git: 0566e750c46c - stable/14 - sched_ule: runq_steal_from(): Suppress first thread special case

The branch stable/14 has been updated by olce:

URL: https://cgit.FreeBSD.org/src/commit/?id=0566e750c46cea02b4fbdda5b31e83dafaeaeb15

commit 0566e750c46cea02b4fbdda5b31e83dafaeaeb15
Author:     Olivier Certner <olce@FreeBSD.org>
AuthorDate: 2024-04-29 06:54:43 +0000
Commit:     Olivier Certner <olce@FreeBSD.org>
CommitDate: 2025-07-28 13:28:18 +0000

    sched_ule: runq_steal_from(): Suppress first thread special case
    
    This special case was introduced as soon as commit "ULE 3.0"
    (ae7a6b38d53f, r171482, from July 2007).  It caused runq_steal_from() to
    ignore the highest-priority thread while stealing.
    
    Its functionality was changed in commit "Rework CPU load balancing in
    SCHED_ULE" (36acfc6507aa, r232207, from February 2012), where the intent
    was to keep track of that first thread and return it if no other one was
    stealable, instead of returning NULL (no steal).  Some bug prevented it
    from working in loaded cases (more than one thread, and all threads but
    the first one not stealable), which was subsequently fixed in commit
    "sched_ule(4): Fix interactive threads stealing." (bd84094a51c4, from
    September 2021).
    
    All the reasons for this mechanism we could second-guess were dubious at
    best.  Jeff Roberson, ULE's main author, says in the differential
    revision that "The point was to move threads that are least likely to
    benefit from affinity because they are unlikely to run soon enough to
    take advantage of it.", to which we responded: "(snip) This may improve
    affinity in some cases, but at the same time we don't really know when
    the next thread on the queue is to run. Not stealing in this case also
    amounts to slightly violating the expected execution ordering and
    fairness.".
    
    As this twist doesn't seem to bring any performance improvement in
    general, let's just remove it.
    
    MFC after:      1 month
    Event:          Kitchener-Waterloo Hackathon 202506
    Sponsored by:   The FreeBSD Foundation
    Differential Revision:  https://reviews.freebsd.org/D45388
    
    (cherry picked from commit fdf31d27476968456a8a389d8152370582756ef1)
---
 sys/kern/sched_ule.c | 14 --------------
 1 file changed, 14 deletions(-)

diff --git a/sys/kern/sched_ule.c b/sys/kern/sched_ule.c
index 2e01cf6c2484..01eaabac3450 100644
--- a/sys/kern/sched_ule.c
+++ b/sys/kern/sched_ule.c
@@ -1185,9 +1185,7 @@ tdq_notify(struct tdq *tdq, int lowpri)
 
 struct runq_steal_pred_data {
 	struct thread	*td;
-	struct thread	*first;
 	int		cpu;
-	bool		use_first_last;
 };
 
 static bool
@@ -1197,11 +1195,6 @@ runq_steal_pred(const int idx, struct rq_queue *const q, void *const data)
 	struct thread *td;
 
 	TAILQ_FOREACH(td, q, td_runq) {
-		if (d->use_first_last && d->first == NULL) {
-			d->first = td;
-			continue;
-		}
-
 		if (THREAD_CAN_MIGRATE(td) && THREAD_CAN_SCHED(td, d->cpu)) {
 			d->td = td;
 			return (true);
@@ -1220,9 +1213,7 @@ runq_steal_from(struct runq *const rq, int cpu, int start_idx)
 {
 	struct runq_steal_pred_data data = {
 		.td = NULL,
-		.first = NULL,
 		.cpu = cpu,
-		.use_first_last = true
 	};
 	int idx;
 
@@ -1238,9 +1229,6 @@ runq_steal_from(struct runq *const rq, int cpu, int start_idx)
 	}
 
 	MPASS(idx == -1 && data.td == NULL);
-	if (data.first != NULL && THREAD_CAN_MIGRATE(data.first) &&
-	    THREAD_CAN_SCHED(data.first, cpu))
-		return (data.first);
 	return (NULL);
 found:
 	MPASS(data.td != NULL);
@@ -1255,9 +1243,7 @@ runq_steal(struct runq *rq, int cpu)
 {
 	struct runq_steal_pred_data data = {
 		.td = NULL,
-		.first = NULL,
 		.cpu = cpu,
-		.use_first_last = false
 	};
 	int idx;