Re: [PATCH v2 00/12] sched: Address schbench regression

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 7/7/25 14:35, Shrikanth Hegde wrote:
> 
> 
> On 7/2/25 17:19, Peter Zijlstra wrote:
>> Hi!
>>
>> Previous version:
>>
>>    https://lkml.kernel.org/r/20250520094538.086709102@infradead.org
>>
>>
>> Changes:
>>   - keep dl_server_stop(), just remove the 'normal' usage of it (juril)
>>   - have the sched_delayed wake list IPIs do select_task_rq() (vingu)
>>   - fixed lockdep splat (dietmar)
>>   - added a few preperatory patches
>>
>>
>> Patches apply on top of tip/master (which includes the disabling of 
>> private futex)
>> and clm's newidle balance patch (which I'm awaiting vingu's ack on).
>>
>> Performance is similar to the last version; as tested on my SPR on 
>> v6.15 base:
>>
> 
> 
> Hi Peter,
> Gave this a spin on a machine with 5 cores (SMT8) PowerPC system.
> 
> I see significant regression in schbench. let me know if i have to test 
> different
> number of threads based on the system size.
> Will go through the series and will try a bisect meanwhile.
> 
> 

Used "./schbench -L -m 4 -M auto -t 64 -n 0 -t 60 -i 60" for git bisect.
Also kept HZ=1000


Git bisect points to
# first bad commit: [dc968ba0544889883d0912360dd72d90f674c140] sched: Add ttwu_queue support for delayed tasks

Note:
at commit: "sched: Change ttwu_runnable() vs sched_delayed" there is a small regression.

-------------------------------------
Numbers at different commits:
-------------------------------------
commit 8784fb5fa2e0042fe3b1632d4876e1037b695f56      <<<< baseline
Merge: 11119b0b378a 94b59d5f567a
Author: Borislav Petkov (AMD) <bp@alien8.de>
Date:   Sat Jul 5 19:24:35 2025 +0200

     Merge irq/drivers into tip/master

Wakeup Latencies percentiles (usec) runtime 30 (s) (39778894 total samples)
           50.0th: 14         (11798914 samples)
           90.0th: 27         (15931329 samples)
         * 99.0th: 42         (3032865 samples)
           99.9th: 64         (346598 samples)

RPS percentiles (requests) runtime 30 (s) (31 total samples)
           20.0th: 1394688    (18 samples)
         * 50.0th: 1394688    (0 samples)
           90.0th: 1398784    (11 samples)

--------------------------------------

commit 88ca74dd6fe5d5b03647afb4698238e4bec3da39 (HEAD)       <<< Still good commit
Author: Peter Zijlstra <peterz@infradead.org>
Date:   Wed Jul 2 13:49:34 2025 +0200

     sched: Use lock guard in sched_ttwu_pending()

Wakeup Latencies percentiles (usec) runtime 30 (s) (40132792 total samples)
           50.0th: 14         (11986044 samples)
           90.0th: 27         (15143836 samples)
         * 99.0th: 46         (3267133 samples)
           99.9th: 72         (333940 samples)
RPS percentiles (requests) runtime 30 (s) (31 total samples)
           20.0th: 1402880    (23 samples)
         * 50.0th: 1402880    (0 samples)
           90.0th: 1406976    (8 samples)

-----------------------------------------------------------------

commit 755d11feca4544b4bc6933dcdef29c41585fa747        <<< There is a small regression.
Author: Peter Zijlstra <peterz@infradead.org>
Date:   Wed Jul 2 13:49:35 2025 +0200

     sched: Change ttwu_runnable() vs sched_delayed

Wakeup Latencies percentiles (usec) runtime 30 (s) (39308991 total samples)
           50.0th: 18         (12991812 samples)
           90.0th: 34         (14381736 samples)
         * 99.0th: 56         (3399332 samples)
           99.9th: 84         (342508 samples)

RPS percentiles (requests) runtime 30 (s) (31 total samples)
           20.0th: 1353728    (21 samples)
         * 50.0th: 1353728    (0 samples)
           90.0th: 1357824    (10 samples)

-----------------------------------------------------------

commit dc968ba0544889883d0912360dd72d90f674c140              <<<< Major regression
Author: Peter Zijlstra <peterz@infradead.org>
Date:   Wed Jul 2 13:49:36 2025 +0200

     sched: Add ttwu_queue support for delayed tasks

Wakeup Latencies percentiles (usec) runtime 30 (s) (19818598 total samples)
           50.0th: 111        (5891601 samples)
           90.0th: 214        (7947099 samples)
         * 99.0th: 283        (1749294 samples)
           99.9th: 329        (177336 samples)

RPS percentiles (requests) runtime 30 (s) (31 total samples)
           20.0th: 654336     (7 samples)
         * 50.0th: 660480     (11 samples)
           90.0th: 666624     (11 samples)