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Date: Mon, 27 May 2024 14:06:50 +0200
From: Daniel Bristot de Oliveira <bristot@...nel.org>
To: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>,
Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
linux-kernel@...r.kernel.org,
Luca Abeni <luca.abeni@...tannapisa.it>,
Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
Thomas Gleixner <tglx@...utronix.de>,
Joel Fernandes <joel@...lfernandes.org>,
Vineeth Pillai <vineeth@...byteword.org>,
Shuah Khan <skhan@...uxfoundation.org>,
bristot@...nel.org,
Phil Auld <pauld@...hat.com>,
Suleiman Souhlal <suleiman@...gle.com>,
Youssef Esmat <youssefesmat@...gle.com>
Subject: [PATCH V7 4/9] sched/fair: Add trivial fair server
From: Peter Zijlstra <peterz@...radead.org>
Use deadline servers to service fair tasks.
This patch adds a fair_server deadline entity which acts as a container
for fair entities and can be used to fix starvation when higher priority
(wrt fair) tasks are monopolizing CPU(s).
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Daniel Bristot de Oliveira <bristot@...nel.org>
---
kernel/sched/core.c | 1 +
kernel/sched/deadline.c | 23 +++++++++++++++++++++++
kernel/sched/fair.c | 34 ++++++++++++++++++++++++++++++++++
kernel/sched/sched.h | 4 ++++
4 files changed, 62 insertions(+)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 6d01863f93ca..53f0470a1d0a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10057,6 +10057,7 @@ void __init sched_init(void)
#endif /* CONFIG_SMP */
hrtick_rq_init(rq);
atomic_set(&rq->nr_iowait, 0);
+ fair_server_init(rq);
#ifdef CONFIG_SCHED_CORE
rq->core = rq;
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index c75d1307d86d..b69d6c3e1587 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1381,6 +1381,13 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64
resched_curr(rq);
}
+ /*
+ * The fair server (sole dl_server) does not account for real-time
+ * workload because it is running fair work.
+ */
+ if (dl_se == &rq->fair_server)
+ return;
+
/*
* Because -- for now -- we share the rt bandwidth, we need to
* account our runtime there too, otherwise actual rt tasks
@@ -1414,15 +1421,31 @@ void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
void dl_server_start(struct sched_dl_entity *dl_se)
{
+ struct rq *rq = dl_se->rq;
+
if (!dl_server(dl_se)) {
+ /* Disabled */
+ dl_se->dl_runtime = 0;
+ dl_se->dl_deadline = 1000 * NSEC_PER_MSEC;
+ dl_se->dl_period = 1000 * NSEC_PER_MSEC;
+
dl_se->dl_server = 1;
setup_new_dl_entity(dl_se);
}
+
+ if (!dl_se->dl_runtime)
+ return;
+
enqueue_dl_entity(dl_se, ENQUEUE_WAKEUP);
+ if (!dl_task(dl_se->rq->curr) || dl_entity_preempt(dl_se, &rq->curr->dl))
+ resched_curr(dl_se->rq);
}
void dl_server_stop(struct sched_dl_entity *dl_se)
{
+ if (!dl_se->dl_runtime)
+ return;
+
dequeue_dl_entity(dl_se, DEQUEUE_SLEEP);
}
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8a5b1ae0aa55..2d5d3e6c1e72 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5766,6 +5766,7 @@ static bool throttle_cfs_rq(struct cfs_rq *cfs_rq)
struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(cfs_rq->tg);
struct sched_entity *se;
long task_delta, idle_task_delta, dequeue = 1;
+ long rq_h_nr_running = rq->cfs.h_nr_running;
raw_spin_lock(&cfs_b->lock);
/* This will start the period timer if necessary */
@@ -5838,6 +5839,9 @@ static bool throttle_cfs_rq(struct cfs_rq *cfs_rq)
sub_nr_running(rq, task_delta);
done:
+ /* Stop the fair server if throttling resulted in no runnable tasks */
+ if (rq_h_nr_running && !rq->cfs.h_nr_running)
+ dl_server_stop(&rq->fair_server);
/*
* Note: distribution will already see us throttled via the
* throttled-list. rq->lock protects completion.
@@ -5855,6 +5859,7 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(cfs_rq->tg);
struct sched_entity *se;
long task_delta, idle_task_delta;
+ long rq_h_nr_running = rq->cfs.h_nr_running;
se = cfs_rq->tg->se[cpu_of(rq)];
@@ -5930,6 +5935,10 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
unthrottle_throttle:
assert_list_leaf_cfs_rq(rq);
+ /* Start the fair server if un-throttling resulted in new runnable tasks */
+ if (!rq_h_nr_running && rq->cfs.h_nr_running)
+ dl_server_start(&rq->fair_server);
+
/* Determine whether we need to wake up potentially idle CPU: */
if (rq->curr == rq->idle && rq->cfs.nr_running)
resched_curr(rq);
@@ -6760,6 +6769,9 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
*/
util_est_enqueue(&rq->cfs, p);
+ if (!throttled_hierarchy(task_cfs_rq(p)) && !rq->cfs.h_nr_running)
+ dl_server_start(&rq->fair_server);
+
/*
* If in_iowait is set, the code below may not trigger any cpufreq
* utilization updates, so do it here explicitly with the IOWAIT flag
@@ -6904,6 +6916,9 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
rq->next_balance = jiffies;
dequeue_throttle:
+ if (!throttled_hierarchy(task_cfs_rq(p)) && !rq->cfs.h_nr_running)
+ dl_server_stop(&rq->fair_server);
+
util_est_update(&rq->cfs, p, task_sleep);
hrtick_update(rq);
}
@@ -8607,6 +8622,25 @@ static struct task_struct *__pick_next_task_fair(struct rq *rq)
return pick_next_task_fair(rq, NULL, NULL);
}
+static bool fair_server_has_tasks(struct sched_dl_entity *dl_se)
+{
+ return !!dl_se->rq->cfs.nr_running;
+}
+
+static struct task_struct *fair_server_pick(struct sched_dl_entity *dl_se)
+{
+ return pick_next_task_fair(dl_se->rq, NULL, NULL);
+}
+
+void fair_server_init(struct rq *rq)
+{
+ struct sched_dl_entity *dl_se = &rq->fair_server;
+
+ init_dl_entity(dl_se);
+
+ dl_server_init(dl_se, rq, fair_server_has_tasks, fair_server_pick);
+}
+
/*
* Account for a descheduled task:
*/
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index a831af102070..39c9669b23a7 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -356,6 +356,8 @@ extern void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
dl_server_has_tasks_f has_tasks,
dl_server_pick_f pick);
+extern void fair_server_init(struct rq *rq);
+
#ifdef CONFIG_CGROUP_SCHED
struct cfs_rq;
@@ -1037,6 +1039,8 @@ struct rq {
struct rt_rq rt;
struct dl_rq dl;
+ struct sched_dl_entity fair_server;
+
#ifdef CONFIG_FAIR_GROUP_SCHED
/* list of leaf cfs_rq on this CPU: */
struct list_head leaf_cfs_rq_list;
--
2.45.1
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