhile.
maybe different allocation scheme or more allocation/free everytime instead of
pre-allocated percpu variables?

First thought of reporting it. Let me know if you need any additional data.