[PATCH v6 10/11] powerpc/mm: Adds counting method to track lockless pagetable walks

[Leonardo Bras <leonardo@linux.ibm.com>]

Implements an additional feature to track lockless pagetable walks,
using a per-cpu counter: lockless_pgtbl_walk_counter.

Before a lockless pagetable walk, preemption is disabled and the
current cpu's counter is increased.
When the lockless pagetable walk finishes, the current cpu counter
is decreased and the preemption is enabled.

With that, it's possible to know in which cpus are happening lockless
pagetable walks, and optimize serialize_against_pte_lookup().

Implementation notes:
- Every counter can be changed only by it's CPU
- It makes use of the original memory barrier in the functions
- Any counter can be read by any CPU

Due to not locking nor using atomic variables, the impact on the
lockless pagetable walk is intended to be minimum.

Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>
---
 arch/powerpc/mm/book3s64/pgtable.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
index 535613030363..bb138b628f86 100644
--- a/arch/powerpc/mm/book3s64/pgtable.c
+++ b/arch/powerpc/mm/book3s64/pgtable.c
@@ -83,6 +83,7 @@ static void do_nothing(void *unused)
 
 }
 
+static DEFINE_PER_CPU(int, lockless_pgtbl_walk_counter);
 /*
  * Serialize against find_current_mm_pte which does lock-less
  * lookup in page tables with local interrupts disabled. For huge pages
@@ -120,6 +121,15 @@ unsigned long __begin_lockless_pgtbl_walk(bool disable_irq)
 	if (disable_irq)
 		local_irq_save(irq_mask);
 
+	/*
+	 * Counts this instance of lockless pagetable walk for this cpu.
+	 * Disables preempt to make sure there is no cpu change between
+	 * begin/end lockless pagetable walk, so that percpu counting
+	 * works fine.
+	 */
+	preempt_disable();
+	(*this_cpu_ptr(&lockless_pgtbl_walk_counter))++;
+
 	/*
 	 * This memory barrier pairs with any code that is either trying to
 	 * delete page tables, or split huge pages. Without this barrier,
@@ -158,6 +168,14 @@ inline void __end_lockless_pgtbl_walk(unsigned long irq_mask, bool enable_irq)
 	 */
 	smp_mb();
 
+	/*
+	 * Removes this instance of lockless pagetable walk for this cpu.
+	 * Enables preempt only after end lockless pagetable walk,
+	 * so that percpu counting works fine.
+	 */
+	(*this_cpu_ptr(&lockless_pgtbl_walk_counter))--;
+	preempt_enable();
+
 	/*
 	 * Interrupts must be disabled during the lockless page table walk.
 	 * That's because the deleting or splitting involves flushing TLBs,
-- 
2.24.1