[PATCH v4 01/11] powerpc/mm: Adds counting method to monitor lockless pgtable walks

[Leonardo Bras]

It's necessary to monitor lockless pagetable walks, in order to avoid doing
THP splitting/collapsing during them.

Some methods rely on local_irq_{save,restore}, but that can be slow on
cases with a lot of cpus are used for the process.

In order to speedup some cases, I propose a refcount-based approach, that
counts the number of lockless pagetable	walks happening on the process.

This method does not exclude the current irq-oriented method. It works as a
complement to skip unnecessary waiting.

start_lockless_pgtbl_walk(mm)
	Insert before starting any lockless pgtable walk
end_lockless_pgtbl_walk(mm)
	Insert after the end of any lockless pgtable walk
	(Mostly after the ptep is last used)
running_lockless_pgtbl_walk(mm)
	Returns the number of lockless pgtable walks running

Signed-off-by: Leonardo Bras <leonardo@xxxxxxxxxxxxx>
---
 arch/powerpc/include/asm/book3s/64/mmu.h |  3 ++
 arch/powerpc/mm/book3s64/mmu_context.c   |  1 +
 arch/powerpc/mm/book3s64/pgtable.c       | 45 ++++++++++++++++++++++++
 3 files changed, 49 insertions(+)

diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h b/arch/powerpc/include/asm/book3s/64/mmu.h
index 23b83d3593e2..13b006e7dde4 100644
--- a/arch/powerpc/include/asm/book3s/64/mmu.h
+++ b/arch/powerpc/include/asm/book3s/64/mmu.h
@@ -116,6 +116,9 @@ typedef struct {
 	/* Number of users of the external (Nest) MMU */
 	atomic_t copros;
 
+	/* Number of running instances of lockless pagetable walk*/
+	atomic_t lockless_pgtbl_walk_count;
+
 	struct hash_mm_context *hash_context;
 
 	unsigned long vdso_base;
diff --git a/arch/powerpc/mm/book3s64/mmu_context.c b/arch/powerpc/mm/book3s64/mmu_context.c
index 2d0cb5ba9a47..3dd01c0ca5be 100644
--- a/arch/powerpc/mm/book3s64/mmu_context.c
+++ b/arch/powerpc/mm/book3s64/mmu_context.c
@@ -200,6 +200,7 @@ int init_new_context(struct task_struct *tsk, struct mm_struct *mm)
 #endif
 	atomic_set(&mm->context.active_cpus, 0);
 	atomic_set(&mm->context.copros, 0);
+	atomic_set(&mm->context.lockless_pgtbl_walk_count, 0);
 
 	return 0;
 }
diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
index 7d0e0d0d22c4..6ba6195bff1b 100644
--- a/arch/powerpc/mm/book3s64/pgtable.c
+++ b/arch/powerpc/mm/book3s64/pgtable.c
@@ -98,6 +98,51 @@ void serialize_against_pte_lookup(struct mm_struct *mm)
 	smp_call_function_many(mm_cpumask(mm), do_nothing, NULL, 1);
 }
 
+/*
+ * Counting method to monitor lockless pagetable walks:
+ * Uses start_lockless_pgtbl_walk and end_lockless_pgtbl_walk to track the
+ * number of lockless pgtable walks happening, and
+ * running_lockless_pgtbl_walk to return this value.
+ */
+
+/* start_lockless_pgtbl_walk: Must be inserted before a function call that does
+ *   lockless pagetable walks, such as __find_linux_pte()
+ */
+void start_lockless_pgtbl_walk(struct mm_struct *mm)
+{
+	atomic_inc(&mm->context.lockless_pgtbl_walk_count);
+	/* Avoid reorder to garantee that the increment will happen before any
+	 * part of the walkless pagetable walk after it.
+	 */
+	smp_mb();
+}
+EXPORT_SYMBOL(start_lockless_pgtbl_walk);
+
+/*
+ * end_lockless_pgtbl_walk: Must be inserted after the last use of a pointer
+ *   returned by a lockless pagetable walk, such as __find_linux_pte()
+*/
+void end_lockless_pgtbl_walk(struct mm_struct *mm)
+{
+	/* Avoid reorder to garantee that it will only decrement after the last
+	 * use of the returned ptep from the lockless pagetable walk.
+	 */
+	smp_mb();
+	atomic_dec(&mm->context.lockless_pgtbl_walk_count);
+}
+EXPORT_SYMBOL(end_lockless_pgtbl_walk);
+
+/*
+ * running_lockless_pgtbl_walk: Returns the number of lockless pagetable walks
+ *   currently running. If it returns 0, there is no running pagetable walk, and
+ *   THP split/collapse can be safely done. This can be used to avoid more
+ *   expensive approaches like serialize_against_pte_lookup()
+ */
+int running_lockless_pgtbl_walk(struct mm_struct *mm)
+{
+	return atomic_read(&mm->context.lockless_pgtbl_walk_count);
+}
+
 /*
  * We use this to invalidate a pmdp entry before switching from a
  * hugepte to regular pmd entry.
-- 
2.20.1