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From:   Peter Oskolkov <posk@posk.io>
To:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org
Cc:     Paul Turner <pjt@google.com>, Ben Segall <bsegall@google.com>,
        Peter Oskolkov <posk@google.com>,
        Peter Oskolkov <posk@posk.io>,
        Joel Fernandes <joel@joelfernandes.org>,
        Andrei Vagin <avagin@google.com>,
        Jim Newsome <jnewsome@torproject.org>,
        Jann Horn <jannh@google.com>,
        Thierry Delisle <tdelisle@uwaterloo.ca>
Subject: [RFC PATCH 1/4 v0.3] sched: add WF_CURRENT_CPU and externise ttwu
Date:   Fri, 16 Jul 2021 11:47:16 -0700
Message-Id: <20210716184719.269033-2-posk@google.com>
In-Reply-To: <20210716184719.269033-1-posk@google.com>
References: <20210716184719.269033-1-posk@google.com>
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Add WF_CURRENT_CPU wake flag that advices the scheduler to
move the wakee to the current CPU. This is useful for fast on-CPU
context switching use cases such as UMCG.

In addition, make ttwu external rather than static so that
the flag could be passed to it from outside of sched/core.c.

Signed-off-by: Peter Oskolkov <posk@google.com>
---
 kernel/sched/core.c  |  3 +--
 kernel/sched/fair.c  |  4 ++++
 kernel/sched/sched.h | 15 +++++++++------
 3 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0c22cd026440..293f5801bf81 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3680,8 +3680,7 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
  * Return: %true if @p->state changes (an actual wakeup was done),
  *	   %false otherwise.
  */
-static int
-try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
+int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 {
 	unsigned long flags;
 	int cpu, success = 0;
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 11d22943753f..16a9c93e6e82 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6836,6 +6836,10 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 	if (wake_flags & WF_TTWU) {
 		record_wakee(p);

+		if ((wake_flags & WF_CURRENT_CPU) &&
+		    cpumask_test_cpu(cpu, p->cpus_ptr))
+			return cpu;
+
 		if (sched_energy_enabled()) {
 			new_cpu = find_energy_efficient_cpu(p, prev_cpu);
 			if (new_cpu >= 0)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 9a1c6aeb9165..80de6836f8ae 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2031,13 +2031,14 @@ static inline int task_on_rq_migrating(struct task_struct *p)
 }

 /* Wake flags. The first three directly map to some SD flag value */
-#define WF_EXEC     0x02 /* Wakeup after exec; maps to SD_BALANCE_EXEC */
-#define WF_FORK     0x04 /* Wakeup after fork; maps to SD_BALANCE_FORK */
-#define WF_TTWU     0x08 /* Wakeup;            maps to SD_BALANCE_WAKE */
+#define WF_EXEC         0x02 /* Wakeup after exec; maps to SD_BALANCE_EXEC */
+#define WF_FORK         0x04 /* Wakeup after fork; maps to SD_BALANCE_FORK */
+#define WF_TTWU         0x08 /* Wakeup;            maps to SD_BALANCE_WAKE */

-#define WF_SYNC     0x10 /* Waker goes to sleep after wakeup */
-#define WF_MIGRATED 0x20 /* Internal use, task got migrated */
-#define WF_ON_CPU   0x40 /* Wakee is on_cpu */
+#define WF_SYNC         0x10 /* Waker goes to sleep after wakeup */
+#define WF_MIGRATED     0x20 /* Internal use, task got migrated */
+#define WF_ON_CPU       0x40 /* Wakee is on_cpu */
+#define WF_CURRENT_CPU  0x80 /* Prefer to move the wakee to the current CPU. */

 #ifdef CONFIG_SMP
 static_assert(WF_EXEC == SD_BALANCE_EXEC);
@@ -3037,6 +3038,8 @@ static inline bool is_per_cpu_kthread(struct task_struct *p)
 extern void swake_up_all_locked(struct swait_queue_head *q);
 extern void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait);

+extern int try_to_wake_up(struct task_struct *tsk, unsigned int state, int wake_flags);
+
 #ifdef CONFIG_PREEMPT_DYNAMIC
 extern int preempt_dynamic_mode;
 extern int sched_dynamic_mode(const char *str);
--
2.25.1