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sched/fair: Rename cfs_rq::avg_vruntime to ::sum_w_vruntime, and helper functions

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The ::avg_vruntime field is a  misnomer: it says it's an
'average vruntime', but in reality it's the momentary sum
of the weighted vruntimes of all queued tasks, which is
at least a division away from being an average.

This is clear from comments about the math of fair scheduling:

    * \Sum (v_i - v0) * w_i := cfs_rq->avg_vruntime

This confusion is increased by the cfs_avg_vruntime() function,
which does perform the division and returns a true average.

The sum of all weighted vruntimes should be named thusly,
so rename the field to ::sum_w_vruntime. (As arguably
::sum_weighted_vruntime would be a bit of a mouthful.)

Understanding the scheduler is hard enough already, without
extra layers of obfuscated naming. ;-)

Also rename related helper functions:

  sum_vruntime_add()    => sum_w_vruntime_add()
  sum_vruntime_sub()    => sum_w_vruntime_sub()
  sum_vruntime_update() => sum_w_vruntime_update()

With the notable exception of cfs_avg_vruntime(), which
was named accurately.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://patch.msgid.link/20251201064647.1851919-7-mingo@kernel.org
This commit is contained in:
Ingo Molnar 2025-12-02 16:09:23 +01:00
parent 4ff674fa98
commit dcbc9d3f0e
2 changed files with 14 additions and 14 deletions
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26
kernel/sched/fair.c
View File

@ -607,7 +607,7 @@ static inline s64 entity_key(struct cfs_rq *cfs_rq, struct sched_entity *se)
* Which we track using:
*
* v0 := cfs_rq->zero_vruntime
* \Sum (v_i - v0) * w_i := cfs_rq->avg_vruntime
* \Sum (v_i - v0) * w_i := cfs_rq->sum_w_vruntime
* \Sum w_i := cfs_rq->sum_weight
*
* Since zero_vruntime closely tracks the per-task service, these
@ -619,32 +619,32 @@ static inline s64 entity_key(struct cfs_rq *cfs_rq, struct sched_entity *se)
* As measured, the max (key * weight) value was ~44 bits for a kernel build.
*/
static void
avg_vruntime_add(struct cfs_rq *cfs_rq, struct sched_entity *se)
sum_w_vruntime_add(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
unsigned long weight = scale_load_down(se->load.weight);
s64 key = entity_key(cfs_rq, se);
cfs_rq->avg_vruntime += key * weight;
cfs_rq->sum_w_vruntime += key * weight;
cfs_rq->sum_weight += weight;
}
static void
avg_vruntime_sub(struct cfs_rq *cfs_rq, struct sched_entity *se)
sum_w_vruntime_sub(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
unsigned long weight = scale_load_down(se->load.weight);
s64 key = entity_key(cfs_rq, se);
cfs_rq->avg_vruntime -= key * weight;
cfs_rq->sum_w_vruntime -= key * weight;
cfs_rq->sum_weight -= weight;
}
static inline
void avg_vruntime_update(struct cfs_rq *cfs_rq, s64 delta)
void sum_w_vruntime_update(struct cfs_rq *cfs_rq, s64 delta)
{
/*
* v' = v + d ==> avg_vruntime' = avg_runtime - d*sum_weight
* v' = v + d ==> sum_w_vruntime' = sum_runtime - d*sum_weight
*/
cfs_rq->avg_vruntime -= cfs_rq->sum_weight * delta;
cfs_rq->sum_w_vruntime -= cfs_rq->sum_weight * delta;
}
/*
@ -654,7 +654,7 @@ void avg_vruntime_update(struct cfs_rq *cfs_rq, s64 delta)
u64 avg_vruntime(struct cfs_rq *cfs_rq)
{
struct sched_entity *curr = cfs_rq->curr;
s64 avg = cfs_rq->avg_vruntime;
s64 avg = cfs_rq->sum_w_vruntime;
long load = cfs_rq->sum_weight;
if (curr && curr->on_rq) {
@ -722,7 +722,7 @@ static void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
static int vruntime_eligible(struct cfs_rq *cfs_rq, u64 vruntime)
{
struct sched_entity *curr = cfs_rq->curr;
s64 avg = cfs_rq->avg_vruntime;
s64 avg = cfs_rq->sum_w_vruntime;
long load = cfs_rq->sum_weight;
if (curr && curr->on_rq) {
@ -745,7 +745,7 @@ static void update_zero_vruntime(struct cfs_rq *cfs_rq)
u64 vruntime = avg_vruntime(cfs_rq);
s64 delta = (s64)(vruntime - cfs_rq->zero_vruntime);
avg_vruntime_update(cfs_rq, delta);
sum_w_vruntime_update(cfs_rq, delta);
cfs_rq->zero_vruntime = vruntime;
}
@ -819,7 +819,7 @@ 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);
sum_w_vruntime_add(cfs_rq, se);
update_zero_vruntime(cfs_rq);
se->min_vruntime = se->vruntime;
se->min_slice = se->slice;
@ -831,7 +831,7 @@ 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);
sum_w_vruntime_sub(cfs_rq, se);
update_zero_vruntime(cfs_rq);
}

2
kernel/sched/sched.h
View File

@ -678,7 +678,7 @@ struct cfs_rq {
unsigned int h_nr_runnable; /* SCHED_{NORMAL,BATCH,IDLE} */
unsigned int h_nr_idle; /* SCHED_IDLE */
s64 avg_vruntime;
s64 sum_w_vruntime;
u64 sum_weight;
u64 zero_vruntime;