mm,vmacache: add debug data

Introduce a CONFIG_DEBUG_VM_VMACACHE option to enable counting the cache
hit rate -- exported in /proc/vmstat.

Any updates to the caching scheme needs this kind of data, thus it can
save some work re-implementing the counting all the time.

Signed-off-by: Davidlohr Bueso <[e-mail]>
Cc: Aswin Chandramouleeswaran <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/vm_event_item.h

···
 		NR_TLB_LOCAL_FLUSH_ALL,
 		NR_TLB_LOCAL_FLUSH_ONE,
 #endif /* CONFIG_DEBUG_TLBFLUSH */
+#ifdef CONFIG_DEBUG_VM_VMACACHE
+		VMACACHE_FIND_CALLS,
+		VMACACHE_FIND_HITS,
+#endif
 		NR_VM_EVENT_ITEMS
 };
 

include/linux/vmstat.h

···
 #define count_vm_tlb_events(x, y) do { (void)(y); } while (0)
 #endif
 
+#ifdef CONFIG_DEBUG_VM_VMACACHE
+#define count_vm_vmacache_event(x) count_vm_event(x)
+#else
+#define count_vm_vmacache_event(x) do {} while (0)
+#endif
+
 #define __count_zone_vm_events(item, zone, delta) \
 		__count_vm_events(item##_NORMAL - ZONE_NORMAL + \
 		zone_idx(zone), delta)

lib/Kconfig.debug

···
 
 	  If unsure, say N.
 
+config DEBUG_VM_VMACACHE
+	bool "Debug VMA caching"
+	depends on DEBUG_VM
+	help
+	  Enable this to turn on VMA caching debug information. Doing so
+	  can cause significant overhead, so only enable it in non-production
+	  environments.
+
+	  If unsure, say N.
+
 config DEBUG_VM_RB
 	bool "Debug VM red-black trees"
 	depends on DEBUG_VM

mm/vmacache.c

···
 	if (!vmacache_valid(mm))
 		return NULL;
 
+	count_vm_vmacache_event(VMACACHE_FIND_CALLS);
+
 	for (i = 0; i < VMACACHE_SIZE; i++) {
 		struct vm_area_struct *vma = current->vmacache[i];
 
···
 			continue;
 		if (WARN_ON_ONCE(vma->vm_mm != mm))
 			break;
-		if (vma->vm_start <= addr && vma->vm_end > addr)
+		if (vma->vm_start <= addr && vma->vm_end > addr) {
+			count_vm_vmacache_event(VMACACHE_FIND_HITS);
 			return vma;
+		}
 	}
 
 	return NULL;
···
 	if (!vmacache_valid(mm))
 		return NULL;
 
+	count_vm_vmacache_event(VMACACHE_FIND_CALLS);
+
 	for (i = 0; i < VMACACHE_SIZE; i++) {
 		struct vm_area_struct *vma = current->vmacache[i];
 
-		if (vma && vma->vm_start == start && vma->vm_end == end)
+		if (vma && vma->vm_start == start && vma->vm_end == end) {
+			count_vm_vmacache_event(VMACACHE_FIND_HITS);
 			return vma;
+		}
 	}
 
 	return NULL;

mm/vmstat.c

···
 	"nr_tlb_local_flush_one",
 #endif /* CONFIG_DEBUG_TLBFLUSH */
 
+#ifdef CONFIG_DEBUG_VM_VMACACHE
+	"vmacache_find_calls",
+	"vmacache_find_hits",
+#endif
 #endif /* CONFIG_VM_EVENTS_COUNTERS */
 };
 #endif /* CONFIG_PROC_FS || CONFIG_SYSFS || CONFIG_NUMA */