[PATCH V3 3/4] sched/numa: implement access PID reset logic

[Raghavendra K T <raghavendra.kt@amd.com>]

 This helps to ensure, only recently accessed PIDs scan the
VMAs.
Current implementation: (idea supported by PeterZ)
 1. Accessing PID information is maintained in two windows.
 access_pids[1] being newest.

 2. Reset old access PID info i.e. access_pid[0] every
(4 * sysctl_numa_balancing_scan_delay) interval after initial
scan delay period expires.

The above interval seemed to be experimentally optimum since it
avoids frequent reset of access info as well as helps clearing
the old access info regularly.
The reset logic is implemented in scan path.

Suggested-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Raghavendra K T <raghavendra.kt@amd.com>
---
 include/linux/mm.h       |  4 ++--
 include/linux/mm_types.h |  3 ++-
 kernel/sched/fair.c      | 23 +++++++++++++++++++++--
 3 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 097680aaca1e..bd07289fc68e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1394,8 +1394,8 @@ static inline void vma_set_access_pid_bit(struct vm_area_struct *vma)
 	unsigned int pid_bit;
 
 	pid_bit = current->pid % BITS_PER_LONG;
-	if (vma->numab_state && !test_bit(pid_bit, &vma->numab_state->access_pids)) {
-		__set_bit(pid_bit, &vma->numab_state->access_pids);
+	if (vma->numab_state && !test_bit(pid_bit, &vma->numab_state->access_pids[1])) {
+		__set_bit(pid_bit, &vma->numab_state->access_pids[1]);
 	}
 }
 #else /* !CONFIG_NUMA_BALANCING */
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 582523e73546..1f1f8bfeae36 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -437,7 +437,8 @@ struct anon_vma_name {
 
 struct vma_numab_state {
 	unsigned long next_scan;
-	unsigned long access_pids;
+	unsigned long next_pid_reset;
+	unsigned long access_pids[2];
 };
 
 /*
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 05490cb2d5c6..f76d5ecaf345 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2918,6 +2918,7 @@ static void reset_ptenuma_scan(struct task_struct *p)
 
 static bool vma_is_accessed(struct vm_area_struct *vma)
 {
+	unsigned long pids;
 	/*
 	 * Allow unconditional access first two times, so that all the (pages)
 	 * of VMAs get prot_none fault introduced irrespective of accesses.
@@ -2927,10 +2928,12 @@ static bool vma_is_accessed(struct vm_area_struct *vma)
 	if (READ_ONCE(current->mm->numa_scan_seq) < 2)
 		return true;
 
-	return test_bit(current->pid % BITS_PER_LONG,
-				&vma->numab_state->access_pids);
+	pids = vma->numab_state->access_pids[0] | vma->numab_state->access_pids[1];
+	return test_bit(current->pid % BITS_PER_LONG, &pids);
 }
 
+#define VMA_PID_RESET_PERIOD (4 * sysctl_numa_balancing_scan_delay)
+
 /*
  * The expensive part of numa migration is done from task_work context.
  * Triggered from task_tick_numa().
@@ -3039,6 +3042,10 @@ static void task_numa_work(struct callback_head *work)
 
 			vma->numab_state->next_scan = now +
 				msecs_to_jiffies(sysctl_numa_balancing_scan_delay);
+
+			/* Reset happens after 4 times scan delay of scan start */
+			vma->numab_state->next_pid_reset =  vma->numab_state->next_scan +
+				msecs_to_jiffies(VMA_PID_RESET_PERIOD);
 		}
 
 		/*
@@ -3053,6 +3060,18 @@ static void task_numa_work(struct callback_head *work)
 		if (!vma_is_accessed(vma))
 			continue;
 
+		/*
+		 * RESET access PIDs regularly for old VMAs. Resetting after checking
+		 * vma for recent access to avoid clearing PID info before access..
+		 */
+		if (mm->numa_scan_seq &&
+				time_after(jiffies, vma->numab_state->next_pid_reset)) {
+			vma->numab_state->next_pid_reset = vma->numab_state->next_pid_reset +
+				msecs_to_jiffies(VMA_PID_RESET_PERIOD);
+			vma->numab_state->access_pids[0] = READ_ONCE(vma->numab_state->access_pids[1]);
+			vma->numab_state->access_pids[1] = 0;
+		}
+
 		do {
 			start = max(start, vma->vm_start);
 			end = ALIGN(start + (pages << PAGE_SHIFT), HPAGE_SIZE);
-- 
2.34.1