From aceccac58ad76305d147165788ea6b939bef179b Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Wed, 12 Nov 2025 12:25:20 +0000 Subject: [PATCH] sched/fair: Enable scheduler feature NEXT_BUDDY The NEXT_BUDDY feature reinforces wakeup preemption to encourage the last wakee to be scheduled sooner on the assumption that the waker/wakee share cache-hot data. In CFS, it was paired with LAST_BUDDY to switch back on the assumption that the pair of tasks still share data but also relied on START_DEBIT and the exact WAKEUP_PREEMPTION implementation to get good results. NEXT_BUDDY has been disabled since commit 0ec9fab3d186 ("sched: Improve latencies and throughput") and LAST_BUDDY was removed in commit 5e963f2bd465 ("sched/fair: Commit to EEVDF"). The reasoning is not clear but as vruntime spread is mentioned so the expectation is that NEXT_BUDDY had an impact on overall fairness. It was not noted why LAST_BUDDY was removed but it is assumed that it's very difficult to reason what LAST_BUDDY's correct and effective behaviour should be while still respecting EEVDFs goals. Peter Zijlstra noted during review; I think I was just struggling to make sense of things and figured less is more and axed it. I have vague memories trying to work through the dynamics of a wakeup-stack and the EEVDF latency requirements and getting a head-ache. NEXT_BUDDY is easier to reason about given that it's a point-in-time decision on the wakees deadline and eligibilty relative to the waker. Enable NEXT_BUDDY as a preparation path to document that the decision to ignore the current implementation is deliberate. While not presented, the results were at best neutral and often much more variable. Signed-off-by: Mel Gorman Signed-off-by: Peter Zijlstra (Intel) Link: https://patch.msgid.link/20251112122521.1331238-2-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 3c12d9f93331d..0607def744af6 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, false) +SCHED_FEAT(NEXT_BUDDY, true) /* * Allow completely ignoring cfs_rq->next; which can be set from various -- 2.47.3