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Date: Wed, 15 Nov 2023 19:33:41 +0800
From: Cruz Zhao <CruzZhao@...ux.alibaba.com>
To: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
vincent.guittot@...aro.org, dietmar.eggemann@....com,
rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
bristot@...hat.com, vschneid@...hat.com, joel@...lfernandes.org
Cc: linux-kernel@...r.kernel.org
Subject: [PATCH 4/4] sched/core: fix cfs_prio_less
The update of vruntime snapshot will cause unfair sched, especially when
tasks enqueue/dequeue frequently.
Consider the following case:
- Task A1 and A2 share a cookie, and task B has another cookie.
- A1 is a short task, waking up frequently but running short everytime.
- A2 and B are long tasks.
- A1 and B runs on ht0 and A2 runs on ht1.
ht0 ht1 fi_before fi update
switch to A1 switch to A2 0 0 1
A1 sleeps
switch to B A2 force idle 0 1 1
A1 wakes up
switch to A1 switch to A1 1 0 1
A1 sleeps
switch to B A2 force idle 0 1 1
In this case, cfs_rq->min_vruntime_fi will update every schedule, and
prio of B and A2 will be pulled to the same level, no matter how long A2
and B have run before, which is not fair enough. Extramely, we observed
that the latency of a task became several minutes due to this reason,
which should be 100ms.
To fix this problem, we compare the priority of ses using core_vruntime
directly, instead of vruntime snapshot.
Fixes: c6047c2e3af6 ("sched/fair: Snapshot the min_vruntime of CPUs on force idle")
Signed-off-by: Cruz Zhao <CruzZhao@...ux.alibaba.com>
---
kernel/sched/core.c | 17 -----------------
kernel/sched/fair.c | 35 +----------------------------------
kernel/sched/sched.h | 2 --
3 files changed, 1 insertion(+), 53 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 647a12af9172..22edf4bcc7e8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6052,8 +6052,6 @@ static inline struct task_struct *pick_task(struct rq *rq)
BUG(); /* The idle class should always have a runnable task. */
}
-extern void task_vruntime_update(struct rq *rq, struct task_struct *p, bool in_fi);
-
static void queue_core_balance(struct rq *rq);
static struct task_struct *
@@ -6154,7 +6152,6 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
* unconstrained picks as well.
*/
WARN_ON_ONCE(fi_before);
- task_vruntime_update(rq, next, false);
goto out_set_next;
}
}
@@ -6204,8 +6201,6 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
if (p == rq_i->idle) {
if (rq_i->nr_running) {
rq->core->core_forceidle_count++;
- if (!fi_before)
- rq->core->core_forceidle_seq++;
}
} else {
occ++;
@@ -6245,17 +6240,6 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
if (!rq_i->core_pick)
continue;
- /*
- * Update for new !FI->FI transitions, or if continuing to be in !FI:
- * fi_before fi update?
- * 0 0 1
- * 0 1 1
- * 1 0 1
- * 1 1 0
- */
- if (!(fi_before && rq->core->core_forceidle_count))
- task_vruntime_update(rq_i, rq_i->core_pick, !!rq->core->core_forceidle_count);
-
rq_i->core_pick->core_occupation = occ;
if (i == cpu) {
@@ -6474,7 +6458,6 @@ static void sched_core_cpu_deactivate(unsigned int cpu)
core_rq->core_pick_seq = rq->core_pick_seq;
core_rq->core_cookie = rq->core_cookie;
core_rq->core_forceidle_count = rq->core_forceidle_count;
- core_rq->core_forceidle_seq = rq->core_forceidle_seq;
core_rq->core_forceidle_occupation = rq->core_forceidle_occupation;
/*
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 60b2fd437474..15c350b7c34a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12382,35 +12382,6 @@ static inline void task_tick_core(struct rq *rq, struct task_struct *curr)
resched_curr(rq);
}
-/*
- * se_fi_update - Update the cfs_rq->min_vruntime_fi in a CFS hierarchy if needed.
- */
-static void se_fi_update(const struct sched_entity *se, unsigned int fi_seq,
- bool forceidle)
-{
- for_each_sched_entity(se) {
- struct cfs_rq *cfs_rq = cfs_rq_of(se);
-
- if (forceidle) {
- if (cfs_rq->forceidle_seq == fi_seq)
- break;
- cfs_rq->forceidle_seq = fi_seq;
- }
-
- cfs_rq->min_vruntime_fi = cfs_rq->min_vruntime;
- }
-}
-
-void task_vruntime_update(struct rq *rq, struct task_struct *p, bool in_fi)
-{
- struct sched_entity *se = &p->se;
-
- if (p->sched_class != &fair_sched_class)
- return;
-
- se_fi_update(se, rq->core->core_forceidle_seq, in_fi);
-}
-
bool cfs_prio_less(const struct task_struct *a, const struct task_struct *b,
bool in_fi)
{
@@ -12438,9 +12409,6 @@ bool cfs_prio_less(const struct task_struct *a, const struct task_struct *b,
seb = parent_entity(seb);
}
- se_fi_update(sea, rq->core->core_forceidle_seq, in_fi);
- se_fi_update(seb, rq->core->core_forceidle_seq, in_fi);
-
cfs_rqa = sea->cfs_rq;
cfs_rqb = seb->cfs_rq;
#else
@@ -12453,8 +12421,7 @@ bool cfs_prio_less(const struct task_struct *a, const struct task_struct *b,
* min_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);
+ delta = (s64)(sea->core_vruntime - seb->core_vruntime);
return delta > 0;
}
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index f9d3701481f1..2ac89eb20973 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -546,7 +546,6 @@ struct cfs_rq {
u64 min_vruntime;
#ifdef CONFIG_SCHED_CORE
u64 core_min_vruntime;
- unsigned int forceidle_seq;
u64 min_vruntime_fi;
struct cfs_rq *core;
#endif
@@ -1134,7 +1133,6 @@ struct rq {
unsigned int core_pick_seq;
unsigned long core_cookie;
unsigned int core_forceidle_count;
- unsigned int core_forceidle_seq;
unsigned int core_forceidle_occupation;
u64 core_forceidle_start;
#endif
--
2.39.3
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