FAILED: patch "[PATCH] sched/eevdf: Fix min_vruntime vs avg_vruntime" failed to apply to 6.6-stable tree

FAILED: patch "[PATCH] sched/eevdf: Fix min_vruntime vs avg_vruntime" failed to apply to 6.6-stable tree

[gregkh]

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 79f3f9bedd149ea438aaeb0fb6a083637affe205
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2025122903-sterile-from-4520@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 79f3f9bedd149ea438aaeb0fb6a083637affe205 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Wed, 2 Apr 2025 20:07:34 +0200
Subject: [PATCH] sched/eevdf: Fix min_vruntime vs avg_vruntime

Basically, from the constraint that the sum of lag is zero, you can
infer that the 0-lag point is the weighted average of the individual
vruntime, which is what we're trying to compute:

        \Sum w_i * v_i
  avg = --------------
           \Sum w_i

Now, since vruntime takes the whole u64 (worse, it wraps), this
multiplication term in the numerator is not something we can compute;
instead we do the min_vruntime (v0 henceforth) thing like:

  v_i = (v_i - v0) + v0

This does two things:
 - it keeps the key: (v_i - v0) 'small';
 - it creates a relative 0-point in the modular space.

If you do that subtitution and work it all out, you end up with:

        \Sum w_i * (v_i - v0)
  avg = --------------------- + v0
              \Sum w_i

Since you cannot very well track a ratio like that (and not suffer
terrible numerical problems) we simpy track the numerator and
denominator individually and only perform the division when strictly
needed.

Notably, the numerator lives in cfs_rq->avg_vruntime and the denominator
lives in cfs_rq->avg_load.

The one extra 'funny' is that these numbers track the entities in the
tree, and current is typically outside of the tree, so avg_vruntime()
adds current when needed before doing the division.

(vruntime_eligible() elides the division by cross-wise multiplication)

Anyway, as mentioned above, we currently use the CFS era min_vruntime
for this purpose. However, this thing can only move forward, while the
above avg can in fact move backward (when a non-eligible task leaves,
the average becomes smaller), this can cause trouble when through
happenstance (or construction) these values drift far enough apart to
wreck the game.

Replace cfs_rq::min_vruntime with cfs_rq::zero_vruntime which is kept
near/at avg_vruntime, following its motion.

The down-side is that this requires computing the avg more often.

Fixes: 147f3efaa241 ("sched/fair: Implement an EEVDF-like scheduling policy")
Reported-by: Zicheng Qu <quzicheng@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251106111741.GC4068168@noisy.programming.kicks-ass.net
Cc: stable@vger.kernel.org

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 02e16b70a790..41caa22e0680 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -796,7 +796,7 @@ static void print_rq(struct seq_file *m, struct rq *rq, int rq_cpu)
 
 void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
 {
-	s64 left_vruntime = -1, min_vruntime, right_vruntime = -1, left_deadline = -1, spread;
+	s64 left_vruntime = -1, zero_vruntime, right_vruntime = -1, left_deadline = -1, spread;
 	struct sched_entity *last, *first, *root;
 	struct rq *rq = cpu_rq(cpu);
 	unsigned long flags;
@@ -819,15 +819,15 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
 	last = __pick_last_entity(cfs_rq);
 	if (last)
 		right_vruntime = last->vruntime;
-	min_vruntime = cfs_rq->min_vruntime;
+	zero_vruntime = cfs_rq->zero_vruntime;
 	raw_spin_rq_unlock_irqrestore(rq, flags);
 
 	SEQ_printf(m, "  .%-30s: %Ld.%06ld\n", "left_deadline",
 			SPLIT_NS(left_deadline));
 	SEQ_printf(m, "  .%-30s: %Ld.%06ld\n", "left_vruntime",
 			SPLIT_NS(left_vruntime));
-	SEQ_printf(m, "  .%-30s: %Ld.%06ld\n", "min_vruntime",
-			SPLIT_NS(min_vruntime));
+	SEQ_printf(m, "  .%-30s: %Ld.%06ld\n", "zero_vruntime",
+			SPLIT_NS(zero_vruntime));
 	SEQ_printf(m, "  .%-30s: %Ld.%06ld\n", "avg_vruntime",
 			SPLIT_NS(avg_vruntime(cfs_rq)));
 	SEQ_printf(m, "  .%-30s: %Ld.%06ld\n", "right_vruntime",
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4a11a832d63e..8d971d48669f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -554,7 +554,7 @@ static inline bool entity_before(const struct sched_entity *a,
 
 static inline s64 entity_key(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
-	return (s64)(se->vruntime - cfs_rq->min_vruntime);
+	return (s64)(se->vruntime - cfs_rq->zero_vruntime);
 }
 
 #define __node_2_se(node) \
@@ -606,13 +606,13 @@ static inline s64 entity_key(struct cfs_rq *cfs_rq, struct sched_entity *se)
  *
  * Which we track using:
  *
- *                    v0 := cfs_rq->min_vruntime
+ *                    v0 := cfs_rq->zero_vruntime
  * \Sum (v_i - v0) * w_i := cfs_rq->avg_vruntime
  *              \Sum w_i := cfs_rq->avg_load
  *
- * Since min_vruntime is a monotonic increasing variable that closely tracks
- * the per-task service, these deltas: (v_i - v), will be in the order of the
- * maximal (virtual) lag induced in the system due to quantisation.
+ * Since zero_vruntime closely tracks the per-task service, these
+ * deltas: (v_i - v), will be in the order of the maximal (virtual) lag
+ * induced in the system due to quantisation.
  *
  * Also, we use scale_load_down() to reduce the size.
  *
@@ -671,7 +671,7 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq)
 		avg = div_s64(avg, load);
 	}
 
-	return cfs_rq->min_vruntime + avg;
+	return cfs_rq->zero_vruntime + avg;
 }
 
 /*
@@ -732,7 +732,7 @@ static int vruntime_eligible(struct cfs_rq *cfs_rq, u64 vruntime)
 		load += weight;
 	}
 
-	return avg >= (s64)(vruntime - cfs_rq->min_vruntime) * load;
+	return avg >= (s64)(vruntime - cfs_rq->zero_vruntime) * load;
 }
 
 int entity_eligible(struct cfs_rq *cfs_rq, struct sched_entity *se)
@@ -740,42 +740,14 @@ int entity_eligible(struct cfs_rq *cfs_rq, struct sched_entity *se)
 	return vruntime_eligible(cfs_rq, se->vruntime);
 }
 
-static u64 __update_min_vruntime(struct cfs_rq *cfs_rq, u64 vruntime)
+static void update_zero_vruntime(struct cfs_rq *cfs_rq)
 {
-	u64 min_vruntime = cfs_rq->min_vruntime;
-	/*
-	 * open coded max_vruntime() to allow updating avg_vruntime
-	 */
-	s64 delta = (s64)(vruntime - min_vruntime);
-	if (delta > 0) {
-		avg_vruntime_update(cfs_rq, delta);
-		min_vruntime = vruntime;
-	}
-	return min_vruntime;
-}
+	u64 vruntime = avg_vruntime(cfs_rq);
+	s64 delta = (s64)(vruntime - cfs_rq->zero_vruntime);
 
-static void update_min_vruntime(struct cfs_rq *cfs_rq)
-{
-	struct sched_entity *se = __pick_root_entity(cfs_rq);
-	struct sched_entity *curr = cfs_rq->curr;
-	u64 vruntime = cfs_rq->min_vruntime;
+	avg_vruntime_update(cfs_rq, delta);
 
-	if (curr) {
-		if (curr->on_rq)
-			vruntime = curr->vruntime;
-		else
-			curr = NULL;
-	}
-
-	if (se) {
-		if (!curr)
-			vruntime = se->min_vruntime;
-		else
-			vruntime = min_vruntime(vruntime, se->min_vruntime);
-	}
-
-	/* ensure we never gain time by being placed backwards. */
-	cfs_rq->min_vruntime = __update_min_vruntime(cfs_rq, vruntime);
+	cfs_rq->zero_vruntime = vruntime;
 }
 
 static inline u64 cfs_rq_min_slice(struct cfs_rq *cfs_rq)
@@ -848,6 +820,7 @@ RB_DECLARE_CALLBACKS(static, min_vruntime_cb, struct sched_entity,
 static void __enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
 	avg_vruntime_add(cfs_rq, se);
+	update_zero_vruntime(cfs_rq);
 	se->min_vruntime = se->vruntime;
 	se->min_slice = se->slice;
 	rb_add_augmented_cached(&se->run_node, &cfs_rq->tasks_timeline,
@@ -859,6 +832,7 @@ static void __dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 	rb_erase_augmented_cached(&se->run_node, &cfs_rq->tasks_timeline,
 				  &min_vruntime_cb);
 	avg_vruntime_sub(cfs_rq, se);
+	update_zero_vruntime(cfs_rq);
 }
 
 struct sched_entity *__pick_root_entity(struct cfs_rq *cfs_rq)
@@ -1226,7 +1200,6 @@ static void update_curr(struct cfs_rq *cfs_rq)
 
 	curr->vruntime += calc_delta_fair(delta_exec, curr);
 	resched = update_deadline(cfs_rq, curr);
-	update_min_vruntime(cfs_rq);
 
 	if (entity_is_task(curr)) {
 		/*
@@ -3808,15 +3781,6 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
 		if (!curr)
 			__enqueue_entity(cfs_rq, se);
 		cfs_rq->nr_queued++;
-
-		/*
-		 * The entity's vruntime has been adjusted, so let's check
-		 * whether the rq-wide min_vruntime needs updated too. Since
-		 * the calculations above require stable min_vruntime rather
-		 * than up-to-date one, we do the update at the end of the
-		 * reweight process.
-		 */
-		update_min_vruntime(cfs_rq);
 	}
 }
 
@@ -5429,15 +5393,6 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 
 	update_cfs_group(se);
 
-	/*
-	 * Now advance min_vruntime if @se was the entity holding it back,
-	 * except when: DEQUEUE_SAVE && !DEQUEUE_MOVE, in this case we'll be
-	 * put back on, and if we advance min_vruntime, we'll be placed back
-	 * further than we started -- i.e. we'll be penalized.
-	 */
-	if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) != DEQUEUE_SAVE)
-		update_min_vruntime(cfs_rq);
-
 	if (flags & DEQUEUE_DELAYED)
 		finish_delayed_dequeue_entity(se);
 
@@ -9015,7 +8970,6 @@ static void yield_task_fair(struct rq *rq)
 	if (entity_eligible(cfs_rq, se)) {
 		se->vruntime = se->deadline;
 		se->deadline += calc_delta_fair(se->slice, se);
-		update_min_vruntime(cfs_rq);
 	}
 }
 
@@ -13078,23 +13032,6 @@ static inline void task_tick_core(struct rq *rq, struct task_struct *curr)
  * Which shows that S and s_i transform alike (which makes perfect sense
  * given that S is basically the (weighted) average of s_i).
  *
- * Then:
- *
- *   x -> s_min := min{s_i}                                   (8)
- *
- * to obtain:
- *
- *               \Sum_i w_i (s_i - s_min)
- *   S = s_min + ------------------------                     (9)
- *                     \Sum_i w_i
- *
- * Which already looks familiar, and is the basis for our current
- * approximation:
- *
- *   S ~= s_min                                              (10)
- *
- * Now, obviously, (10) is absolute crap :-), but it sorta works.
- *
  * So the thing to remember is that the above is strictly UP. It is
  * possible to generalize to multiple runqueues -- however it gets really
  * yuck when you have to add affinity support, as illustrated by our very
@@ -13116,23 +13053,23 @@ static inline void task_tick_core(struct rq *rq, struct task_struct *curr)
  * Let, for our runqueue 'k':
  *
  *   T_k = \Sum_i w_i s_i
- *   W_k = \Sum_i w_i      ; for all i of k                  (11)
+ *   W_k = \Sum_i w_i      ; for all i of k                  (8)
  *
  * Then we can write (6) like:
  *
  *         T_k
- *   S_k = ---                                               (12)
+ *   S_k = ---                                               (9)
  *         W_k
  *
  * From which immediately follows that:
  *
  *           T_k + T_l
- *   S_k+l = ---------                                       (13)
+ *   S_k+l = ---------                                       (10)
  *           W_k + W_l
  *
  * On which we can define a combined lag:
  *
- *   lag_k+l(i) := S_k+l - s_i                               (14)
+ *   lag_k+l(i) := S_k+l - s_i                               (11)
  *
  * And that gives us the tools to compare tasks across a combined runqueue.
  *
@@ -13143,7 +13080,7 @@ static inline void task_tick_core(struct rq *rq, struct task_struct *curr)
  *     using (7); this only requires storing single 'time'-stamps.
  *
  *  b) when comparing tasks between 2 runqueues of which one is forced-idle,
- *     compare the combined lag, per (14).
+ *     compare the combined lag, per (11).
  *
  * Now, of course cgroups (I so hate them) make this more interesting in
  * that a) seems to suggest we need to iterate all cgroup on a CPU at such
@@ -13191,12 +13128,11 @@ static inline void task_tick_core(struct rq *rq, struct task_struct *curr)
  * every tick. This limits the observed divergence due to the work
  * conservancy.
  *
- * On top of that, we can improve upon things by moving away from our
- * horrible (10) hack and moving to (9) and employing (13) here.
+ * On top of that, we can improve upon things by employing (10) here.
  */
 
 /*
- * se_fi_update - Update the cfs_rq->min_vruntime_fi in a CFS hierarchy if needed.
+ * se_fi_update - Update the cfs_rq->zero_vruntime_fi in a CFS hierarchy if needed.
  */
 static void se_fi_update(const struct sched_entity *se, unsigned int fi_seq,
 			 bool forceidle)
@@ -13210,7 +13146,7 @@ static void se_fi_update(const struct sched_entity *se, unsigned int fi_seq,
 			cfs_rq->forceidle_seq = fi_seq;
 		}
 
-		cfs_rq->min_vruntime_fi = cfs_rq->min_vruntime;
+		cfs_rq->zero_vruntime_fi = cfs_rq->zero_vruntime;
 	}
 }
 
@@ -13263,11 +13199,11 @@ bool cfs_prio_less(const struct task_struct *a, const struct task_struct *b,
 
 	/*
 	 * Find delta after normalizing se's vruntime with its cfs_rq's
-	 * min_vruntime_fi, which would have been updated in prior calls
+	 * zero_vruntime_fi, which would have been updated in prior calls
 	 * to se_fi_update().
 	 */
 	delta = (s64)(sea->vruntime - seb->vruntime) +
-		(s64)(cfs_rqb->min_vruntime_fi - cfs_rqa->min_vruntime_fi);
+		(s64)(cfs_rqb->zero_vruntime_fi - cfs_rqa->zero_vruntime_fi);
 
 	return delta > 0;
 }
@@ -13513,7 +13449,7 @@ static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first)
 void init_cfs_rq(struct cfs_rq *cfs_rq)
 {
 	cfs_rq->tasks_timeline = RB_ROOT_CACHED;
-	cfs_rq->min_vruntime = (u64)(-(1LL << 20));
+	cfs_rq->zero_vruntime = (u64)(-(1LL << 20));
 	raw_spin_lock_init(&cfs_rq->removed.lock);
 }
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 82e74e8ca2ea..5a3cf81c27be 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -681,10 +681,10 @@ struct cfs_rq {
 	s64			avg_vruntime;
 	u64			avg_load;
 
-	u64			min_vruntime;
+	u64			zero_vruntime;
 #ifdef CONFIG_SCHED_CORE
 	unsigned int		forceidle_seq;
-	u64			min_vruntime_fi;
+	u64			zero_vruntime_fi;
 #endif
 
 	struct rb_root_cached	tasks_timeline;

[PATCH 6.6.y 1/3] sched/fair: Forfeit vruntime on yield

[Sasha Levin]

From: Fernand Sieber <sieberf@amazon.com>

[ Upstream commit 79104becf42baeeb4a3f2b106f954b9fc7c10a3c ]

If a task yields, the scheduler may decide to pick it again. The task in
turn may decide to yield immediately or shortly after, leading to a tight
loop of yields.

If there's another runnable task as this point, the deadline will be
increased by the slice at each loop. This can cause the deadline to runaway
pretty quickly, and subsequent elevated run delays later on as the task
doesn't get picked again. The reason the scheduler can pick the same task
again and again despite its deadline increasing is because it may be the
only eligible task at that point.

Fix this by making the task forfeiting its remaining vruntime and pushing
the deadline one slice ahead. This implements yield behavior more
authentically.

We limit the forfeiting to eligible tasks. This is because core scheduling
prefers running ineligible tasks rather than force idling. As such, without
the condition, we can end up on a yield loop which makes the vruntime
increase rapidly, leading to anomalous run delays later down the line.

Fixes: 147f3efaa24182 ("sched/fair: Implement an EEVDF-like scheduling  policy")
Signed-off-by: Fernand Sieber <sieberf@amazon.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250401123622.584018-1-sieberf@amazon.com
Link: https://lore.kernel.org/r/20250911095113.203439-1-sieberf@amazon.com
Link: https://lore.kernel.org/r/20250916140228.452231-1-sieberf@amazon.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/sched/fair.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7f23b866c3d4..cf3a51f323e3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8614,7 +8614,19 @@ static void yield_task_fair(struct rq *rq)
 	 */
 	rq_clock_skip_update(rq);
 
-	se->deadline += calc_delta_fair(se->slice, se);
+	/*
+	 * Forfeit the remaining vruntime, only if the entity is eligible. This
+	 * condition is necessary because in core scheduling we prefer to run
+	 * ineligible tasks rather than force idling. If this happens we may
+	 * end up in a loop where the core scheduler picks the yielding task,
+	 * which yields immediately again; without the condition the vruntime
+	 * ends up quickly running away.
+	 */
+	if (entity_eligible(cfs_rq, se)) {
+		se->vruntime = se->deadline;
+		se->deadline += calc_delta_fair(se->slice, se);
+		update_min_vruntime(cfs_rq);
+	}
 }
 
 static bool yield_to_task_fair(struct rq *rq, struct task_struct *p)
-- 
2.51.0

[PATCH 6.6.y 2/3] sched/core: Add comment explaining force-idle vruntime snapshots

[Sasha Levin]

From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit 9359d9785d85bb53f1ff1738a59aeeec4b878906 ]

I always end up having to re-read these emails every time I look at
this code. And a future patch is going to change this story a little.
This means it is past time to stick them in a comment so it can be
modified and stay current.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200506143506.GH5298@hirez.programming.kicks-ass.net
Link: https://lkml.kernel.org/r/20200515103844.GG2978@hirez.programming.kicks-ass.net
Link: https://patch.msgid.link/20251106111603.GB4068168@noisy.programming.kicks-ass.net
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/sched/fair.c | 181 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 181 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index cf3a51f323e3..e020bcf8b007 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12566,6 +12566,187 @@ static inline void task_tick_core(struct rq *rq, struct task_struct *curr)
 		resched_curr(rq);
 }
 
+/*
+ * Consider any infeasible weight scenario. Take for instance two tasks,
+ * each bound to their respective sibling, one with weight 1 and one with
+ * weight 2. Then the lower weight task will run ahead of the higher weight
+ * task without bound.
+ *
+ * This utterly destroys the concept of a shared time base.
+ *
+ * Remember; all this is about a proportionally fair scheduling, where each
+ * tasks receives:
+ *
+ *              w_i
+ *   dt_i = ---------- dt                                     (1)
+ *          \Sum_j w_j
+ *
+ * which we do by tracking a virtual time, s_i:
+ *
+ *          1
+ *   s_i = --- d[t]_i                                         (2)
+ *         w_i
+ *
+ * Where d[t] is a delta of discrete time, while dt is an infinitesimal.
+ * The immediate corollary is that the ideal schedule S, where (2) to use
+ * an infinitesimal delta, is:
+ *
+ *           1
+ *   S = ---------- dt                                        (3)
+ *       \Sum_i w_i
+ *
+ * From which we can define the lag, or deviation from the ideal, as:
+ *
+ *   lag(i) = S - s_i                                         (4)
+ *
+ * And since the one and only purpose is to approximate S, we get that:
+ *
+ *   \Sum_i w_i lag(i) := 0                                   (5)
+ *
+ * If this were not so, we no longer converge to S, and we can no longer
+ * claim our scheduler has any of the properties we derive from S. This is
+ * exactly what you did above, you broke it!
+ *
+ *
+ * Let's continue for a while though; to see if there is anything useful to
+ * be learned. We can combine (1)-(3) or (4)-(5) and express S in s_i:
+ *
+ *       \Sum_i w_i s_i
+ *   S = --------------                                       (6)
+ *         \Sum_i w_i
+ *
+ * Which gives us a way to compute S, given our s_i. Now, if you've read
+ * our code, you know that we do not in fact do this, the reason for this
+ * is two-fold. Firstly, computing S in that way requires a 64bit division
+ * for every time we'd use it (see 12), and secondly, this only describes
+ * the steady-state, it doesn't handle dynamics.
+ *
+ * Anyway, in (6):  s_i -> x + (s_i - x), to get:
+ *
+ *           \Sum_i w_i (s_i - x)
+ *   S - x = --------------------                             (7)
+ *              \Sum_i w_i
+ *
+ * Which shows that S and s_i transform alike (which makes perfect sense
+ * given that S is basically the (weighted) average of s_i).
+ *
+ * Then:
+ *
+ *   x -> s_min := min{s_i}                                   (8)
+ *
+ * to obtain:
+ *
+ *               \Sum_i w_i (s_i - s_min)
+ *   S = s_min + ------------------------                     (9)
+ *                     \Sum_i w_i
+ *
+ * Which already looks familiar, and is the basis for our current
+ * approximation:
+ *
+ *   S ~= s_min                                              (10)
+ *
+ * Now, obviously, (10) is absolute crap :-), but it sorta works.
+ *
+ * So the thing to remember is that the above is strictly UP. It is
+ * possible to generalize to multiple runqueues -- however it gets really
+ * yuck when you have to add affinity support, as illustrated by our very
+ * first counter-example.
+ *
+ * Luckily I think we can avoid needing a full multi-queue variant for
+ * core-scheduling (or load-balancing). The crucial observation is that we
+ * only actually need this comparison in the presence of forced-idle; only
+ * then do we need to tell if the stalled rq has higher priority over the
+ * other.
+ *
+ * [XXX assumes SMT2; better consider the more general case, I suspect
+ * it'll work out because our comparison is always between 2 rqs and the
+ * answer is only interesting if one of them is forced-idle]
+ *
+ * And (under assumption of SMT2) when there is forced-idle, there is only
+ * a single queue, so everything works like normal.
+ *
+ * Let, for our runqueue 'k':
+ *
+ *   T_k = \Sum_i w_i s_i
+ *   W_k = \Sum_i w_i      ; for all i of k                  (11)
+ *
+ * Then we can write (6) like:
+ *
+ *         T_k
+ *   S_k = ---                                               (12)
+ *         W_k
+ *
+ * From which immediately follows that:
+ *
+ *           T_k + T_l
+ *   S_k+l = ---------                                       (13)
+ *           W_k + W_l
+ *
+ * On which we can define a combined lag:
+ *
+ *   lag_k+l(i) := S_k+l - s_i                               (14)
+ *
+ * And that gives us the tools to compare tasks across a combined runqueue.
+ *
+ *
+ * Combined this gives the following:
+ *
+ *  a) when a runqueue enters force-idle, sync it against it's sibling rq(s)
+ *     using (7); this only requires storing single 'time'-stamps.
+ *
+ *  b) when comparing tasks between 2 runqueues of which one is forced-idle,
+ *     compare the combined lag, per (14).
+ *
+ * Now, of course cgroups (I so hate them) make this more interesting in
+ * that a) seems to suggest we need to iterate all cgroup on a CPU at such
+ * boundaries, but I think we can avoid that. The force-idle is for the
+ * whole CPU, all it's rqs. So we can mark it in the root and lazily
+ * propagate downward on demand.
+ */
+
+/*
+ * So this sync is basically a relative reset of S to 0.
+ *
+ * So with 2 queues, when one goes idle, we drop them both to 0 and one
+ * then increases due to not being idle, and the idle one builds up lag to
+ * get re-elected. So far so simple, right?
+ *
+ * When there's 3, we can have the situation where 2 run and one is idle,
+ * we sync to 0 and let the idle one build up lag to get re-election. Now
+ * suppose another one also drops idle. At this point dropping all to 0
+ * again would destroy the built-up lag from the queue that was already
+ * idle, not good.
+ *
+ * So instead of syncing everything, we can:
+ *
+ *   less := !((s64)(s_a - s_b) <= 0)
+ *
+ *   (v_a - S_a) - (v_b - S_b) == v_a - v_b - S_a + S_b
+ *                             == v_a - (v_b - S_a + S_b)
+ *
+ * IOW, we can recast the (lag) comparison to a one-sided difference.
+ * So if then, instead of syncing the whole queue, sync the idle queue
+ * against the active queue with S_a + S_b at the point where we sync.
+ *
+ * (XXX consider the implication of living in a cyclic group: N / 2^n N)
+ *
+ * This gives us means of syncing single queues against the active queue,
+ * and for already idle queues to preserve their build-up lag.
+ *
+ * Of course, then we get the situation where there's 2 active and one
+ * going idle, who do we pick to sync against? Theory would have us sync
+ * against the combined S, but as we've already demonstrated, there is no
+ * such thing in infeasible weight scenarios.
+ *
+ * One thing I've considered; and this is where that core_active rudiment
+ * came from, is having active queues sync up between themselves after
+ * every tick. This limits the observed divergence due to the work
+ * conservancy.
+ *
+ * On top of that, we can improve upon things by moving away from our
+ * horrible (10) hack and moving to (9) and employing (13) here.
+ */
+
 /*
  * se_fi_update - Update the cfs_rq->min_vruntime_fi in a CFS hierarchy if needed.
  */
-- 
2.51.0

[PATCH 6.6.y 3/3] sched/eevdf: Fix min_vruntime vs avg_vruntime

[Sasha Levin]

From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit 79f3f9bedd149ea438aaeb0fb6a083637affe205 ]

Basically, from the constraint that the sum of lag is zero, you can
infer that the 0-lag point is the weighted average of the individual
vruntime, which is what we're trying to compute:

        \Sum w_i * v_i
  avg = --------------
           \Sum w_i

Now, since vruntime takes the whole u64 (worse, it wraps), this
multiplication term in the numerator is not something we can compute;
instead we do the min_vruntime (v0 henceforth) thing like:

  v_i = (v_i - v0) + v0

This does two things:
 - it keeps the key: (v_i - v0) 'small';
 - it creates a relative 0-point in the modular space.

If you do that subtitution and work it all out, you end up with:

        \Sum w_i * (v_i - v0)
  avg = --------------------- + v0
              \Sum w_i

Since you cannot very well track a ratio like that (and not suffer
terrible numerical problems) we simpy track the numerator and
denominator individually and only perform the division when strictly
needed.

Notably, the numerator lives in cfs_rq->avg_vruntime and the denominator
lives in cfs_rq->avg_load.

The one extra 'funny' is that these numbers track the entities in the
tree, and current is typically outside of the tree, so avg_vruntime()
adds current when needed before doing the division.

(vruntime_eligible() elides the division by cross-wise multiplication)

Anyway, as mentioned above, we currently use the CFS era min_vruntime
for this purpose. However, this thing can only move forward, while the
above avg can in fact move backward (when a non-eligible task leaves,
the average becomes smaller), this can cause trouble when through
happenstance (or construction) these values drift far enough apart to
wreck the game.

Replace cfs_rq::min_vruntime with cfs_rq::zero_vruntime which is kept
near/at avg_vruntime, following its motion.

The down-side is that this requires computing the avg more often.

Fixes: 147f3efaa241 ("sched/fair: Implement an EEVDF-like scheduling policy")
Reported-by: Zicheng Qu <quzicheng@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251106111741.GC4068168@noisy.programming.kicks-ass.net
Cc: stable@vger.kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/sched/debug.c |   8 +--
 kernel/sched/fair.c  | 114 +++++++++----------------------------------
 kernel/sched/sched.h |   6 +--
 3 files changed, 31 insertions(+), 97 deletions(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 115e266db76b..e44bf608cee7 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -628,7 +628,7 @@ static void print_rq(struct seq_file *m, struct rq *rq, int rq_cpu)
 
 void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
 {
-	s64 left_vruntime = -1, min_vruntime, right_vruntime = -1, spread;
+	s64 left_vruntime = -1, zero_vruntime, right_vruntime = -1, spread;
 	struct sched_entity *last, *first;
 	struct rq *rq = cpu_rq(cpu);
 	unsigned long flags;
@@ -650,13 +650,13 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
 	last = __pick_last_entity(cfs_rq);
 	if (last)
 		right_vruntime = last->vruntime;
-	min_vruntime = cfs_rq->min_vruntime;
+	zero_vruntime = cfs_rq->zero_vruntime;
 	raw_spin_rq_unlock_irqrestore(rq, flags);
 
 	SEQ_printf(m, "  .%-30s: %Ld.%06ld\n", "left_vruntime",
 			SPLIT_NS(left_vruntime));
-	SEQ_printf(m, "  .%-30s: %Ld.%06ld\n", "min_vruntime",
-			SPLIT_NS(min_vruntime));
+	SEQ_printf(m, "  .%-30s: %Ld.%06ld\n", "zero_vruntime",
+			SPLIT_NS(zero_vruntime));
 	SEQ_printf(m, "  .%-30s: %Ld.%06ld\n", "avg_vruntime",
 			SPLIT_NS(avg_vruntime(cfs_rq)));
 	SEQ_printf(m, "  .%-30s: %Ld.%06ld\n", "right_vruntime",
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e020bcf8b007..46de719b669d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -571,7 +571,7 @@ static inline bool entity_before(const struct sched_entity *a,
 
 static inline s64 entity_key(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
-	return (s64)(se->vruntime - cfs_rq->min_vruntime);
+	return (s64)(se->vruntime - cfs_rq->zero_vruntime);
 }
 
 #define __node_2_se(node) \
@@ -623,13 +623,13 @@ static inline s64 entity_key(struct cfs_rq *cfs_rq, struct sched_entity *se)
  *
  * Which we track using:
  *
- *                    v0 := cfs_rq->min_vruntime
+ *                    v0 := cfs_rq->zero_vruntime
  * \Sum (v_i - v0) * w_i := cfs_rq->avg_vruntime
  *              \Sum w_i := cfs_rq->avg_load
  *
- * Since min_vruntime is a monotonic increasing variable that closely tracks
- * the per-task service, these deltas: (v_i - v), will be in the order of the
- * maximal (virtual) lag induced in the system due to quantisation.
+ * Since zero_vruntime closely tracks the per-task service, these
+ * deltas: (v_i - v), will be in the order of the maximal (virtual) lag
+ * induced in the system due to quantisation.
  *
  * Also, we use scale_load_down() to reduce the size.
  *
@@ -688,7 +688,7 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq)
 		avg = div_s64(avg, load);
 	}
 
-	return cfs_rq->min_vruntime + avg;
+	return cfs_rq->zero_vruntime + avg;
 }
 
 /*
@@ -757,44 +757,14 @@ int entity_eligible(struct cfs_rq *cfs_rq, struct sched_entity *se)
 	return avg >= entity_key(cfs_rq, se) * load;
 }
 
-static u64 __update_min_vruntime(struct cfs_rq *cfs_rq, u64 vruntime)
+static void update_zero_vruntime(struct cfs_rq *cfs_rq)
 {
-	u64 min_vruntime = cfs_rq->min_vruntime;
-	/*
-	 * open coded max_vruntime() to allow updating avg_vruntime
-	 */
-	s64 delta = (s64)(vruntime - min_vruntime);
-	if (delta > 0) {
-		avg_vruntime_update(cfs_rq, delta);
-		min_vruntime = vruntime;
-	}
-	return min_vruntime;
-}
+	u64 vruntime = avg_vruntime(cfs_rq);
+	s64 delta = (s64)(vruntime - cfs_rq->zero_vruntime);
 
-static void update_min_vruntime(struct cfs_rq *cfs_rq)
-{
-	struct sched_entity *se = __pick_first_entity(cfs_rq);
-	struct sched_entity *curr = cfs_rq->curr;
-
-	u64 vruntime = cfs_rq->min_vruntime;
-
-	if (curr) {
-		if (curr->on_rq)
-			vruntime = curr->vruntime;
-		else
-			curr = NULL;
-	}
-
-	if (se) {
-		if (!curr)
-			vruntime = se->vruntime;
-		else
-			vruntime = min_vruntime(vruntime, se->vruntime);
-	}
+	avg_vruntime_update(cfs_rq, delta);
 
-	/* ensure we never gain time by being placed backwards. */
-	u64_u32_store(cfs_rq->min_vruntime,
-		      __update_min_vruntime(cfs_rq, vruntime));
+	cfs_rq->zero_vruntime = vruntime;
 }
 
 static inline bool __entity_less(struct rb_node *a, const struct rb_node *b)
@@ -837,6 +807,7 @@ RB_DECLARE_CALLBACKS(static, min_deadline_cb, struct sched_entity,
 static void __enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
 	avg_vruntime_add(cfs_rq, se);
+	update_zero_vruntime(cfs_rq);
 	se->min_deadline = se->deadline;
 	rb_add_augmented_cached(&se->run_node, &cfs_rq->tasks_timeline,
 				__entity_less, &min_deadline_cb);
@@ -847,6 +818,7 @@ static void __dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 	rb_erase_augmented_cached(&se->run_node, &cfs_rq->tasks_timeline,
 				  &min_deadline_cb);
 	avg_vruntime_sub(cfs_rq, se);
+	update_zero_vruntime(cfs_rq);
 }
 
 struct sched_entity *__pick_first_entity(struct cfs_rq *cfs_rq)
@@ -1212,7 +1184,6 @@ static void update_curr(struct cfs_rq *cfs_rq)
 
 	curr->vruntime += calc_delta_fair(delta_exec, curr);
 	update_deadline(cfs_rq, curr);
-	update_min_vruntime(cfs_rq);
 
 	if (entity_is_task(curr))
 		update_curr_task(task_of(curr), delta_exec);
@@ -3889,15 +3860,6 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
 		update_load_add(&cfs_rq->load, se->load.weight);
 		if (!curr)
 			__enqueue_entity(cfs_rq, se);
-
-		/*
-		 * The entity's vruntime has been adjusted, so let's check
-		 * whether the rq-wide min_vruntime needs updated too. Since
-		 * the calculations above require stable min_vruntime rather
-		 * than up-to-date one, we do the update at the end of the
-		 * reweight process.
-		 */
-		update_min_vruntime(cfs_rq);
 	}
 }
 
@@ -5422,15 +5384,6 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 
 	update_cfs_group(se);
 
-	/*
-	 * Now advance min_vruntime if @se was the entity holding it back,
-	 * except when: DEQUEUE_SAVE && !DEQUEUE_MOVE, in this case we'll be
-	 * put back on, and if we advance min_vruntime, we'll be placed back
-	 * further than we started -- ie. we'll be penalized.
-	 */
-	if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) != DEQUEUE_SAVE)
-		update_min_vruntime(cfs_rq);
-
 	if (cfs_rq->nr_running == 0)
 		update_idle_cfs_rq_clock_pelt(cfs_rq);
 }
@@ -8625,7 +8578,6 @@ static void yield_task_fair(struct rq *rq)
 	if (entity_eligible(cfs_rq, se)) {
 		se->vruntime = se->deadline;
 		se->deadline += calc_delta_fair(se->slice, se);
-		update_min_vruntime(cfs_rq);
 	}
 }
 
@@ -12630,23 +12582,6 @@ static inline void task_tick_core(struct rq *rq, struct task_struct *curr)
  * Which shows that S and s_i transform alike (which makes perfect sense
  * given that S is basically the (weighted) average of s_i).
  *
- * Then:
- *
- *   x -> s_min := min{s_i}                                   (8)
- *
- * to obtain:
- *
- *               \Sum_i w_i (s_i - s_min)
- *   S = s_min + ------------------------                     (9)
- *                     \Sum_i w_i
- *
- * Which already looks familiar, and is the basis for our current
- * approximation:
- *
- *   S ~= s_min                                              (10)
- *
- * Now, obviously, (10) is absolute crap :-), but it sorta works.
- *
  * So the thing to remember is that the above is strictly UP. It is
  * possible to generalize to multiple runqueues -- however it gets really
  * yuck when you have to add affinity support, as illustrated by our very
@@ -12668,23 +12603,23 @@ static inline void task_tick_core(struct rq *rq, struct task_struct *curr)
  * Let, for our runqueue 'k':
  *
  *   T_k = \Sum_i w_i s_i
- *   W_k = \Sum_i w_i      ; for all i of k                  (11)
+ *   W_k = \Sum_i w_i      ; for all i of k                  (8)
  *
  * Then we can write (6) like:
  *
  *         T_k
- *   S_k = ---                                               (12)
+ *   S_k = ---                                               (9)
  *         W_k
  *
  * From which immediately follows that:
  *
  *           T_k + T_l
- *   S_k+l = ---------                                       (13)
+ *   S_k+l = ---------                                       (10)
  *           W_k + W_l
  *
  * On which we can define a combined lag:
  *
- *   lag_k+l(i) := S_k+l - s_i                               (14)
+ *   lag_k+l(i) := S_k+l - s_i                               (11)
  *
  * And that gives us the tools to compare tasks across a combined runqueue.
  *
@@ -12695,7 +12630,7 @@ static inline void task_tick_core(struct rq *rq, struct task_struct *curr)
  *     using (7); this only requires storing single 'time'-stamps.
  *
  *  b) when comparing tasks between 2 runqueues of which one is forced-idle,
- *     compare the combined lag, per (14).
+ *     compare the combined lag, per (11).
  *
  * Now, of course cgroups (I so hate them) make this more interesting in
  * that a) seems to suggest we need to iterate all cgroup on a CPU at such
@@ -12743,12 +12678,11 @@ static inline void task_tick_core(struct rq *rq, struct task_struct *curr)
  * every tick. This limits the observed divergence due to the work
  * conservancy.
  *
- * On top of that, we can improve upon things by moving away from our
- * horrible (10) hack and moving to (9) and employing (13) here.
+ * On top of that, we can improve upon things by employing (10) here.
  */
 
 /*
- * se_fi_update - Update the cfs_rq->min_vruntime_fi in a CFS hierarchy if needed.
+ * se_fi_update - Update the cfs_rq->zero_vruntime_fi in a CFS hierarchy if needed.
  */
 static void se_fi_update(const struct sched_entity *se, unsigned int fi_seq,
 			 bool forceidle)
@@ -12762,7 +12696,7 @@ static void se_fi_update(const struct sched_entity *se, unsigned int fi_seq,
 			cfs_rq->forceidle_seq = fi_seq;
 		}
 
-		cfs_rq->min_vruntime_fi = cfs_rq->min_vruntime;
+		cfs_rq->zero_vruntime_fi = cfs_rq->zero_vruntime;
 	}
 }
 
@@ -12815,11 +12749,11 @@ bool cfs_prio_less(const struct task_struct *a, const struct task_struct *b,
 
 	/*
 	 * Find delta after normalizing se's vruntime with its cfs_rq's
-	 * min_vruntime_fi, which would have been updated in prior calls
+	 * zero_vruntime_fi, which would have been updated in prior calls
 	 * to se_fi_update().
 	 */
 	delta = (s64)(sea->vruntime - seb->vruntime) +
-		(s64)(cfs_rqb->min_vruntime_fi - cfs_rqa->min_vruntime_fi);
+		(s64)(cfs_rqb->zero_vruntime_fi - cfs_rqa->zero_vruntime_fi);
 
 	return delta > 0;
 }
@@ -13048,7 +12982,7 @@ static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first)
 void init_cfs_rq(struct cfs_rq *cfs_rq)
 {
 	cfs_rq->tasks_timeline = RB_ROOT_CACHED;
-	u64_u32_store(cfs_rq->min_vruntime, (u64)(-(1LL << 20)));
+	u64_u32_store(cfs_rq->zero_vruntime, (u64)(-(1LL << 20)));
 #ifdef CONFIG_SMP
 	raw_spin_lock_init(&cfs_rq->removed.lock);
 #endif
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 64634314a89c..a1c6ccf7249d 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -554,14 +554,14 @@ struct cfs_rq {
 	u64			avg_load;
 
 	u64			exec_clock;
-	u64			min_vruntime;
+	u64			zero_vruntime;
 #ifdef CONFIG_SCHED_CORE
 	unsigned int		forceidle_seq;
-	u64			min_vruntime_fi;
+	u64			zero_vruntime_fi;
 #endif
 
 #ifndef CONFIG_64BIT
-	u64			min_vruntime_copy;
+	u64			zero_vruntime_copy;
 #endif
 
 	struct rb_root_cached	tasks_timeline;
-- 
2.51.0