From debd41081ecdd2d858a7a4c0d7ddb89257fdb418 Mon Sep 17 00:00:00 2001
From: Stefan Kalkowski <stefan.kalkowski@genode-labs.com>
Date: Wed, 7 Sep 2022 16:45:27 +0200
Subject: [PATCH] base-hw: optimize & cleanup scheduler

* Remove useless `ready_check` function
* Implement fine-grained checking when schedule updating is necessary

Ref genodelabs/genode#4151
---
 repos/base-hw/src/core/kernel/cpu.cc          | 11 +--
 .../base-hw/src/core/kernel/cpu_scheduler.cc  | 71 ++++++++-----------
 repos/base-hw/src/core/kernel/cpu_scheduler.h |  7 +-
 3 files changed, 33 insertions(+), 56 deletions(-)

diff --git a/repos/base-hw/src/core/kernel/cpu.cc b/repos/base-hw/src/core/kernel/cpu.cc
index 02e190d63e..cd7e57b5b9 100644
--- a/repos/base-hw/src/core/kernel/cpu.cc
+++ b/repos/base-hw/src/core/kernel/cpu.cc
@@ -121,14 +121,9 @@ Cpu::Idle_thread::Idle_thread(Board::Address_space_id_allocator &addr_space_id_a
 
 void Cpu::schedule(Job * const job)
 {
-	if (_id == executing_id())
-		_scheduler.ready(job->share());
-	else {
-		_scheduler.ready_check(job->share());
-
-		if (_scheduler.need_to_schedule())
-			trigger_ip_interrupt();
-	}
+	_scheduler.ready(job->share());
+	if (_id != executing_id() && _scheduler.need_to_schedule())
+		trigger_ip_interrupt();
 }
 
 
diff --git a/repos/base-hw/src/core/kernel/cpu_scheduler.cc b/repos/base-hw/src/core/kernel/cpu_scheduler.cc
index a9ac614e5d..b56d771656 100644
--- a/repos/base-hw/src/core/kernel/cpu_scheduler.cc
+++ b/repos/base-hw/src/core/kernel/cpu_scheduler.cc
@@ -191,45 +191,17 @@ void Cpu_scheduler::update(time_t time)
 }
 
 
-void Cpu_scheduler::ready_check(Share &s1)
-{
-	assert(_head);
-
-	ready(s1);
-
-	if (_need_to_schedule)
-		return;
-
-	Share * s2 = _head;
-	if (!s1._claim) {
-		_need_to_schedule = s2 == &_idle;
-	} else if (!_head_claims) {
-		_need_to_schedule = true;
-	} else if (s1._prio != s2->_prio) {
-		_need_to_schedule = s1._prio > s2->_prio;
-	} else {
-		for (
-			; s2 && s2 != &s1;
-			s2 =
-				Double_list<Cpu_share>::next(&s2->_claim_item) != nullptr ?
-					&Double_list<Cpu_share>::next(&s2->_claim_item)->payload() :
-					nullptr) ;
-
-		_need_to_schedule = !s2;
-	}
-}
-
-
 void Cpu_scheduler::ready(Share &s)
 {
 	assert(!s._ready && &s != &_idle);
 
-	_need_to_schedule = true;
-
 	s._ready = 1;
 	s._fill = _fill;
 	_fills.insert_tail(&s._fill_item);
 
+	if (_head == &_idle)
+		_need_to_schedule = true;
+
 	if (!s._quota)
 		return;
 
@@ -239,6 +211,28 @@ void Cpu_scheduler::ready(Share &s)
 		_rcl[s._prio].insert_head(&s._claim_item);
 	else
 		_rcl[s._prio].insert_tail(&s._claim_item);
+
+	/*
+	 * Check whether we need to re-schedule
+	 */
+	if (_need_to_schedule)
+		return;
+
+	/* current head has no quota left */
+	if (!_head_claims) {
+		_need_to_schedule = true;
+		return;
+	}
+
+	/* if current head has different priority */
+	if (s._prio != _head->_prio) {
+		_need_to_schedule = s._prio > _head->_prio;
+		return;
+	}
+
+	/* if current head has same priority, the ready share gets active */
+	if (s._claim)
+		_need_to_schedule = true;
 }
 
 
@@ -246,7 +240,8 @@ void Cpu_scheduler::unready(Share &s)
 {
 	assert(s._ready && &s != &_idle);
 
-	_need_to_schedule = true;
+	if (&s == _head)
+		_need_to_schedule = true;
 
 	s._ready = 0;
 	_fills.remove(&s._fill_item);
@@ -270,21 +265,15 @@ void Cpu_scheduler::remove(Share &s)
 {
 	assert(&s != &_idle);
 
-	_need_to_schedule = true;
+	if (s._ready) unready(s);
 
 	if (&s == _head)
 		_head = nullptr;
 
-	if (s._ready)
-		_fills.remove(&s._fill_item);
-
 	if (!s._quota)
 		return;
 
-	if (s._ready)
-		_rcl[s._prio].remove(&s._claim_item);
-	else
-		_ucl[s._prio].remove(&s._claim_item);
+	_ucl[s._prio].remove(&s._claim_item);
 }
 
 
@@ -292,8 +281,6 @@ void Cpu_scheduler::insert(Share &s)
 {
 	assert(!s._ready);
 
-	_need_to_schedule = true;
-
 	if (!s._quota)
 		return;
 
diff --git a/repos/base-hw/src/core/kernel/cpu_scheduler.h b/repos/base-hw/src/core/kernel/cpu_scheduler.h
index ad8624a257..70e921f44c 100644
--- a/repos/base-hw/src/core/kernel/cpu_scheduler.h
+++ b/repos/base-hw/src/core/kernel/cpu_scheduler.h
@@ -180,15 +180,10 @@ class Kernel::Cpu_scheduler
 		void timeout()          { _need_to_schedule = true; }
 
 		/**
-		 * Update head according to the consumed time
+		 * Update head according to the current (absolute) time
 		 */
 		void update(time_t time);
 
-		/**
-		 * Set 's1' ready and return wether this outdates current head
-		 */
-		void ready_check(Share &s1);
-
 		/**
 		 * Set share 's' ready
 		 */