[PATCH 6.12 488/567] sched/eevdf: Fix min_vruntime vs avg_vruntime

Greg Kroah-Hartman
2026-01-06T17:41:19Z

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
[ Adjust context in comments + init_cfs_rq ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/sched/debug.c | 8 ++--
kernel/sched/fair.c | 84 +++++++++++----------------------------------------
kernel/sched/sched.h | 4 +-
3 files changed, 25 insertions(+), 71 deletions(-)

--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -804,7 +804,7 @@ static void print_rq(struct seq_file *m,

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;
@@ -827,15 +827,15 @@ void print_cfs_rq(struct seq_file *m, in
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",
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -553,7 +553,7 @@ static inline bool entity_before(const s

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) \
@@ -605,13 +605,13 @@ static inline s64 entity_key(struct cfs_
*
* 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.
*
@@ -670,7 +670,7 @@ 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;
}

/*
@@ -736,7 +736,7 @@ static int vruntime_eligible(struct cfs_
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)
@@ -744,42 +744,14 @@ int entity_eligible(struct cfs_rq *cfs_r
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;
-}
-
-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;
+ u64 vruntime = avg_vruntime(cfs_rq);
+ s64 delta = (s64)(vruntime - cfs_rq->zero_vruntime);

- if (curr) {
- if (curr->on_rq)
- vruntime = curr->vruntime;
- else
- curr = NULL;
- }
+ avg_vruntime_update(cfs_rq, delta);

- 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)
@@ -852,6 +824,7 @@ RB_DECLARE_CALLBACKS(static, min_vruntim
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,
@@ -863,6 +836,7 @@ static void __dequeue_entity(struct cfs_
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)
@@ -1243,7 +1217,6 @@ static void update_curr(struct cfs_rq *c

curr->vruntime += calc_delta_fair(delta_exec, curr);
resched = update_deadline(cfs_rq, curr);
- update_min_vruntime(cfs_rq);

if (entity_is_task(curr)) {
struct task_struct *p = task_of(curr);
@@ -3937,15 +3910,6 @@ static void reweight_entity(struct cfs_r
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);
}
}

@@ -5614,15 +5578,6 @@ dequeue_entity(struct cfs_rq *cfs_rq, st

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);

@@ -9165,7 +9120,6 @@ static void yield_task_fair(struct rq *r
if (entity_eligible(cfs_rq, se)) {
se->vruntime = se->deadline;
se->deadline += calc_delta_fair(se->slice, se);
- update_min_vruntime(cfs_rq);
}
}

@@ -13093,7 +13047,7 @@ static inline void task_tick_core(struct
}

/*
- * 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)
@@ -13107,7 +13061,7 @@ static void se_fi_update(const struct sc
cfs_rq->forceidle_seq = fi_seq;
}

- cfs_rq->min_vruntime_fi = cfs_rq->min_vruntime;
+ cfs_rq->zero_vruntime_fi = cfs_rq->zero_vruntime;
}
}

@@ -13160,11 +13114,11 @@ bool cfs_prio_less(const struct task_str

/*
* 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;
}
@@ -13402,7 +13356,7 @@ static void set_next_task_fair(struct rq
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));
#ifdef CONFIG_SMP
raw_spin_lock_init(&cfs_rq->removed.lock);
#endif
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -660,10 +660,10 @@ 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;