Re: [PATCH v2] sched: fair: Prevent negative lag increase during delayed dequeue

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 31, 2026 at 06:23:52PM +0200, Vincent Guittot wrote:
> Delayed dequeue feature aims to reduce the negative lag of a dequeued task
> while sleeping but it can happens that newly enqueued tasks will move
> backward the avg vruntime and increase its negative lag.
> When the delayed dequeued task wakes up, it has more neg lag compared to
> being dequeued immediately or to other tasks that have been dequeued just
> before theses new enqueues.
> 
> Ensure that the negative lag of a delayed dequeued task doesn't increase
> during its delayed dequeued phase while waiting for its neg lag to
> diseappear. Similarly, we remove any positive lag that the delayed
> dequeued task could have gain during thsi period.
> 
> Short slice tasks are particularly impacted in overloaded system.
> 
> Test on snapdragon rb5:
> 
> hackbench -T -p -l 16000000 -g 2 1> /dev/null &
> cyclictest -t 1 -i 2777 -D 333 --policy=fair --mlock  -h 20000 -q
> 
> The scheduling latency of cyclictest is:
> 
>                        tip/sched/core  tip/sched/core    +this patch
> cyclictest slice  (ms) (default)2.8             8               8
> hackbench slice   (ms) (default)2.8            20              20
> Total Samples          |   115632          119733          119806
> Average           (us) |      364              64(-82%)        61(- 5%)
> Median (P50)      (us) |       60              56(- 7%)        56(  0%)
> 90th Percentile   (us) |     1166              62(-95%)        62(  0%)
> 99th Percentile   (us) |     4192              73(-98%)        72(- 1%)
> 99.9th Percentile (us) |     8528            2707(-68%)      1300(-52%)
> Maximum           (us) |    17735           14273(-20%)     13525(- 5%)
> 

Anyway, I can confirm this works quite well. The latency-slice numbers
are far more stable now.

Thanks for digging into that!