Re: [PATCH] sched/fair: Disable scheduler feature NEXT_BUDDY

[Madadi Vineeth Reddy <vineethr@linux.ibm.com>]

On 20/01/26 17:03, Mel Gorman wrote:
> NEXT_BUDDY was disabled with the introduction of EEVDF and enabled again
> after NEXT_BUDDY was rewritten for EEVDF by commit e837456fdca8 ("sched/fair:
> Reimplement NEXT_BUDDY to align with EEVDF goals"). It was not expected
> that this would be a universal win without a crystal ball instruction
> but the reported regressions are a concern [1][2] even if gains were
> also reported. Specifically;
> 
> o mysql with client/server running on different servers regresses
> o specjbb reports lower peak metrics
> o daytrader regresses
> 
> The mysql is realistic and a concern. It needs to be confirmed if
> specjbb is simply shifting the point where peak performance is measured
> but still a concern. daytrader is considered to be representative of a
> real workload.
> 
> Access to test machines is currently problematic for verifying any fix to
> this problem. Disable NEXT_BUDDY for now by default until the root causes
> are addressed.
> 
> Link: https://lore.kernel.org/lkml/4b96909a-f1ac-49eb-b814-97b8adda6229@arm.com [1]
> Link: https://lore.kernel.org/lkml/ec3ea66f-3a0d-4b5a-ab36-ce778f159b5b@linux.ibm.com [2]
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---
>  kernel/sched/features.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> index 980d92bab8ab..136a6584be79 100644
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -29,7 +29,7 @@ SCHED_FEAT(PREEMPT_SHORT, true)
>   * wakeup-preemption), since its likely going to consume data we
>   * touched, increases cache locality.
>   */
> -SCHED_FEAT(NEXT_BUDDY, true)
> +SCHED_FEAT(NEXT_BUDDY, false)

Thanks Mel, this should fix the issue for now. I spent some time in limiting the
number of times NEXT_BUDDY is selected consecutively which improved the Daytrader
performance by around 2%. (out of the 7% regression). Will continue exploring it.

Tested-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>

Thanks,
Vineeth

>  
>  /*
>   * Allow completely ignoring cfs_rq->next; which can be set from various