linux-kernel - [PATCH v2 22/35] sched: default preemption policy for PREEMPT

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Message-Id: <20240528003521.979836-23-ankur.a.arora@oracle.com>
Date: Mon, 27 May 2024 17:35:08 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: linux-kernel@...r.kernel.org
Cc: tglx@...utronix.de, peterz@...radead.org, torvalds@...ux-foundation.org,
        paulmck@...nel.org, rostedt@...dmis.org, mark.rutland@....com,
        juri.lelli@...hat.com, joel@...lfernandes.org, raghavendra.kt@....com,
        sshegde@...ux.ibm.com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com, Ankur Arora <ankur.a.arora@...cle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH v2 22/35] sched: default preemption policy for PREEMPT_AUTO

Add resched_opt_translate() which determines the particular
need-resched flag based on scheduling policy.

Preemption models other than PREEMPT_AUTO: continue to use
tif_resched(RESCHED_NOW).

PREEMPT_AUTO: use tif_resched(RESCHED_LAZY) to reschedule at
the next exit-to-user.

Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Originally-by: Thomas Gleixner <tglx@...utronix.de>
Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
---
 kernel/sched/core.c  | 30 ++++++++++++++++++++++++------
 kernel/sched/sched.h | 12 +++++++++++-
 2 files changed, 35 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 27b908cc9134..ee846dc9133b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1032,20 +1032,38 @@ void wake_up_q(struct wake_q_head *head)
 }
 
 /*
- * resched_curr - mark rq's current task 'to be rescheduled now'.
+ * For preemption models other than PREEMPT_AUTO: always schedule
+ * eagerly.
  *
- * On UP this means the setting of the need_resched flag, on SMP it
- * might also involve a cross-CPU call to trigger the scheduler on
- * the target CPU.
+ * For PREEMPT_AUTO: allow everything else to finish its time quanta, and
+ * mark for rescheduling at the next exit to user.
  */
-void resched_curr(struct rq *rq)
+static resched_t resched_opt_translate(struct task_struct *curr,
+				       enum resched_opt opt)
+{
+	if (!IS_ENABLED(CONFIG_PREEMPT_AUTO))
+		return RESCHED_NOW;
+
+	return RESCHED_LAZY;
+}
+
+/*
+ * __resched_curr - mark rq's current task 'to be rescheduled now'.
+ *
+ * On UP this means the setting of the appropriate need_resched flag.
+ * On SMP, in addition it might also involve a cross-CPU call to
+ * trigger the scheduler on the target CPU.
+ */
+void __resched_curr(struct rq *rq, enum resched_opt opt)
 {
 	struct task_struct *curr = rq->curr;
-	resched_t rs = RESCHED_NOW;
+	resched_t rs;
 	int cpu;
 
 	lockdep_assert_rq_held(rq);
 
+	rs = resched_opt_translate(curr, opt);
+
 	/*
 	 * TIF_NEED_RESCHED is the higher priority bit, so if it is already
 	 * set, nothing more to be done.
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c9239c0b0095..7013bd054a2f 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2464,7 +2464,17 @@ extern void init_sched_fair_class(void);
 
 extern void reweight_task(struct task_struct *p, int prio);
 
-extern void resched_curr(struct rq *rq);
+enum resched_opt {
+	RESCHED_DEFAULT,
+};
+
+extern void __resched_curr(struct rq *rq, enum resched_opt opt);
+
+static inline void resched_curr(struct rq *rq)
+{
+	__resched_curr(rq, RESCHED_DEFAULT);
+}
+
 extern void resched_cpu(int cpu);
 
 extern struct rt_bandwidth def_rt_bandwidth;
-- 
2.31.1