Re: [PATCH 04/15] powerpc/time: Prepare to stop elapsing in dynticks-idle

Christophe Leroy (CS GROUP)
2026-02-25T09:46:07Z

Hi Hegde,

Le 25/02/2026 à 08:46, Shrikanth Hegde a écrit :
> Hi Christophe,
>
> On 2/24/26 9:11 PM, Christophe Leroy (CS GROUP) wrote:
>> Hi Hegde,
>>
>> Le 19/02/2026 à 19:30, Shrikanth Hegde a écrit :
>>>
>>>
>>> On 2/6/26 7:52 PM, Frederic Weisbecker wrote:
>>>> Currently the tick subsystem stores the idle cputime accounting in
>>>> private fields, allowing cohabitation with architecture idle vtime
>>>> accounting. The former is fetched on online CPUs, the latter on offline
>>>> CPUs.
>>>>
>>>> For consolidation purpose, architecture vtime accounting will continue
>>>> to account the cputime but will make a break when the idle tick is
>>>> stopped. The dyntick cputime accounting will then be relayed by the
>>>> tick
>>>> subsystem so that the idle cputime is still seen advancing coherently
>>>> even when the tick isn't there to flush the idle vtime.
>>>>
>>>> Prepare for that and introduce three new APIs which will be used in
>>>> subsequent patches:
>>>>
>>>> _ vtime_dynticks_start() is deemed to be called when idle enters in
>>>>    dyntick mode. The idle cputime that elapsed so far is accumulated.
>>>>
>>>> - vtime_dynticks_stop() is deemed to be called when idle exits from
>>>>    dyntick mode. The vtime entry clocks are fast-forward to current
>>>> time
>>>>    so that idle accounting restarts elapsing from now.
>>>>
>>>> - vtime_reset() is deemed to be called from dynticks idle IRQ entry to
>>>>    fast-forward the clock to current time so that the IRQ time is still
>>>>    accounted by vtime while nohz cputime is paused.
>>>>
>>>> Also accumulated vtime won't be flushed from dyntick-idle ticks to
>>>> avoid
>>>> accounting twice the idle cputime, along with nohz accounting.
>>>>
>>>> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
>>>
>>> Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
>>>
>>>> ---
>>>>   arch/powerpc/kernel/time.c | 41 ++++++++++++++++++++++++++++++++++
>>>> ++++
>>>>   include/linux/vtime.h      |  6 ++++++
>>>>   2 files changed, 47 insertions(+)
>>>>
>>>> diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
>>>> index 4bbeb8644d3d..18506740f4a4 100644
>>>> --- a/arch/powerpc/kernel/time.c
>>>> +++ b/arch/powerpc/kernel/time.c
>>>> @@ -376,6 +376,47 @@ void vtime_task_switch(struct task_struct *prev)
>>>>           acct->starttime = acct0->starttime;
>>>>       }
>>>>   }
>>>> +
>>>> +#ifdef CONFIG_NO_HZ_COMMON
>>>> +/**
>>>> + * vtime_reset - Fast forward vtime entry clocks
>>>> + *
>>>> + * Called from dynticks idle IRQ entry to fast-forward the clocks
>>>> to current time
>>>> + * so that the IRQ time is still accounted by vtime while nohz
>>>> cputime is paused.
>>>> + */
>>>> +void vtime_reset(void)
>>>> +{
>>>> +    struct cpu_accounting_data *acct = get_accounting(current);
>>>> +
>>>> +    acct->starttime = mftb();
>>>
>>> I figured out why those huge values happen.
>>>
>>> This happens because mftb is from when the system is booted.
>>> I was doing kexec to start the new kernel and mftb wasn't getting
>>> reset.
>>>
>>> I thought about this. This is concern for pseries too, where LPAR's
>>> restart but system won't restart and mftb will continue to run
>>> instead of
>>> reset.
>>>
>>> I think we should be using sched_clock instead of mftb here.
>>> Though we need it a few more places and some cosmetic changes around it.
>>>
>>> Note: Some values being huge exists without series for few CPUs, with
>>> series it
>>> shows up in most of the CPUs.
>>>
>>> So I am planning send out fix below fix separately keeping your
>>> series as dependency.
>>>
>>> ---
>>>   arch/powerpc/include/asm/accounting.h |  4 ++--
>>>   arch/powerpc/include/asm/cputime.h    | 14 +++++++-------
>>>   arch/powerpc/kernel/time.c            | 22 +++++++++++-----------
>>>   3 files changed, 20 insertions(+), 20 deletions(-)
>>>
>>> diff --git a/arch/powerpc/include/asm/accounting.h b/arch/powerpc/
>>> include/asm/accounting.h
>>> index 6d79c31700e2..50f120646e6d 100644
>>> --- a/arch/powerpc/include/asm/accounting.h
>>> +++ b/arch/powerpc/include/asm/accounting.h
>>> @@ -21,8 +21,8 @@ struct cpu_accounting_data {
>>>       unsigned long steal_time;
>>>       unsigned long idle_time;
>>>       /* Internal counters */
>>> -    unsigned long starttime;    /* TB value snapshot */
>>> -    unsigned long starttime_user;    /* TB value on exit to usermode */
>>> +    unsigned long starttime;    /* Time value snapshot */
>>> +    unsigned long starttime_user;    /* Time value on exit to
>>> usermode */
>>>   #ifdef CONFIG_ARCH_HAS_SCALED_CPUTIME
>>>       unsigned long startspurr;    /* SPURR value snapshot */
>>>       unsigned long utime_sspurr;    /* ->user_time when ->startspurr
>>> set */
>>> diff --git a/arch/powerpc/include/asm/cputime.h b/arch/powerpc/
>>> include/ asm/cputime.h
>>> index aff858ca99c0..eb6b629b113f 100644
>>> --- a/arch/powerpc/include/asm/cputime.h
>>> +++ b/arch/powerpc/include/asm/cputime.h
>>> @@ -20,9 +20,9 @@
>>>   #include <asm/time.h>
>>>   #include <asm/param.h>
>>>   #include <asm/firmware.h>
>>> +#include <linux/sched/clock.h>
>>>
>>>   #ifdef __KERNEL__
>>> -#define cputime_to_nsecs(cputime) tb_to_ns(cputime)
>>>
>>>   /*
>>>    * PPC64 uses PACA which is task independent for storing accounting
>>> data while
>>> @@ -44,20 +44,20 @@
>>>    */
>>>   static notrace inline void account_cpu_user_entry(void)
>>>   {
>>> -    unsigned long tb = mftb();
>>> +    unsigned long now = sched_clock();
>>
>> Now way !
>>
>> By doing that you'll kill performance for no reason. All we need when
>> accounting time spent in kernel or in user is the difference between
>> time at entry and time at exit, no mater what the time was at boot time.
>>
>
> No. With this patch there will not be any performance difference.
> All it does is, instead of using mftb uses sched_clock at those places.
>
>
> In arch/powerpc/kernel/time.c we have sched_clock().
> notrace unsigned long long sched_clock(void)
> {
>         return mulhdu(get_tb() - boot_tb, tb_to_ns_scale) <<
> tb_to_ns_shift;
> }
>
> It does the same mftb call, and accounts only the time after boot, which is
> what /proc/stat should do as well.
>
> "
> the amount of time, measured in units of USER_HZ
> (1/100ths of a second on most architectures
>
> user   (1) Time spent in user mode.
>
> idle   (4) Time spent in the idle task.  This value
>        should be USER_HZ times the second entry in
>        the /proc/uptime pseudo-file.
> "
> /proc/uptime is based on sched_clock, so i infer /proc/stat also should
> show
> values w.r.t to boot of the OS.
>
>
>> Also sched_clock() returns nanoseconds which implies calculation from
>> timebase. This is pointless CPU consumption. The current
>> implementation calculates nanoseconds at task switch when calling
>> vtime_flush().Your change will now do it at every kernel entry and
>> kernel exit by calling sched_clock().
>
> This change doesn't add any additional paths. Even without patches, mftb
> would have
> been called in every kernel entry/exit.  See mftb usage
> account_cpu_user_exit/enter
>
> Now instead of mftb sched_clock is used, that's all. No additional
> entry/exit points.
> And previously when accounting we would have done cputime_to_nsecs, now
> that conversion
> is done automatically in sched_clock. So overall computation-wise it
> should be same.
>
> What i am missing to see it here?

Ok, lets try to explain in more details:

While a process is running, it will enter and leave the kernel multiple
times, without task switch. For instance for system calls or for interrupts.

At every kernel entry and exit, account_cpu_user_entry() and
account_cpu_user_exit() are called. That's a very hot path.

I have added the following functions to see what the code looks like:

+
+void my_account_cpu_user_entry(void);
+void my_account_cpu_user_entry(void)
+{
+ account_cpu_user_entry();
+}
+
+void my_account_cpu_user_exit(void);
+void my_account_cpu_user_exit(void)
+{
+ account_cpu_user_exit();
+}

What we have today is very optimised:

00000148 <my_account_cpu_user_entry>:
148: 7d 0c 42 e6 mftb r8
14c: 80 e2 00 08 lwz r7,8(r2)
150: 81 22 00 28 lwz r9,40(r2)
154: 91 02 00 24 stw r8,36(r2)
158: 7d 29 38 50 subf r9,r9,r7
15c: 7d 29 42 14 add r9,r9,r8
160: 91 22 00 08 stw r9,8(r2)
164: 4e 80 00 20 blr

00000168 <my_account_cpu_user_exit>:
168: 7d 0c 42 e6 mftb r8
16c: 80 e2 00 0c lwz r7,12(r2)
170: 81 22 00 24 lwz r9,36(r2)
174: 91 02 00 28 stw r8,40(r2)
178: 7d 29 38 50 subf r9,r9,r7
17c: 7d 29 42 14 add r9,r9,r8
180: 91 22 00 0c stw r9,12(r2)
184: 4e 80 00 20 blr

With your change we now get a call to sched_clock() instead of a simple
mftb,

00000154 <my_account_cpu_user_entry>:
154: 94 21 ff f0 stwu r1,-16(r1)
158: 7c 08 02 a6 mflr r0
15c: 90 01 00 14 stw r0,20(r1)
160: 48 00 00 01 bl 160 <my_account_cpu_user_entry+0xc>
160: R_PPC_REL24 sched_clock
164: 81 02 00 08 lwz r8,8(r2)
168: 81 22 00 28 lwz r9,40(r2)
16c: 90 82 00 24 stw r4,36(r2)
170: 7d 29 40 50 subf r9,r9,r8
174: 7d 29 22 14 add r9,r9,r4
178: 91 22 00 08 stw r9,8(r2)
17c: 80 01 00 14 lwz r0,20(r1)
180: 38 21 00 10 addi r1,r1,16
184: 7c 08 03 a6 mtlr r0
188: 4e 80 00 20 blr

0000018c <my_account_cpu_user_exit>:
18c: 94 21 ff f0 stwu r1,-16(r1)
190: 7c 08 02 a6 mflr r0
194: 90 01 00 14 stw r0,20(r1)
198: 48 00 00 01 bl 198 <my_account_cpu_user_exit+0xc>
198: R_PPC_REL24 sched_clock
19c: 81 02 00 0c lwz r8,12(r2)
1a0: 81 22 00 24 lwz r9,36(r2)
1a4: 90 82 00 28 stw r4,40(r2)
1a8: 7d 29 40 50 subf r9,r9,r8
1ac: 7d 29 22 14 add r9,r9,r4
1b0: 91 22 00 0c stw r9,12(r2)
1b4: 80 01 00 14 lwz r0,20(r1)
1b8: 38 21 00 10 addi r1,r1,16
1bc: 7c 08 03 a6 mtlr r0
1c0: 4e 80 00 20 blr

And sched_clock() is heavy, first it has the sequence mftbu/mftb/mftbu,
and then it does awful lot of calculations including many multiply:

000004d8 <sched_clock>:
4d8: 7d 2d 42 e6 mftbu r9
4dc: 7d 0c 42 e6 mftb r8
4e0: 7d 4d 42 e6 mftbu r10
4e4: 7c 09 50 40 cmplw r9,r10
4e8: 40 82 ff f0 bne 4d8 <sched_clock>
4ec: 3d 40 00 00 lis r10,0
4ee: R_PPC_ADDR16_HA .data..ro_after_init
4f0: 38 ca 00 00 addi r6,r10,0
4f2: R_PPC_ADDR16_LO .data..ro_after_init
4f4: 3c e0 00 00 lis r7,0
4f6: R_PPC_ADDR16_HA .data..read_mostly
4f8: 38 87 00 00 addi r4,r7,0
4fa: R_PPC_ADDR16_LO .data..read_mostly
4fc: 80 66 00 04 lwz r3,4(r6)
500: 80 e7 00 00 lwz r7,0(r7)
502: R_PPC_ADDR16_LO .data..read_mostly
504: 80 c4 00 04 lwz r6,4(r4)
508: 81 4a 00 00 lwz r10,0(r10)
50a: R_PPC_ADDR16_LO .data..ro_after_init
50c: 7c 63 40 10 subfc r3,r3,r8
510: 7d 0a 49 10 subfe r8,r10,r9
514: 7d 27 19 d6 mullw r9,r7,r3
518: 7d 43 30 16 mulhwu r10,r3,r6
51c: 7c 08 31 d6 mullw r0,r8,r6
520: 7d 4a 48 14 addc r10,r10,r9
524: 7c 67 18 16 mulhwu r3,r7,r3
528: 39 20 00 00 li r9,0
52c: 7c c8 30 16 mulhwu r6,r8,r6
530: 7c a9 49 14 adde r5,r9,r9
534: 7d 67 41 d6 mullw r11,r7,r8
538: 7d 4a 00 14 addc r10,r10,r0
53c: 7c a5 01 94 addze r5,r5
540: 7c 63 30 14 addc r3,r3,r6
544: 7d 29 49 14 adde r9,r9,r9
548: 80 84 00 08 lwz r4,8(r4)
54c: 7c 63 58 14 addc r3,r3,r11
550: 7c e7 40 16 mulhwu r7,r7,r8
554: 7d 29 01 94 addze r9,r9
558: 7c 63 28 14 addc r3,r3,r5
55c: 7d 29 39 14 adde r9,r9,r7
560: 35 44 ff e0 addic. r10,r4,-32
564: 41 80 00 10 blt 574 <sched_clock+0x9c>
568: 7c 63 50 30 slw r3,r3,r10
56c: 38 80 00 00 li r4,0
570: 4e 80 00 20 blr
574: 21 04 00 1f subfic r8,r4,31
578: 54 6a f8 7e srwi r10,r3,1
57c: 7d 29 20 30 slw r9,r9,r4
580: 7d 4a 44 30 srw r10,r10,r8
584: 7c 64 20 30 slw r4,r3,r4
588: 7d 43 4b 78 or r3,r10,r9
58c: 4e 80 00 20 blr

I think the difference is obvious, no need of benchmarking. We shall
refrain from calling sched_clock() at every kernel entry/exit.
Converting from timebase to nanoseconds only need to be done in
vtime_flush() called by vtime_task_switch() during task switch.

Hope it is more explicit now.

Christophe