Re: [PATCH v2 00/10] sched: Flatten the pick

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 13, 2026 at 01:35:10PM +0200, Peter Zijlstra wrote:
> On Tue, May 12, 2026 at 10:42:33AM +0200, Vincent Guittot wrote:
> 
> > I haven't reviewed the patches yet but I ran some tests with it while
> > testing sched latency related changes for short slice wakeup
> > preemption. I have some large hackbench regressions with this series
> > on HMP system with and without EAS. those figures are unexpected
> > because the benchs run on root cfs
> > 
> > One example with hackbench 8 groups thread pipe
> > tip/sched/core  tip/sched/core          +this patchset          +this patchset
> > slice 2.8ms     16ms                    2.8ms                   16ms
> > dragonboard rb5 with EAS
> > 0,748(+/-4,6%)  0,621(+/-3.6%) +17%     1,915(+/-7.9%) -156%
> > 0,689(+/- 9.1%) +8%
> > 
> > radxa orion6 HMP without EAS
> > 0,588(+/-5.8%)  0,677(+/-5.9%) -15%     1,505(+/-10%) -156%
> > 1,071(+/-5.9%) -82%
> > 
> > Increasing the slice partly removes regressions but tis is surprising
> > because the bench runs at root cfs and I thought that results will not
> > change in such a case
> 
> D'oh :/
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e54da4c6c945..77d0e1937f2c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9071,7 +9071,7 @@ static void wakeup_preempt_fair(struct rq *rq, struct task_struct *p, int wake_f
>  	enum preempt_wakeup_action preempt_action = PREEMPT_WAKEUP_PICK;
>  	struct task_struct *donor = rq->donor;
>  	struct sched_entity *nse, *se = &donor->se, *pse = &p->se;
> -	struct cfs_rq *cfs_rq = task_cfs_rq(donor);
> +	struct cfs_rq *cfs_rq = &rq->cfs;
>  	int cse_is_idle, pse_is_idle;
>  
>  	/*

With that fixed, I now get:

	vanilla	slice(*)

FPS min	  3.0	11.1
    avg  44.7	57.3
    max  88.1	96.2

FT  min   9.1	 8.0
    avg  41.4	21.0
    max 157.2   53.9

FPS (Frames Per Second)
FT  (FrameTime)


Which I suppose shows we now preempt less. Its still significantly
better with reduced slice, but not as good as it was.