sched/numa: Complete scanning of partial VMAs regardless of PID activity

NUMA Balancing skips VMAs when the current task has not trapped a NUMA fault within the VMA. If the VMA is skipped then mm->numa_scan_offset advances and a task that is trapping faults within the VMA may never fully update PTEs within the VMA. Force tasks to update PTEs for partially scanned PTEs. The VMA will be tagged for NUMA hints by some task but this removes some of the benefit of tracking PID activity within a VMA. A follow-on patch will mitigate this problem. The test cases and machines evaluated did not trigger the corner case so the performance results are neutral with only small changes within the noise from normal test-to-test variance. However, the next patch makes the corner case easier to trigger. Signed-off-by: Mel Gorman <[email protected]> Signed-off-by: Ingo Molnar <[email protected]> Tested-by: Raghavendra K T <[email protected]> Link: https://lore.kernel.org/r/[email protected]
noroot · Oct 10, 2023 · b7a5b53 · b7a5b53
1 parent 2e2675d
commit b7a5b53
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Showing 3 changed files with 18 additions and 4 deletions.
diff --git a/include/linux/sched/numa_balancing.h b/include/linux/sched/numa_balancing.h
@@ -21,6 +21,7 @@ enum numa_vmaskip_reason {
 	NUMAB_SKIP_INACCESSIBLE,
 	NUMAB_SKIP_SCAN_DELAY,
 	NUMAB_SKIP_PID_INACTIVE,
+	NUMAB_SKIP_IGNORE_PID,
 };
 
 #ifdef CONFIG_NUMA_BALANCING

diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
@@ -670,7 +670,8 @@ DEFINE_EVENT(sched_numa_pair_template, sched_swap_numa,
 	EM( NUMAB_SKIP_SHARED_RO,		"shared_ro" )	\
 	EM( NUMAB_SKIP_INACCESSIBLE,		"inaccessible" )	\
 	EM( NUMAB_SKIP_SCAN_DELAY,		"scan_delay" )	\
-	EMe(NUMAB_SKIP_PID_INACTIVE,		"pid_inactive" )
+	EM( NUMAB_SKIP_PID_INACTIVE,		"pid_inactive" )	\
+	EMe(NUMAB_SKIP_IGNORE_PID,		"ignore_pid_inactive" )
 
 /* Redefine for export. */
 #undef EM

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
@@ -3113,7 +3113,7 @@ static void reset_ptenuma_scan(struct task_struct *p)
 	p->mm->numa_scan_offset = 0;
 }
 
-static bool vma_is_accessed(struct vm_area_struct *vma)
+static bool vma_is_accessed(struct mm_struct *mm, struct vm_area_struct *vma)
 {
 	unsigned long pids;
 	/*
@@ -3126,7 +3126,19 @@ static bool vma_is_accessed(struct vm_area_struct *vma)
 		return true;
 
 	pids = vma->numab_state->pids_active[0] | vma->numab_state->pids_active[1];
-	return test_bit(hash_32(current->pid, ilog2(BITS_PER_LONG)), &pids);
+	if (test_bit(hash_32(current->pid, ilog2(BITS_PER_LONG)), &pids))
+		return true;
+
+	/*
+	 * Complete a scan that has already started regardless of PID access, or
+	 * some VMAs may never be scanned in multi-threaded applications:
+	 */
+	if (mm->numa_scan_offset > vma->vm_start) {
+		trace_sched_skip_vma_numa(mm, vma, NUMAB_SKIP_IGNORE_PID);
+		return true;
+	}
+
+	return false;
 }
 
 #define VMA_PID_RESET_PERIOD (4 * sysctl_numa_balancing_scan_delay)
@@ -3270,7 +3282,7 @@ static void task_numa_work(struct callback_head *work)
 		}
 
 		/* Do not scan the VMA if task has not accessed */
-		if (!vma_is_accessed(vma)) {
+		if (!vma_is_accessed(mm, vma)) {
 			trace_sched_skip_vma_numa(mm, vma, NUMAB_SKIP_PID_INACTIVE);
 			continue;
 		}