Re: [REGRESSION] sched/idle: Sysbench threads regression after f4c31b07b136

[Joseph Salisbury <joseph.salisbury@oracle.com>]

Hi Rafael, Christian,

On 7/6/26 10:29 AM, Christian Loehle wrote:
> On 7/2/26 19:47, Rafael J. Wysocki (Intel) wrote:
>> Hi,
>>
>> On Thu, Jul 2, 2026 at 6:30 PM Joseph Salisbury
>> <joseph.salisbury@oracle.com> wrote:
>>> Hi Rafael,
>>>
>>> We are seeing a reproducible MySQL Sysbench threads regression.  A
>>> bisect indicated the following commit as the first bad commit:
>>> f4c31b07b136 ("sched: idle: Consolidate the handling of two special cases")
>>>
>>> The regression was found in Oracle kernel performance testing on OCI VM
>>> shapes:
>>>
>>> VM Details:
>>> * VM.Standard2.1:
>>>         x86 OCI VM shape, 1 OCPU / 2 hardware threads, about 14.5 GB RAM
>>>
>>>    * VM.Standard.A1.Flex.2:
>>>         Arm/Ampere A1 flexible VM shape, 2 vCPU threads, about 10.9 GB RAM
>>>
>>>
>>> The ResultsDB runs show the regression in the Sysbench threads metric:
>>>
>>>     - VM.Standard2.1:       333 -> 236  (-29.1%)
>>>     - VM.Standard.A1.Flex.2: 1286 -> 1152 (-10.4%)
>>>
>>> A test kernel was built with f4c31b07b136 reverted and the performance
>>> regression was recovered.
>>>
>>>   From the code, it is possible the regression is due to the new
>>> previous-wakeup heuristic in the special idle cases.  Before the commit:
>>>
>>>     - no cpuidle driver:
>>>         tick_nohz_idle_stop_tick()
>>>         default_idle_call()
>>>
>>>     - one idle state:
>>>         tick_nohz_idle_retain_tick()
>>>         cpuidle state 0
>> I think that this is your case and the tick stops for you sometimes
>> now while it had never stopped before.
>>
>> Can you confirm?
The guest-visible data does not show the single-idle-state cpuidle case.
Both affected guests report:

/sys/devices/system/cpu/cpuidle/current_driver = none
/sys/devices/system/cpu/cpuidle/current_governor = menu

There are also no /sys/devices/system/cpu/cpu*/cpuidle entries on either 
guest.  So from the guest data, this looks like the no-cpuidle-driver 
special case rather than the one-idle-state case.

The full revert of f4c31b07b136 recovered the regression. I also tested 
Rafael's suggested one-line change, applied as:

     -        idle_call_stop_or_retain_tick(stop_tick);
    +        idle_call_stop_or_retain_tick(false);

That test kernel still showed regressed performance on VM.Standard2.1.


>>
>> Overall, it would be good to know the idle state lists for both the VM
>> and the host.
For the VMs, there are no guest cpuidle state lists exposed because the 
cpuidle driver is "none".

I do not currently have the host-side idle-state lists from the OCI 
hosts.  I can try to get that data if it would still be useful.
>>
> +1, but also which HZ are you using?
VM.Standard2.1, x86_64:

CONFIG_HZ_1000=y
CONFIG_HZ=1000

VM.Standard.A1.Flex.2, aarch64:

CONFIG_HZ_250=y
CONFIG_HZ=250

> Both systems reported have 2 logical CPUs then?

Yes:

VM.Standard2.1:

CPU(s): 2
Thread(s) per core: 2
Core(s) per socket: 1
Socket(s): 1

VM.Standard.A1.Flex.2:

CPU(s): 2
Thread(s) per core: 1
Core(s) per socket: 2
Socket(s): 1

> Were higher core counts also
> tested and how does it affect them?
Yes. Higher-core-count runs were checked. The regression appears limited 
to the smaller core-count shapes.

The current data shows regressions on:

- VM.Standard2.1
- VM.Standard.A1.Flex.2
- VM.Standard.E4.Flex.1

The larger tested shapes did not show the same regression pattern. The 
test runs use one sysbench thread per online CPU/core count as encoded 
in the metric name.