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

Shrikanth Hegde
2026-02-25T07:47:31Z

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?

>
> Another point is that sched_clock() returns a long long not a long.

Thanks for pointing that out.

Ok. Let me change some of those variables into unsigned long long.
Compiler didn't warn me, so i didn't see it.

>
> And also sched_clock() uses get_tb() which does mftb and mftbu. Which is
> pointless for calculating time deltas unless your application spends
> hours without being re-scheduled.
>

I didn't get this. At current also, we use mftb, that functionality should be the same.
Could you please explain how?

>
>>       struct cpu_accounting_data *acct = raw_get_accounting(current);
>>
>> -    acct->utime += (tb - acct->starttime_user);
>> -    acct->starttime = tb;
>> +    acct->utime += (now - acct->starttime_user);
>> +    acct->starttime = now;
>>   }
>>
>>   static notrace inline void account_cpu_user_exit(void)
>>   {
>> -    unsigned long tb = mftb();
>> +    unsigned long now = sched_clock();
>>       struct cpu_accounting_data *acct = raw_get_accounting(current);
>>
>> -    acct->stime += (tb - acct->starttime);
>> -    acct->starttime_user = tb;
>> +    acct->stime += (now - acct->starttime);
>> +    acct->starttime_user = now;
>>   }
>>
>>   static notrace inline void account_stolen_time(void)
>> diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
>> index 18506740f4a4..fb67cdae3bcb 100644
>> --- a/arch/powerpc/kernel/time.c
>> +++ b/arch/powerpc/kernel/time.c
>> @@ -215,7 +215,7 @@ static unsigned long vtime_delta(struct
>> cpu_accounting_data *acct,
>>
>>       WARN_ON_ONCE(!irqs_disabled());
>>
>> -    now = mftb();
>> +    now = sched_clock();
>>       stime = now - acct->starttime;
>>       acct->starttime = now;
>>
>> @@ -299,9 +299,9 @@ static void vtime_flush_scaled(struct task_struct
>> *tsk,
>>   {
>>   #ifdef CONFIG_ARCH_HAS_SCALED_CPUTIME
>>       if (acct->utime_scaled)
>> -        tsk->utimescaled += cputime_to_nsecs(acct->utime_scaled);
>> +        tsk->utimescaled += acct->utime_scaled;
>>       if (acct->stime_scaled)
>> -        tsk->stimescaled += cputime_to_nsecs(acct->stime_scaled);
>> +        tsk->stimescaled += acct->stime_scaled;
>>
>>       acct->utime_scaled = 0;
>>       acct->utime_sspurr = 0;
>> @@ -321,28 +321,28 @@ void vtime_flush(struct task_struct *tsk)
>>       struct cpu_accounting_data *acct = get_accounting(tsk);
>>
>>       if (acct->utime)
>> -        account_user_time(tsk, cputime_to_nsecs(acct->utime));
>> +        account_user_time(tsk, acct->utime);
>>
>>       if (acct->gtime)
>> -        account_guest_time(tsk, cputime_to_nsecs(acct->gtime));
>> +        account_guest_time(tsk, acct->gtime);
>>
>>       if (IS_ENABLED(CONFIG_PPC_SPLPAR) && acct->steal_time) {
>> -        account_steal_time(cputime_to_nsecs(acct->steal_time));
>> +        account_steal_time(acct->steal_time);
>>           acct->steal_time = 0;
>>       }
>>
>>       if (acct->idle_time)
>> -        account_idle_time(cputime_to_nsecs(acct->idle_time));
>> +        account_idle_time(acct->idle_time);
>>
>>       if (acct->stime)
>> -        account_system_index_time(tsk, cputime_to_nsecs(acct->stime),
>> +        account_system_index_time(tsk, acct->stime,
>>                         CPUTIME_SYSTEM);
>>
>>       if (acct->hardirq_time)
>> -        account_system_index_time(tsk, cputime_to_nsecs(acct-
>>  >hardirq_time),
>> +        account_system_index_time(tsk, acct->hardirq_time,
>>                         CPUTIME_IRQ);
>>       if (acct->softirq_time)
>> -        account_system_index_time(tsk, cputime_to_nsecs(acct-
>>  >softirq_time),
>> +        account_system_index_time(tsk, acct->softirq_time,
>>                         CPUTIME_SOFTIRQ);
>>
>>       vtime_flush_scaled(tsk, acct);
>> @@ -388,7 +388,7 @@ void vtime_reset(void)
>>   {
>>       struct cpu_accounting_data *acct = get_accounting(current);
>>
>> -    acct->starttime = mftb();
>> +    acct->starttime = sched_clock();
>>   #ifdef CONFIG_ARCH_HAS_SCALED_CPUTIME
>>       acct->startspurr = read_spurr(acct->starttime);
>>   #endif
>

PS: I measured the performance with hackbench. I don't see any degradation.