From 1f787a6826207e9aeb7e3283ce65a31842c35401 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Tue, 28 Sep 2021 14:24:32 +0200
Subject: [PATCH] sched: Move mmdrop to RCU on RT

References: bsc#1189998 (PREEMPT_RT prerequisite backports)
Patch-mainline: v5.16-rc1
Git-commit: 8d491de6edc27138806cae6e8eca455beb325b62

mmdrop() is invoked from finish_task_switch() by the incoming task to drop
the mm which was handed over by the previous task. mmdrop() can be quite
expensive which prevents an incoming real-time task from getting useful
work done.

Provide mmdrop_sched() which maps to mmdrop() on !RT kernels. On RT kernels
it delagates the eventually required invocation of __mmdrop() to RCU.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210928122411.648582026@linutronix.de
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 include/linux/mm_types.h |  4 ++++
 include/linux/sched/mm.h | 29 +++++++++++++++++++++++++++++
 kernel/sched/core.c      |  2 +-
 3 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 7f8ee09c711f..e9672de22cf2 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -12,6 +12,7 @@
 #include <linux/completion.h>
 #include <linux/cpumask.h>
 #include <linux/uprobes.h>
+#include <linux/rcupdate.h>
 #include <linux/page-flags-layout.h>
 #include <linux/workqueue.h>
 #include <linux/seqlock.h>
@@ -572,6 +573,9 @@ struct mm_struct {
 		bool tlb_flush_batched;
 #endif
 		struct uprobes_state uprobes_state;
+#ifdef CONFIG_PREEMPT_RT
+		struct rcu_head delayed_drop;
+#endif
 #ifdef CONFIG_HUGETLB_PAGE
 		atomic_long_t hugetlb_usage;
 #endif
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 8894825cc4db..c57b8356bfac 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -49,6 +49,35 @@ static inline void mmdrop(struct mm_struct *mm)
 		__mmdrop(mm);
 }
 
+#ifdef CONFIG_PREEMPT_RT
+/*
+ * RCU callback for delayed mm drop. Not strictly RCU, but call_rcu() is
+ * by far the least expensive way to do that.
+ */
+static inline void __mmdrop_delayed(struct rcu_head *rhp)
+{
+	struct mm_struct *mm = container_of(rhp, struct mm_struct, delayed_drop);
+
+	__mmdrop(mm);
+}
+
+/*
+ * Invoked from finish_task_switch(). Delegates the heavy lifting on RT
+ * kernels via RCU.
+ */
+static inline void mmdrop_sched(struct mm_struct *mm)
+{
+	/* Provides a full memory barrier. See mmdrop() */
+	if (atomic_dec_and_test(&mm->mm_count))
+		call_rcu(&mm->delayed_drop, __mmdrop_delayed);
+}
+#else
+static inline void mmdrop_sched(struct mm_struct *mm)
+{
+	mmdrop(mm);
+}
+#endif
+
 /**
  * mmget() - Pin the address space associated with a &struct mm_struct.
  * @mm: The address space to pin.
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index eebfc940b191..d22489d3245a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4651,7 +4651,7 @@ static struct rq *finish_task_switch(struct task_struct *prev)
 	 */
 	if (mm) {
 		membarrier_mm_sync_core_before_usermode(mm);
-		mmdrop(mm);
+		mmdrop_sched(mm);
 	}
 	if (unlikely(prev_state == TASK_DEAD)) {
 		if (prev->sched_class->task_dead)