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Subject[PATCH v2 4/6] locking/lockdep: Reuse freed chain_hlocks entries
Date
Once a lock class is zapped, all the lock chains that include the zapped
class are essentially useless. The lock_chain structure itself can be
reused, but not the corresponding chain_hlocks[] entries. Over time,
we will run out of chain_hlocks entries while there are still plenty
of other lockdep array entries available.

To fix this imbalance, we have to make chain_hlocks entries reusable
just like the others. As the freed chain_hlocks entries are in blocks of
various lengths. A simple bitmap like the one used in the other reusable
lockdep arrays isn't applicable. Instead the freed chain_hlocks entries
are formed into a chain block and put into a free bucketed list according
to the block size. On allocation, the corresponding chain block bucket
is searched first before allocating directly from the chain_hlocks array.

By reusing the chain_hlocks entries, we are able to handle workloads
that add and zap a lot of lock classes without the risk of overflowing
the chain_hlocks array as long as the total number of outstanding lock
classes at any time remain within a reasonable limit.

Two new tracking counters, nr_free_chain_hlocks & nr_large_chain_blocks,
are added to track the total number of chain_hlocks entries in the
free bucketed lists and the number of large chain blocks in buckets[0]
respectively.

The nr_free_chain_hlocks counters enables us to see how efficient the
current workload is able to reuse the freed chain_hlocks entries in
the bucketed lists. The nr_large_chain_blocks counter enables to see if
we should increase the number of buckets (MAX_CHAIN_BUCKETS) available
as linear search is used to find a matching chain block which is much
slower than the O(1) time to find a chain block with size not bigger
than MAX_CHAIN_BUCKETS.

An internal nfsd test that ran for more than an hour could cause the
following message to be displayed.

[ 4318.443670] BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!

The patched kernel was able to complete the test with a lot of
chain_hlocks entries to spare:

# cat /proc/lockdep_stats
:
dependency chains: 24861 [max: 65536]
dependency chain hlocks: 102232 [max: 327680]
:
zapped classes: 1538
zapped lock chains: 57077
free chain hlocks: 335
large chain blocks: 0

By modifying MAX_CHAIN_BUCKETS to 3 in order to exercise the large
chain blocks allocation and freeing code paths, the stats were:

# cat /proc/lockdep_stats
:
dependency chains: 24561 [max: 65536]
dependency chain hlocks: 103098 [max: 327680]
:
zapped classes: 1544
zapped lock chains: 57067
free chain hlocks: 383
large chain blocks: 64

By monitoring the output of lockdep_stats, the number of free
chain hlocks and large chain blocks grew and shrank (sometimes to 0)
repetitively as the test was progressing. There was no movement in the
large chain blocks count with a MAX_CHAIN_BUCKETS value of 8.

Signed-off-by: Waiman Long <longman@redhat.com>
---
kernel/locking/lockdep.c | 232 ++++++++++++++++++++++++++---
kernel/locking/lockdep_internals.h | 2 +
kernel/locking/lockdep_proc.c | 4 +
3 files changed, 219 insertions(+), 19 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 8293f7e80ee6..f7dcd4e77a70 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -2627,6 +2627,208 @@ static DECLARE_BITMAP(lock_chains_in_use, MAX_LOCKDEP_CHAINS);
static u16 chain_hlocks[MAX_LOCKDEP_CHAIN_HLOCKS];
unsigned long nr_zapped_lock_chains;
unsigned int nr_chain_hlocks;
+unsigned int nr_free_chain_hlocks; /* Free cfhain_hlocks in buckets */
+unsigned int nr_large_chain_blocks; /* size > MAX_CHAIN_BUCKETS */
+
+#ifdef CONFIG_PROVE_LOCKING
+/*
+ * As chain_hlocks entries are allocated and freed, the freed entries are
+ * grouped into chain blocks that can be reused again. These chain blocks
+ * are put into a bucketed list according to their block size.
+ *
+ * Two APIs are provided for chain block allocation and freeing:
+ * - int alloc_chain_hlocks(int size)
+ * - void free_chain_hlocks(int base, int size)
+ *
+ * A minimum allocation size of 2 entries is enforced. When a lock chain
+ * is freed, the corresponding chain_hlocks[] entries are put into one
+ * of the bucketed lists:
+ * 2 <= size <= MAX_CHAIN_BUCKETS => buckets[size-1]
+ * > MAX_CHAIN_BUCKETS => buckets[0]
+ *
+ * When an allocation request is made, alloc_chain_hlocks() will first
+ * look up the matching bucketed list to find a free chain block. If not
+ * found, allocation will be made directly from the chain_hlocks array
+ * from the current nr_chain_hlocks value.
+ *
+ * Note that a chain_hlocks entry contain the class index which is
+ * crrently limited to 13 bits.
+ *
+ * The first 2 chain_hlocks entries in the chain block in the bucket
+ * list contains the following meta data:
+ * entry[0]:
+ * Bit 15 - always set to 1 (it is not a class index)
+ * Bits 0-14 - upper 15 bits of the next block index
+ * entry[1] - lower 16 bits of next block index
+ *
+ * A next block index of all 1 bits means it is the end of the list.
+ *
+ * On the unsized bucket (buckets[0]), the 3rd and 4th entries contain
+ * the chain block size.
+ *
+ * entry[2] - upper 16 bits of the chain block size
+ * entry[3] - lower 16 bits of the chain block size
+ *
+ * TODO:
+ * Evaluate if merging neighoring chain blocks and breaking a large chain
+ * block into smaller ones can be helpful in reducing the number of free
+ * held_hlocks entries stored in the bucketed lists.
+ */
+#define MAX_CHAIN_BUCKETS 8
+#define CHAIN_HLOCKS_MASK 0xffff
+#define CHAIN_BLK_FLAG (1U << 15)
+#define CHAIN_BLK_LIST_END ((u16)-1)
+
+/*
+ * An empty bucket has a value of -1. Otherwise, it represents an offset
+ * to the chain_hlocks array.
+ */
+static int chain_block_buckets[MAX_CHAIN_BUCKETS];
+
+static inline void init_chain_block_buckets(void)
+{
+ int i;
+
+ for (i = 0; i < MAX_CHAIN_BUCKETS; i++)
+ chain_block_buckets[i] = -1;
+}
+
+/*
+ * Return offset of next chain block or -1 if end of list.
+ */
+static inline int next_chain_block(int offset)
+{
+ if (chain_hlocks[offset] == CHAIN_BLK_LIST_END)
+ return -1;
+ return ((chain_hlocks[offset] & ~CHAIN_BLK_FLAG) << 16) |
+ chain_hlocks[offset + 1];
+}
+
+static inline void set_chain_block(int offset, int size, int next)
+{
+ if (unlikely(offset < 0)) {
+ chain_block_buckets[0] = next;
+ return;
+ }
+ chain_hlocks[offset] = (next >> 16) | CHAIN_BLK_FLAG;
+ chain_hlocks[offset + 1] = next & CHAIN_HLOCKS_MASK;
+ if (size > MAX_CHAIN_BUCKETS) {
+ chain_hlocks[offset + 2] = size >> 16;
+ chain_hlocks[offset + 3] = size & CHAIN_HLOCKS_MASK;
+ }
+}
+
+/*
+ * This function should only be called for chain block in buckets[0].
+ */
+static inline int chain_block_size(int offset)
+{
+ return (chain_hlocks[offset + 2] << 16) | chain_hlocks[offset + 3];
+}
+
+/*
+ * Return offset of a chain block of the right size or -1 if not found.
+ */
+static inline int alloc_chain_hlocks_from_buckets(int size)
+{
+ int prev, curr, next;
+
+ if (!nr_free_chain_hlocks)
+ return -1;
+
+ if (size <= MAX_CHAIN_BUCKETS) {
+ curr = chain_block_buckets[size - 1];
+ if (curr < 0)
+ return -1;
+
+ chain_block_buckets[size - 1] = next_chain_block(curr);
+ nr_free_chain_hlocks -= size;
+ return curr;
+ }
+
+ /*
+ * Look for a free chain block of the given size
+ *
+ * It is rare to have a lock chain with depth > MAX_CHAIN_BUCKETS.
+ * It is also more expensive as we may iterate the whole list
+ * without finding one.
+ */
+ prev = -1;
+ curr = chain_block_buckets[0];
+ while (curr >= 0) {
+ next = next_chain_block(curr);
+ if (chain_block_size(curr) == size) {
+ set_chain_block(prev, 0, next);
+ nr_free_chain_hlocks -= size;
+ nr_large_chain_blocks--;
+ return curr;
+ }
+ prev = curr;
+ curr = next;
+ }
+ return -1;
+}
+
+static inline void free_chain_hlocks(int base, int size)
+{
+ if (size < 2)
+ size = 2;
+
+ nr_free_chain_hlocks += size;
+ if (size <= MAX_CHAIN_BUCKETS) {
+ set_chain_block(base, 0, chain_block_buckets[size - 1]);
+ chain_block_buckets[size - 1] = base;
+ return;
+ }
+ set_chain_block(base, size, chain_block_buckets[0]);
+ chain_block_buckets[0] = base;
+ nr_large_chain_blocks++;
+}
+#else
+static inline void init_chain_block_buckets(void) { }
+static inline int alloc_chain_hlocks_from_buckets(int size)
+{
+ return -1;
+}
+#endif /* CONFIG_PROVE_LOCKING */
+
+/*
+ * The graph lock must be held before calling this function.
+ *
+ * Return: an offset to chain_hlocks if successful, or
+ * -1 with graph lock released
+ */
+static int alloc_chain_hlocks(int size)
+{
+ int curr;
+
+ if (size < 2)
+ size = 2;
+
+ curr = alloc_chain_hlocks_from_buckets(size);
+ if (curr >= 0)
+ return curr;
+
+ BUILD_BUG_ON((1UL << 24) <= ARRAY_SIZE(chain_hlocks));
+ BUILD_BUG_ON((1UL << 6) <= ARRAY_SIZE(current->held_locks));
+ BUILD_BUG_ON((1UL << 8*sizeof(chain_hlocks[0])) <=
+ ARRAY_SIZE(lock_classes));
+
+ /*
+ * Allocate directly from chain_hlocks.
+ */
+ if (likely(nr_chain_hlocks + size <= MAX_LOCKDEP_CHAIN_HLOCKS)) {
+ curr = nr_chain_hlocks;
+ nr_chain_hlocks += size;
+ return curr;
+ }
+ if (!debug_locks_off_graph_unlock())
+ return -1;
+
+ print_lockdep_off("BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!");
+ dump_stack();
+ return -1;
+}

struct lock_class *lock_chain_get_class(struct lock_chain *chain, int i)
{
@@ -2822,28 +3024,17 @@ static inline int add_chain_cache(struct task_struct *curr,
chain->irq_context = hlock->irq_context;
i = get_first_held_lock(curr, hlock);
chain->depth = curr->lockdep_depth + 1 - i;
+ j = alloc_chain_hlocks(chain->depth);
+ if (j < 0)
+ return 0;

- BUILD_BUG_ON((1UL << 24) <= ARRAY_SIZE(chain_hlocks));
- BUILD_BUG_ON((1UL << 6) <= ARRAY_SIZE(curr->held_locks));
- BUILD_BUG_ON((1UL << 8*sizeof(chain_hlocks[0])) <= ARRAY_SIZE(lock_classes));
-
- if (likely(nr_chain_hlocks + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS)) {
- chain->base = nr_chain_hlocks;
- for (j = 0; j < chain->depth - 1; j++, i++) {
- int lock_id = curr->held_locks[i].class_idx;
- chain_hlocks[chain->base + j] = lock_id;
- }
- chain_hlocks[chain->base + j] = class - lock_classes;
- nr_chain_hlocks += chain->depth;
- } else {
- if (!debug_locks_off_graph_unlock())
- return 0;
+ chain->base = j;
+ for (j = 0; j < chain->depth - 1; j++, i++) {
+ int lock_id = curr->held_locks[i].class_idx;

- print_lockdep_off("BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!");
- dump_stack();
- return 0;
+ chain_hlocks[chain->base + j] = lock_id;
}
-
+ chain_hlocks[chain->base + j] = class - lock_classes;
hlist_add_head_rcu(&chain->entry, hash_head);
debug_atomic_inc(chain_lookup_misses);
inc_chains();
@@ -4786,6 +4977,7 @@ static void remove_class_from_lock_chain(struct pending_free *pf,
return;

free_lock_chain:
+ free_chain_hlocks(chain->base, chain->depth);
/* Overwrite the chain key for concurrent RCU readers. */
WRITE_ONCE(chain->chain_key, INITIAL_CHAIN_KEY);
/*
@@ -5178,6 +5370,8 @@ EXPORT_SYMBOL_GPL(lockdep_unregister_key);

void __init lockdep_init(void)
{
+ init_chain_block_buckets();
+
printk("Lock dependency validator: Copyright (c) 2006 Red Hat, Inc., Ingo Molnar\n");

printk("... MAX_LOCKDEP_SUBCLASSES: %lu\n", MAX_LOCKDEP_SUBCLASSES);
diff --git a/kernel/locking/lockdep_internals.h b/kernel/locking/lockdep_internals.h
index 35758e469ff7..91bbc5684103 100644
--- a/kernel/locking/lockdep_internals.h
+++ b/kernel/locking/lockdep_internals.h
@@ -135,6 +135,8 @@ extern unsigned int nr_hardirq_chains;
extern unsigned int nr_softirq_chains;
extern unsigned int nr_process_chains;
extern unsigned int nr_chain_hlocks;
+extern unsigned int nr_free_chain_hlocks;
+extern unsigned int nr_large_chain_blocks;

extern unsigned int max_lockdep_depth;
extern unsigned int max_bfs_queue_depth;
diff --git a/kernel/locking/lockdep_proc.c b/kernel/locking/lockdep_proc.c
index 7c15dd03cf49..125c65e20bff 100644
--- a/kernel/locking/lockdep_proc.c
+++ b/kernel/locking/lockdep_proc.c
@@ -347,6 +347,10 @@ static int lockdep_stats_show(struct seq_file *m, void *v)
nr_zapped_classes);
seq_printf(m, " zapped lock chains: %11lu\n",
nr_zapped_lock_chains);
+ seq_printf(m, " free chain hlocks: %11u\n",
+ nr_free_chain_hlocks);
+ seq_printf(m, " large chain blocks: %11u\n",
+ nr_large_chain_blocks);
return 0;
}

--
2.18.1
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