linux-kernel - [RFC PATCH 39/86] sched: handle lazy resched in set_nr_*

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Message-Id: <20231107215742.363031-40-ankur.a.arora@oracle.com>
Date:   Tue,  7 Nov 2023 13:57:25 -0800
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,
        linux-mm@...ck.org, x86@...nel.org, akpm@...ux-foundation.org,
        luto@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com,
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        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
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        David.Laight@...LAB.COM, richard@....at, mjguzik@...il.com,
        Ankur Arora <ankur.a.arora@...cle.com>
Subject: [RFC PATCH 39/86] sched: handle lazy resched in set_nr_*_polling()

To trigger a reschedule on a target runqueue a few things need
to happen first:

  1. set_tsk_need_resched(target_rq->curr, RESCHED_eager)
  2. ensure that the target CPU sees the need-resched bit
  3. preempt_fold_need_resched()

Most of this is done via some combination of: resched_curr(),
set_nr_if_polling(), and set_nr_and_not_polling().

Update the last two to also handle TIF_NEED_RESCHED_LAZY.

One thing to note is that TIF_NEED_RESCHED_LAZY has run to completion
semantics, so unlike TIF_NEED_RESCHED, we don't need to ensure that
the caller sees it, and of course there is no preempt folding.

Originally-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
---
 kernel/sched/core.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e2215c417323..01df5ac2982c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -892,14 +892,15 @@ static inline void hrtick_rq_init(struct rq *rq)
 
 #if defined(CONFIG_SMP) && defined(TIF_POLLING_NRFLAG)
 /*
- * Atomically set TIF_NEED_RESCHED and test for TIF_POLLING_NRFLAG,
+ * Atomically set TIF_NEED_RESCHED[_LAZY] and test for TIF_POLLING_NRFLAG,
  * this avoids any races wrt polling state changes and thereby avoids
  * spurious IPIs.
  */
-static inline bool set_nr_and_not_polling(struct task_struct *p)
+static inline bool set_nr_and_not_polling(struct task_struct *p, resched_t rs)
 {
 	struct thread_info *ti = task_thread_info(p);
-	return !(fetch_or(&ti->flags, _TIF_NEED_RESCHED) & _TIF_POLLING_NRFLAG);
+
+	return !(fetch_or(&ti->flags, _tif_resched(rs)) & _TIF_POLLING_NRFLAG);
 }
 
 /*
@@ -916,7 +917,7 @@ static bool set_nr_if_polling(struct task_struct *p)
 	for (;;) {
 		if (!(val & _TIF_POLLING_NRFLAG))
 			return false;
-		if (val & _TIF_NEED_RESCHED)
+		if (val & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY))
 			return true;
 		if (try_cmpxchg(&ti->flags, &val, val | _TIF_NEED_RESCHED))
 			break;
@@ -925,9 +926,9 @@ static bool set_nr_if_polling(struct task_struct *p)
 }
 
 #else
-static inline bool set_nr_and_not_polling(struct task_struct *p)
+static inline bool set_nr_and_not_polling(struct task_struct *p, resched_t rs)
 {
-	set_tsk_need_resched(p, RESCHED_eager);
+	set_tsk_need_resched(p, rs);
 	return true;
 }
 
@@ -1050,7 +1051,7 @@ void resched_curr(struct rq *rq)
 		return;
 	}
 
-	if (set_nr_and_not_polling(curr))
+	if (set_nr_and_not_polling(curr, RESCHED_eager))
 		smp_send_reschedule(cpu);
 	else
 		trace_sched_wake_idle_without_ipi(cpu);
@@ -1126,7 +1127,7 @@ static void wake_up_idle_cpu(int cpu)
 	if (cpu == smp_processor_id())
 		return;
 
-	if (set_nr_and_not_polling(rq->idle))
+	if (set_nr_and_not_polling(rq->idle, RESCHED_eager))
 		smp_send_reschedule(cpu);
 	else
 		trace_sched_wake_idle_without_ipi(cpu);
-- 
2.31.1