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From: Qais Yousef <qyousef@layalina.io>
To: Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Christian Brauner <brauner@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jens Axboe <axboe@kernel.dk>,
	linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org,
	linux-trace-kernel@vger.kernel.org,
	linux-mm@kvack.org,
	Qais Yousef <qyousef@layalina.io>
Subject: [PATCH] sched/rt: Clean up usage of rt_task()
Date: Wed, 15 May 2024 00:41:12 +0100
Message-Id: <20240514234112.792989-1-qyousef@layalina.io>
Precedence: bulk
MIME-Version: 1.0
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rt_task() checks if a task has RT priority. But depends on your
dictionary, this could mean it belongs to RT class, or is a 'realtime'
task, which includes RT and DL classes.

Since this has caused some confusion already on discussion [1], it
seemed a clean up is due.

I define the usage of rt_task() to be tasks that belong to RT class.
Make sure that it returns true only for RT class and audit the users and
replace them with the new realtime_task() which returns true for RT and
DL classes - the old behavior. Introduce similar realtime_prio() to
create similar distinction to rt_prio() and update the users.

Move MAX_DL_PRIO to prio.h so it can be used in the new definitions.

Document the functions to make it more obvious what is the difference
between them. PI-boosted tasks is a factor that must be taken into
account when choosing which function to use.

Rename task_is_realtime() to task_has_realtime_policy() as the old name
is confusing against the new realtime_task().

No functional changes were intended.

[1] https://lore.kernel.org/lkml/20240506100509.GL40213@noisy.programming.kicks-ass.net/

Signed-off-by: Qais Yousef <qyousef@layalina.io>
---
 fs/select.c                       |  2 +-
 include/linux/ioprio.h            |  2 +-
 include/linux/sched/deadline.h    |  6 ++++--
 include/linux/sched/prio.h        |  1 +
 include/linux/sched/rt.h          | 27 ++++++++++++++++++++++++++-
 kernel/locking/rtmutex.c          |  4 ++--
 kernel/locking/rwsem.c            |  4 ++--
 kernel/locking/ww_mutex.h         |  2 +-
 kernel/sched/core.c               |  6 +++---
 kernel/time/hrtimer.c             |  6 +++---
 kernel/trace/trace_sched_wakeup.c |  2 +-
 mm/page-writeback.c               |  4 ++--
 mm/page_alloc.c                   |  2 +-
 13 files changed, 48 insertions(+), 20 deletions(-)

diff --git a/fs/select.c b/fs/select.c
index 9515c3fa1a03..8d5c1419416c 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -82,7 +82,7 @@ u64 select_estimate_accuracy(struct timespec64 *tv)
 	 * Realtime tasks get a slack of 0 for obvious reasons.
 	 */
 
-	if (rt_task(current))
+	if (realtime_task(current))
 		return 0;
 
 	ktime_get_ts64(&now);
diff --git a/include/linux/ioprio.h b/include/linux/ioprio.h
index db1249cd9692..6c00342b6166 100644
--- a/include/linux/ioprio.h
+++ b/include/linux/ioprio.h
@@ -40,7 +40,7 @@ static inline int task_nice_ioclass(struct task_struct *task)
 {
 	if (task->policy == SCHED_IDLE)
 		return IOPRIO_CLASS_IDLE;
-	else if (task_is_realtime(task))
+	else if (task_has_realtime_policy(task))
 		return IOPRIO_CLASS_RT;
 	else
 		return IOPRIO_CLASS_BE;
diff --git a/include/linux/sched/deadline.h b/include/linux/sched/deadline.h
index df3aca89d4f5..5cb88b748ad6 100644
--- a/include/linux/sched/deadline.h
+++ b/include/linux/sched/deadline.h
@@ -10,8 +10,6 @@
 
 #include <linux/sched.h>
 
-#define MAX_DL_PRIO		0
-
 static inline int dl_prio(int prio)
 {
 	if (unlikely(prio < MAX_DL_PRIO))
@@ -19,6 +17,10 @@ static inline int dl_prio(int prio)
 	return 0;
 }
 
+/*
+ * Returns true if a task has a priority that belongs to DL class. PI-boosted
+ * tasks will return true. Use dl_policy() to ignore PI-boosted tasks.
+ */
 static inline int dl_task(struct task_struct *p)
 {
 	return dl_prio(p->prio);
diff --git a/include/linux/sched/prio.h b/include/linux/sched/prio.h
index ab83d85e1183..6ab43b4f72f9 100644
--- a/include/linux/sched/prio.h
+++ b/include/linux/sched/prio.h
@@ -14,6 +14,7 @@
  */
 
 #define MAX_RT_PRIO		100
+#define MAX_DL_PRIO		0
 
 #define MAX_PRIO		(MAX_RT_PRIO + NICE_WIDTH)
 #define DEFAULT_PRIO		(MAX_RT_PRIO + NICE_WIDTH / 2)
diff --git a/include/linux/sched/rt.h b/include/linux/sched/rt.h
index b2b9e6eb9683..b31be3c50152 100644
--- a/include/linux/sched/rt.h
+++ b/include/linux/sched/rt.h
@@ -7,18 +7,43 @@
 struct task_struct;
 
 static inline int rt_prio(int prio)
+{
+	if (unlikely(prio < MAX_RT_PRIO && prio >= MAX_DL_PRIO))
+		return 1;
+	return 0;
+}
+
+static inline int realtime_prio(int prio)
 {
 	if (unlikely(prio < MAX_RT_PRIO))
 		return 1;
 	return 0;
 }
 
+/*
+ * Returns true if a task has a priority that belongs to RT class. PI-boosted
+ * tasks will return true. Use rt_policy() to ignore PI-boosted tasks.
+ */
 static inline int rt_task(struct task_struct *p)
 {
 	return rt_prio(p->prio);
 }
 
-static inline bool task_is_realtime(struct task_struct *tsk)
+/*
+ * Returns true if a task has a priority that belongs to RT or DL classes.
+ * PI-boosted tasks will return true. Use task_has_realtime_policy() to ignore
+ * PI-boosted tasks.
+ */
+static inline int realtime_task(struct task_struct *p)
+{
+	return realtime_prio(p->prio);
+}
+
+/*
+ * Returns true if a task has a policy that belongs to RT or DL classes.
+ * PI-boosted tasks will return false.
+ */
+static inline bool task_has_realtime_policy(struct task_struct *tsk)
 {
 	int policy = tsk->policy;
 
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 88d08eeb8bc0..55c9dab37f33 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -347,7 +347,7 @@ static __always_inline int __waiter_prio(struct task_struct *task)
 {
 	int prio = task->prio;
 
-	if (!rt_prio(prio))
+	if (!realtime_prio(prio))
 		return DEFAULT_PRIO;
 
 	return prio;
@@ -435,7 +435,7 @@ static inline bool rt_mutex_steal(struct rt_mutex_waiter *waiter,
 	 * Note that RT tasks are excluded from same priority (lateral)
 	 * steals to prevent the introduction of an unbounded latency.
 	 */
-	if (rt_prio(waiter->tree.prio) || dl_prio(waiter->tree.prio))
+	if (realtime_prio(waiter->tree.prio))
 		return false;
 
 	return rt_waiter_node_equal(&waiter->tree, &top_waiter->tree);
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index c6d17aee4209..ad8d4438bc91 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -631,7 +631,7 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
 			 * if it is an RT task or wait in the wait queue
 			 * for too long.
 			 */
-			if (has_handoff || (!rt_task(waiter->task) &&
+			if (has_handoff || (!realtime_task(waiter->task) &&
 					    !time_after(jiffies, waiter->timeout)))
 				return false;
 
@@ -914,7 +914,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 		if (owner_state != OWNER_WRITER) {
 			if (need_resched())
 				break;
-			if (rt_task(current) &&
+			if (realtime_task(current) &&
 			   (prev_owner_state != OWNER_WRITER))
 				break;
 		}
diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
index 3ad2cc4823e5..fa4b416a1f62 100644
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -237,7 +237,7 @@ __ww_ctx_less(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b)
 	int a_prio = a->task->prio;
 	int b_prio = b->task->prio;
 
-	if (rt_prio(a_prio) || rt_prio(b_prio)) {
+	if (realtime_prio(a_prio) || realtime_prio(b_prio)) {
 
 		if (a_prio > b_prio)
 			return true;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 1a914388144a..27f15de3d099 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -162,7 +162,7 @@ static inline int __task_prio(const struct task_struct *p)
 	if (p->sched_class == &stop_sched_class) /* trumps deadline */
 		return -2;
 
-	if (rt_prio(p->prio)) /* includes deadline */
+	if (realtime_prio(p->prio)) /* includes deadline */
 		return p->prio; /* [-1, 99] */
 
 	if (p->sched_class == &idle_sched_class)
@@ -2198,7 +2198,7 @@ static int effective_prio(struct task_struct *p)
 	 * keep the priority unchanged. Otherwise, update priority
 	 * to the normal priority:
 	 */
-	if (!rt_prio(p->prio))
+	if (!realtime_prio(p->prio))
 		return p->normal_prio;
 	return p->prio;
 }
@@ -10282,7 +10282,7 @@ void normalize_rt_tasks(void)
 		schedstat_set(p->stats.sleep_start, 0);
 		schedstat_set(p->stats.block_start, 0);
 
-		if (!dl_task(p) && !rt_task(p)) {
+		if (!realtime_task(p)) {
 			/*
 			 * Renice negative nice level userspace
 			 * tasks back to 0:
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 70625dff62ce..4150e98847fa 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1996,7 +1996,7 @@ static void __hrtimer_init_sleeper(struct hrtimer_sleeper *sl,
 	 * expiry.
 	 */
 	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
-		if (task_is_realtime(current) && !(mode & HRTIMER_MODE_SOFT))
+		if (task_has_realtime_policy(current) && !(mode & HRTIMER_MODE_SOFT))
 			mode |= HRTIMER_MODE_HARD;
 	}
 
@@ -2096,7 +2096,7 @@ long hrtimer_nanosleep(ktime_t rqtp, const enum hrtimer_mode mode,
 	u64 slack;
 
 	slack = current->timer_slack_ns;
-	if (rt_task(current))
+	if (realtime_task(current))
 		slack = 0;
 
 	hrtimer_init_sleeper_on_stack(&t, clockid, mode);
@@ -2301,7 +2301,7 @@ schedule_hrtimeout_range_clock(ktime_t *expires, u64 delta,
 	 * Override any slack passed by the user if under
 	 * rt contraints.
 	 */
-	if (rt_task(current))
+	if (realtime_task(current))
 		delta = 0;
 
 	hrtimer_init_sleeper_on_stack(&t, clock_id, mode);
diff --git a/kernel/trace/trace_sched_wakeup.c b/kernel/trace/trace_sched_wakeup.c
index 0469a04a355f..19d737742e29 100644
--- a/kernel/trace/trace_sched_wakeup.c
+++ b/kernel/trace/trace_sched_wakeup.c
@@ -545,7 +545,7 @@ probe_wakeup(void *ignore, struct task_struct *p)
 	 *  - wakeup_dl handles tasks belonging to sched_dl class only.
 	 */
 	if (tracing_dl || (wakeup_dl && !dl_task(p)) ||
-	    (wakeup_rt && !dl_task(p) && !rt_task(p)) ||
+	    (wakeup_rt && !realtime_task(p)) ||
 	    (!dl_task(p) && (p->prio >= wakeup_prio || p->prio >= current->prio)))
 		return;
 
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 3e19b87049db..7372e40f225d 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -418,7 +418,7 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
 	if (bg_thresh >= thresh)
 		bg_thresh = thresh / 2;
 	tsk = current;
-	if (rt_task(tsk)) {
+	if (realtime_task(tsk)) {
 		bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
 		thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
 	}
@@ -468,7 +468,7 @@ static unsigned long node_dirty_limit(struct pglist_data *pgdat)
 	else
 		dirty = vm_dirty_ratio * node_memory / 100;
 
-	if (rt_task(tsk))
+	if (realtime_task(tsk))
 		dirty += dirty / 4;
 
 	return dirty;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 14d39f34d336..0af24a60ade0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3877,7 +3877,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask, unsigned int order)
 		 */
 		if (alloc_flags & ALLOC_MIN_RESERVE)
 			alloc_flags &= ~ALLOC_CPUSET;
-	} else if (unlikely(rt_task(current)) && in_task())
+	} else if (unlikely(realtime_task(current)) && in_task())
 		alloc_flags |= ALLOC_MIN_RESERVE;
 
 	alloc_flags = gfp_to_alloc_flags_cma(gfp_mask, alloc_flags);
-- 
2.34.1