[PATCH v3] vmstat: Kernel stack usage histogram

[Pasha Tatashin <pasha.tatashin@soleen.com>]

Provide a kernel stack usage histogram to aid in optimizing kernel stack
sizes and minimizing memory waste in large-scale environments. The
histogram divides stack usage into power-of-two buckets and reports the
results in /proc/vmstat. This information is especially valuable in
environments with millions of machines, where even small optimizations
can have a significant impact.

The histogram data is presented in /proc/vmstat with entries like
"kstack_1k", "kstack_2k", and so on, indicating the number of threads
that exited with stack usage falling within each respective bucket.

Example outputs:
Intel:
$ grep kstack /proc/vmstat
kstack_1k 3
kstack_2k 188
kstack_4k 11391
kstack_8k 243
kstack_16k 0

ARM with 64K page_size:
$ grep kstack /proc/vmstat
kstack_1k 1
kstack_2k 340
kstack_4k 25212
kstack_8k 1659
kstack_16k 0
kstack_32k 0
kstack_64k 0

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
Changelog:
v3:
- Changed from page counts to power-of-two buckets, this is helpful for
builds with large base pages (i.e. arm64 with 64K pages) to evaluate
kernel stack internal fragmentation.

 include/linux/sched/task_stack.h | 49 ++++++++++++++++++++++++++++++--
 include/linux/vm_event_item.h    | 42 +++++++++++++++++++++++++++
 include/linux/vmstat.h           | 16 -----------
 mm/vmstat.c                      | 24 ++++++++++++++++
 4 files changed, 113 insertions(+), 18 deletions(-)

diff --git a/include/linux/sched/task_stack.h b/include/linux/sched/task_stack.h
index ccd72b978e1f..65e8c9fb7f9b 100644
--- a/include/linux/sched/task_stack.h
+++ b/include/linux/sched/task_stack.h
@@ -95,9 +95,51 @@ static inline int object_is_on_stack(const void *obj)
 extern void thread_stack_cache_init(void);
 
 #ifdef CONFIG_DEBUG_STACK_USAGE
+#ifdef CONFIG_VM_EVENT_COUNTERS
+#include <linux/vm_event_item.h>
+
+/* Count the maximum pages reached in kernel stacks */
+static inline void kstack_histogram(unsigned long used_stack)
+{
+	if (used_stack <= 1024)
+		this_cpu_inc(vm_event_states.event[KSTACK_1K]);
+#if THREAD_SIZE > 1024
+	else if (used_stack <= 2048)
+		this_cpu_inc(vm_event_states.event[KSTACK_2K]);
+#endif
+#if THREAD_SIZE > 2048
+	else if (used_stack <= 4096)
+		this_cpu_inc(vm_event_states.event[KSTACK_4K]);
+#endif
+#if THREAD_SIZE > 4096
+	else if (used_stack <= 8192)
+		this_cpu_inc(vm_event_states.event[KSTACK_8K]);
+#endif
+#if THREAD_SIZE > 8192
+	else if (used_stack <= 16384)
+		this_cpu_inc(vm_event_states.event[KSTACK_16K]);
+#endif
+#if THREAD_SIZE > 16384
+	else if (used_stack <= 32768)
+		this_cpu_inc(vm_event_states.event[KSTACK_32K]);
+#endif
+#if THREAD_SIZE > 32768
+	else if (used_stack <= 65536)
+		this_cpu_inc(vm_event_states.event[KSTACK_64K]);
+#endif
+#if THREAD_SIZE > 65536
+	else
+		this_cpu_inc(vm_event_states.event[KSTACK_REST]);
+#endif
+}
+#else /* !CONFIG_VM_EVENT_COUNTERS */
+static inline void kstack_histogram(unsigned long used_stack) {}
+#endif /* CONFIG_VM_EVENT_COUNTERS */
+
 static inline unsigned long stack_not_used(struct task_struct *p)
 {
 	unsigned long *n = end_of_stack(p);
+	unsigned long unused_stack;
 
 	do { 	/* Skip over canary */
 # ifdef CONFIG_STACK_GROWSUP
@@ -108,10 +150,13 @@ static inline unsigned long stack_not_used(struct task_struct *p)
 	} while (!*n);
 
 # ifdef CONFIG_STACK_GROWSUP
-	return (unsigned long)end_of_stack(p) - (unsigned long)n;
+	unused_stack = (unsigned long)end_of_stack(p) - (unsigned long)n;
 # else
-	return (unsigned long)n - (unsigned long)end_of_stack(p);
+	unused_stack = (unsigned long)n - (unsigned long)end_of_stack(p);
 # endif
+	kstack_histogram(THREAD_SIZE - unused_stack);
+
+	return unused_stack;
 }
 #endif
 extern void set_task_stack_end_magic(struct task_struct *tsk);
diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index 747943bc8cc2..73fa5fbf33a3 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h
@@ -154,9 +154,51 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 		VMA_LOCK_RETRY,
 		VMA_LOCK_MISS,
 #endif
+#ifdef CONFIG_DEBUG_STACK_USAGE
+		KSTACK_1K,
+#if THREAD_SIZE > 1024
+		KSTACK_2K,
+#endif
+#if THREAD_SIZE > 2048
+		KSTACK_4K,
+#endif
+#if THREAD_SIZE > 4096
+		KSTACK_8K,
+#endif
+#if THREAD_SIZE > 8192
+		KSTACK_16K,
+#endif
+#if THREAD_SIZE > 16384
+		KSTACK_32K,
+#endif
+#if THREAD_SIZE > 32768
+		KSTACK_64K,
+#endif
+#if THREAD_SIZE > 65536
+		KSTACK_REST,
+#endif
+#endif /* CONFIG_DEBUG_STACK_USAGE */
 		NR_VM_EVENT_ITEMS
 };
 
+#ifdef CONFIG_VM_EVENT_COUNTERS
+/*
+ * Light weight per cpu counter implementation.
+ *
+ * Counters should only be incremented and no critical kernel component
+ * should rely on the counter values.
+ *
+ * Counters are handled completely inline. On many platforms the code
+ * generated will simply be the increment of a global address.
+ */
+
+struct vm_event_state {
+	unsigned long event[NR_VM_EVENT_ITEMS];
+};
+
+DECLARE_PER_CPU(struct vm_event_state, vm_event_states);
+#endif
+
 #ifndef CONFIG_TRANSPARENT_HUGEPAGE
 #define THP_FILE_ALLOC ({ BUILD_BUG(); 0; })
 #define THP_FILE_FALLBACK ({ BUILD_BUG(); 0; })
diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
index 343906a98d6e..18d4a97d3afd 100644
--- a/include/linux/vmstat.h
+++ b/include/linux/vmstat.h
@@ -41,22 +41,6 @@ enum writeback_stat_item {
 };
 
 #ifdef CONFIG_VM_EVENT_COUNTERS
-/*
- * Light weight per cpu counter implementation.
- *
- * Counters should only be incremented and no critical kernel component
- * should rely on the counter values.
- *
- * Counters are handled completely inline. On many platforms the code
- * generated will simply be the increment of a global address.
- */
-
-struct vm_event_state {
-	unsigned long event[NR_VM_EVENT_ITEMS];
-};
-
-DECLARE_PER_CPU(struct vm_event_state, vm_event_states);
-
 /*
  * vm counters are allowed to be racy. Use raw_cpu_ops to avoid the
  * local_irq_disable overhead.
diff --git a/mm/vmstat.c b/mm/vmstat.c
index db79935e4a54..21932bd6a449 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1413,6 +1413,30 @@ const char * const vmstat_text[] = {
 	"vma_lock_retry",
 	"vma_lock_miss",
 #endif
+#ifdef CONFIG_DEBUG_STACK_USAGE
+	"kstack_1k",
+#if THREAD_SIZE > 1024
+	"kstack_2k",
+#endif
+#if THREAD_SIZE > 2048
+	"kstack_4k",
+#endif
+#if THREAD_SIZE > 4096
+	"kstack_8k",
+#endif
+#if THREAD_SIZE > 8192
+	"kstack_16k",
+#endif
+#if THREAD_SIZE > 16384
+	"kstack_32k",
+#endif
+#if THREAD_SIZE > 32768
+	"kstack_64k",
+#endif
+#if THREAD_SIZE > 65536
+	"kstack_rest",
+#endif
+#endif
 #endif /* CONFIG_VM_EVENT_COUNTERS || CONFIG_MEMCG */
 };
 #endif /* CONFIG_PROC_FS || CONFIG_SYSFS || CONFIG_NUMA || CONFIG_MEMCG */
-- 
2.45.1.288.g0e0cd299f1-goog