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Message-ID: <169165145788.27769.5271183004663340258.tip-bot2@tip-bot2>
Date: Thu, 10 Aug 2023 07:10:57 -0000
From: "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject: [tip: sched/core] sched/fair: Add lag based placement
The following commit has been merged into the sched/core branch of tip:
Commit-ID: 86bfbb7ce4f67a88df2639198169b685668e7349
Gitweb: https://git.kernel.org/tip/86bfbb7ce4f67a88df2639198169b685668e7349
Author: Peter Zijlstra <peterz@...radead.org>
AuthorDate: Wed, 31 May 2023 13:58:42 +02:00
Committer: Ingo Molnar <mingo@...nel.org>
CommitterDate: Wed, 19 Jul 2023 09:43:58 +02:00
sched/fair: Add lag based placement
With the introduction of avg_vruntime, it is possible to approximate
lag (the entire purpose of introducing it in fact). Use this to do lag
based placement over sleep+wake.
Specifically, the FAIR_SLEEPERS thing places things too far to the
left and messes up the deadline aspect of EEVDF.
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Link: https://lore.kernel.org/r/20230531124603.794929315@infradead.org
---
include/linux/sched.h | 3 +-
kernel/sched/core.c | 1 +-
kernel/sched/fair.c | 168 ++++++++++++++++++++++++++++++---------
kernel/sched/features.h | 8 ++-
4 files changed, 141 insertions(+), 39 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2aab7be..ba1828b 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -554,8 +554,9 @@ struct sched_entity {
u64 exec_start;
u64 sum_exec_runtime;
- u64 vruntime;
u64 prev_sum_exec_runtime;
+ u64 vruntime;
+ s64 vlag;
u64 nr_migrations;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 83e3654..84b0d47 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4501,6 +4501,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
p->se.prev_sum_exec_runtime = 0;
p->se.nr_migrations = 0;
p->se.vruntime = 0;
+ p->se.vlag = 0;
INIT_LIST_HEAD(&p->se.group_node);
#ifdef CONFIG_FAIR_GROUP_SCHED
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fc43482..dd12ada 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -715,6 +715,15 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq)
return cfs_rq->min_vruntime + avg;
}
+/*
+ * lag_i = S - s_i = w_i * (V - v_i)
+ */
+void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
+{
+ SCHED_WARN_ON(!se->on_rq);
+ se->vlag = avg_vruntime(cfs_rq) - se->vruntime;
+}
+
static u64 __update_min_vruntime(struct cfs_rq *cfs_rq, u64 vruntime)
{
u64 min_vruntime = cfs_rq->min_vruntime;
@@ -3492,6 +3501,8 @@ dequeue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) { }
static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
unsigned long weight)
{
+ unsigned long old_weight = se->load.weight;
+
if (se->on_rq) {
/* commit outstanding execution time */
if (cfs_rq->curr == se)
@@ -3504,6 +3515,14 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
update_load_set(&se->load, weight);
+ if (!se->on_rq) {
+ /*
+ * Because we keep se->vlag = V - v_i, while: lag_i = w_i*(V - v_i),
+ * we need to scale se->vlag when w_i changes.
+ */
+ se->vlag = div_s64(se->vlag * old_weight, weight);
+ }
+
#ifdef CONFIG_SMP
do {
u32 divider = get_pelt_divider(&se->avg);
@@ -4853,49 +4872,119 @@ static void
place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
{
u64 vruntime = avg_vruntime(cfs_rq);
+ s64 lag = 0;
- /* sleeps up to a single latency don't count. */
- if (!initial) {
- unsigned long thresh;
+ /*
+ * Due to how V is constructed as the weighted average of entities,
+ * adding tasks with positive lag, or removing tasks with negative lag
+ * will move 'time' backwards, this can screw around with the lag of
+ * other tasks.
+ *
+ * EEVDF: placement strategy #1 / #2
+ */
+ if (sched_feat(PLACE_LAG) && cfs_rq->nr_running > 1) {
+ struct sched_entity *curr = cfs_rq->curr;
+ unsigned long load;
- if (se_is_idle(se))
- thresh = sysctl_sched_min_granularity;
- else
- thresh = sysctl_sched_latency;
+ lag = se->vlag;
/*
- * Halve their sleep time's effect, to allow
- * for a gentler effect of sleepers:
+ * If we want to place a task and preserve lag, we have to
+ * consider the effect of the new entity on the weighted
+ * average and compensate for this, otherwise lag can quickly
+ * evaporate.
+ *
+ * Lag is defined as:
+ *
+ * lag_i = S - s_i = w_i * (V - v_i)
+ *
+ * To avoid the 'w_i' term all over the place, we only track
+ * the virtual lag:
+ *
+ * vl_i = V - v_i <=> v_i = V - vl_i
+ *
+ * And we take V to be the weighted average of all v:
+ *
+ * V = (\Sum w_j*v_j) / W
+ *
+ * Where W is: \Sum w_j
+ *
+ * Then, the weighted average after adding an entity with lag
+ * vl_i is given by:
+ *
+ * V' = (\Sum w_j*v_j + w_i*v_i) / (W + w_i)
+ * = (W*V + w_i*(V - vl_i)) / (W + w_i)
+ * = (W*V + w_i*V - w_i*vl_i) / (W + w_i)
+ * = (V*(W + w_i) - w_i*l) / (W + w_i)
+ * = V - w_i*vl_i / (W + w_i)
+ *
+ * And the actual lag after adding an entity with vl_i is:
+ *
+ * vl'_i = V' - v_i
+ * = V - w_i*vl_i / (W + w_i) - (V - vl_i)
+ * = vl_i - w_i*vl_i / (W + w_i)
+ *
+ * Which is strictly less than vl_i. So in order to preserve lag
+ * we should inflate the lag before placement such that the
+ * effective lag after placement comes out right.
+ *
+ * As such, invert the above relation for vl'_i to get the vl_i
+ * we need to use such that the lag after placement is the lag
+ * we computed before dequeue.
+ *
+ * vl'_i = vl_i - w_i*vl_i / (W + w_i)
+ * = ((W + w_i)*vl_i - w_i*vl_i) / (W + w_i)
+ *
+ * (W + w_i)*vl'_i = (W + w_i)*vl_i - w_i*vl_i
+ * = W*vl_i
+ *
+ * vl_i = (W + w_i)*vl'_i / W
*/
- if (sched_feat(GENTLE_FAIR_SLEEPERS))
- thresh >>= 1;
-
- vruntime -= thresh;
- }
-
- /*
- * Pull vruntime of the entity being placed to the base level of
- * cfs_rq, to prevent boosting it if placed backwards.
- * However, min_vruntime can advance much faster than real time, with
- * the extreme being when an entity with the minimal weight always runs
- * on the cfs_rq. If the waking entity slept for a long time, its
- * vruntime difference from min_vruntime may overflow s64 and their
- * comparison may get inversed, so ignore the entity's original
- * vruntime in that case.
- * The maximal vruntime speedup is given by the ratio of normal to
- * minimal weight: scale_load_down(NICE_0_LOAD) / MIN_SHARES.
- * When placing a migrated waking entity, its exec_start has been set
- * from a different rq. In order to take into account a possible
- * divergence between new and prev rq's clocks task because of irq and
- * stolen time, we take an additional margin.
- * So, cutting off on the sleep time of
- * 2^63 / scale_load_down(NICE_0_LOAD) ~ 104 days
- * should be safe.
- */
- if (entity_is_long_sleeper(se))
- se->vruntime = vruntime;
- else
- se->vruntime = max_vruntime(se->vruntime, vruntime);
+ load = cfs_rq->avg_load;
+ if (curr && curr->on_rq)
+ load += curr->load.weight;
+
+ lag *= load + se->load.weight;
+ if (WARN_ON_ONCE(!load))
+ load = 1;
+ lag = div_s64(lag, load);
+
+ vruntime -= lag;
+ }
+
+ if (sched_feat(FAIR_SLEEPERS)) {
+
+ /* sleeps up to a single latency don't count. */
+ if (!initial) {
+ unsigned long thresh;
+
+ if (se_is_idle(se))
+ thresh = sysctl_sched_min_granularity;
+ else
+ thresh = sysctl_sched_latency;
+
+ /*
+ * Halve their sleep time's effect, to allow
+ * for a gentler effect of sleepers:
+ */
+ if (sched_feat(GENTLE_FAIR_SLEEPERS))
+ thresh >>= 1;
+
+ vruntime -= thresh;
+ }
+
+ /*
+ * Pull vruntime of the entity being placed to the base level of
+ * cfs_rq, to prevent boosting it if placed backwards. If the entity
+ * slept for a long time, don't even try to compare its vruntime with
+ * the base as it may be too far off and the comparison may get
+ * inversed due to s64 overflow.
+ */
+ if (!entity_is_long_sleeper(se))
+ vruntime = max_vruntime(se->vruntime, vruntime);
+ }
+
+ se->vruntime = vruntime;
}
static void check_enqueue_throttle(struct cfs_rq *cfs_rq);
@@ -5077,6 +5166,9 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
clear_buddies(cfs_rq, se);
+ if (flags & DEQUEUE_SLEEP)
+ update_entity_lag(cfs_rq, se);
+
if (se != cfs_rq->curr)
__dequeue_entity(cfs_rq, se);
se->on_rq = 0;
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index fa828b3..7958a10 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -1,12 +1,20 @@
/* SPDX-License-Identifier: GPL-2.0 */
+
/*
* Only give sleepers 50% of their service deficit. This allows
* them to run sooner, but does not allow tons of sleepers to
* rip the spread apart.
*/
+SCHED_FEAT(FAIR_SLEEPERS, false)
SCHED_FEAT(GENTLE_FAIR_SLEEPERS, true)
/*
+ * Using the avg_vruntime, do the right thing and preserve lag across
+ * sleep+wake cycles. EEVDF placement strategy #1, #2 if disabled.
+ */
+SCHED_FEAT(PLACE_LAG, true)
+
+/*
* Prefer to schedule the task we woke last (assuming it failed
* wakeup-preemption), since its likely going to consume data we
* touched, increases cache locality.
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