[PATCH 04/15] powerpc/time: Prepare to stop elapsing in dynticks-idle
Shrikanth Hegde
sshegde at linux.ibm.com
Wed Feb 25 21:34:45 AEDT 2026
Hi Christophe.
On 2/25/26 3:15 PM, Christophe Leroy (CS GROUP) wrote:
> 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 at kernel.org>
>>>>
>>>> Reviewed-by: Shrikanth Hegde <sshegde at 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.
>
Got it. The main concern was around with additional computation that sched_clock,
not any additional paths per se.
yes, that would be possible,
How about we do below? This adds only one subtraction.
This achieves the same outcome.
---
diff --git a/arch/powerpc/include/asm/cputime.h b/arch/powerpc/include/asm/cputime.h
index aff858ca99c0..7afba0202568 100644
--- a/arch/powerpc/include/asm/cputime.h
+++ b/arch/powerpc/include/asm/cputime.h
@@ -44,7 +44,7 @@
*/
static notrace inline void account_cpu_user_entry(void)
{
- unsigned long tb = mftb();
+ unsigned long tb = mftb() - get_boot_tb();
struct cpu_accounting_data *acct = raw_get_accounting(current);
acct->utime += (tb - acct->starttime_user);
@@ -53,7 +53,7 @@ static notrace inline void account_cpu_user_entry(void)
static notrace inline void account_cpu_user_exit(void)
{
- unsigned long tb = mftb();
+ unsigned long tb = mftb() - get_boot_tb();
struct cpu_accounting_data *acct = raw_get_accounting(current);
acct->stime += (tb - acct->starttime);
diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 18506740f4a4..ff5524e6cdc7 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 = mftb() - get_boot_tb();
stime = now - acct->starttime;
acct->starttime = now;
@@ -388,7 +388,7 @@ void vtime_reset(void)
{
struct cpu_accounting_data *acct = get_accounting(current);
- acct->starttime = mftb();
+ acct->starttime = mftb() - get_boot_tb();
#ifdef CONFIG_ARCH_HAS_SCALED_CPUTIME
acct->startspurr = read_spurr(acct->starttime);
#endif
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