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Subject[RFC PATCH v1 3/8] sched/cpufreq_schedutil: make worker kthread be SCHED_DEADLINE
Date
Worker kthread needs to be able to change frequency for all other
threads.

Make it special, just under STOP class.

Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Luca Abeni <luca.abeni@santannapisa.it>
Cc: Claudio Scordino <claudio@evidence.eu.com>
---
Changes from RFCv0:

- use a high bit of sched_flags and don't expose the new flag to
userspace (also add derisory comments) (Peter)
---
include/linux/sched.h | 1 +
kernel/sched/core.c | 15 +++++++++++++--
kernel/sched/cpufreq_schedutil.c | 15 ++++++++++++---
kernel/sched/deadline.c | 13 +++++++++++++
kernel/sched/sched.h | 20 +++++++++++++++++++-
5 files changed, 58 insertions(+), 6 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1f0f427e0292..0ba767fdedae 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1359,6 +1359,7 @@ extern int idle_cpu(int cpu);
extern int sched_setscheduler(struct task_struct *, int, const struct sched_param *);
extern int sched_setscheduler_nocheck(struct task_struct *, int, const struct sched_param *);
extern int sched_setattr(struct task_struct *, const struct sched_attr *);
+extern int sched_setattr_nocheck(struct task_struct *, const struct sched_attr *);
extern struct task_struct *idle_task(int cpu);

/**
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5186797908dc..0e40c7ff2bf4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3998,7 +3998,9 @@ static int __sched_setscheduler(struct task_struct *p,
}

if (attr->sched_flags &
- ~(SCHED_FLAG_RESET_ON_FORK | SCHED_FLAG_RECLAIM))
+ ~(SCHED_FLAG_RESET_ON_FORK |
+ SCHED_FLAG_RECLAIM |
+ SCHED_FLAG_SPECIAL))
return -EINVAL;

/*
@@ -4065,6 +4067,9 @@ static int __sched_setscheduler(struct task_struct *p,
}

if (user) {
+ if (attr->sched_flags & SCHED_FLAG_SPECIAL)
+ return -EPERM;
+
retval = security_task_setscheduler(p);
if (retval)
return retval;
@@ -4120,7 +4125,8 @@ static int __sched_setscheduler(struct task_struct *p,
}
#endif
#ifdef CONFIG_SMP
- if (dl_bandwidth_enabled() && dl_policy(policy)) {
+ if (dl_bandwidth_enabled() && dl_policy(policy) &&
+ !(attr->sched_flags & SCHED_FLAG_SPECIAL)) {
cpumask_t *span = rq->rd->span;

/*
@@ -4250,6 +4256,11 @@ int sched_setattr(struct task_struct *p, const struct sched_attr *attr)
}
EXPORT_SYMBOL_GPL(sched_setattr);

+int sched_setattr_nocheck(struct task_struct *p, const struct sched_attr *attr)
+{
+ return __sched_setscheduler(p, attr, false, true);
+}
+
/**
* sched_setscheduler_nocheck - change the scheduling policy and/or RT priority of a thread from kernelspace.
* @p: the task in question.
diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index f2494d1fc8ef..ba6227625f24 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -424,7 +424,16 @@ static void sugov_policy_free(struct sugov_policy *sg_policy)
static int sugov_kthread_create(struct sugov_policy *sg_policy)
{
struct task_struct *thread;
- struct sched_param param = { .sched_priority = MAX_USER_RT_PRIO / 2 };
+ struct sched_attr attr = {
+ .size = sizeof(struct sched_attr),
+ .sched_policy = SCHED_DEADLINE,
+ .sched_flags = SCHED_FLAG_SPECIAL,
+ .sched_nice = 0,
+ .sched_priority = 0,
+ .sched_runtime = 0,
+ .sched_deadline = 0,
+ .sched_period = 0,
+ };
struct cpufreq_policy *policy = sg_policy->policy;
int ret;

@@ -442,10 +451,10 @@ static int sugov_kthread_create(struct sugov_policy *sg_policy)
return PTR_ERR(thread);
}

- ret = sched_setscheduler_nocheck(thread, SCHED_FIFO, &param);
+ ret = sched_setattr_nocheck(thread, &attr);
if (ret) {
kthread_stop(thread);
- pr_warn("%s: failed to set SCHED_FIFO\n", __func__);
+ pr_warn("%s: failed to set SCHED_DEADLINE\n", __func__);
return ret;
}

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 6912f7f35f9b..4ec82fa56b04 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -220,6 +220,9 @@ static void task_non_contending(struct task_struct *p)
if (dl_se->dl_runtime == 0)
return;

+ if (unlikely(dl_entity_is_special(dl_se)))
+ return;
+
WARN_ON(hrtimer_active(&dl_se->inactive_timer));
WARN_ON(dl_se->dl_non_contending);

@@ -1150,6 +1153,9 @@ static void update_curr_dl(struct rq *rq)

sched_rt_avg_update(rq, delta_exec);

+ if (unlikely(dl_entity_is_special(dl_se)))
+ return;
+
if (unlikely(dl_se->flags & SCHED_FLAG_RECLAIM))
delta_exec = grub_reclaim(delta_exec, rq, &curr->dl);
dl_se->runtime -= delta_exec;
@@ -2447,6 +2453,9 @@ int sched_dl_overflow(struct task_struct *p, int policy,
u64 new_bw = dl_policy(policy) ? to_ratio(period, runtime) : 0;
int cpus, err = -1;

+ if (attr->sched_flags & SCHED_FLAG_SPECIAL)
+ return 0;
+
/* !deadline task may carry old deadline bandwidth */
if (new_bw == p->dl.dl_bw && task_has_dl_policy(p))
return 0;
@@ -2533,6 +2542,10 @@ void __getparam_dl(struct task_struct *p, struct sched_attr *attr)
*/
bool __checkparam_dl(const struct sched_attr *attr)
{
+ /* special dl tasks don't actually use any parameter */
+ if (attr->sched_flags & SCHED_FLAG_SPECIAL)
+ return true;
+
/* deadline != 0 */
if (attr->sched_deadline == 0)
return false;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index d8798bb54ace..1be5dac728a4 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -156,12 +156,30 @@ static inline int task_has_dl_policy(struct task_struct *p)
}

/*
+ * !! For sched_setattr_nocheck() (kernel) only !!
+ *
+ * This is actually gross. :(
+ *
+ * It is used to make schedutil kworker(s) higher priority than SCHED_DEADLINE
+ * tasks, but still be able to sleep. We need this on platforms that cannot
+ * atomically change clock frequency. Remove once fast switching will be
+ * available on such platforms.
+ */
+#define SCHED_FLAG_SPECIAL 0x10000000
+
+static inline int dl_entity_is_special(struct sched_dl_entity *dl_se)
+{
+ return dl_se->flags & SCHED_FLAG_SPECIAL;
+}
+
+/*
* Tells if entity @a should preempt entity @b.
*/
static inline bool
dl_entity_preempt(struct sched_dl_entity *a, struct sched_dl_entity *b)
{
- return dl_time_before(a->deadline, b->deadline);
+ return dl_entity_is_special(a) ||
+ dl_time_before(a->deadline, b->deadline);
}

/*
--
2.11.0
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