lkml.org 
[lkml]   [2020]   [Feb]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH v5 6/7] 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 chain_hlocks
entries are put into bucketed lists (MAX_CHAIN_BUCKETS) of chain blocks.
Bucket 0 is the dump bucket which houses chain blocks of size larger
than MAX_CHAIN_BUCKETS. Initially, the whole array is in one chain
block in bucket 0.

Allocation requests for the chain_hlocks are fulfilled first by looking
for chain block of matching size. If not found, a larger chain block
is split. As the minimum size of a chain block is 2 chain_hlocks[]
entries. That will be the minimum allocation size. In other word,
allocation requests for one chain_hlocks[] entry will cause 2-entry block
to be returned and hence 1 entry will be wasted. When the chain_hlocks[]
array is nearly full or highly fragmented, there is a slight chance
that 1-entry fragment will be created during the chain block splitting
process. Those 1-entry fragments will be discarded and hence leaked.

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 replaces nr_chain_hlocks.

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 and kept on
loading and unloading kernel modules 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: 26723 [max: 65536]
dependency chain hlocks: 115052 [max: 327680]
:
zapped classes: 1538
zapped lock chains: 56990
large chain blocks: 0

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Waiman Long <longman@redhat.com>
---
kernel/locking/lockdep.c | 242 +++++++++++++++++++++++++++--
kernel/locking/lockdep_internals.h | 3 +-
kernel/locking/lockdep_proc.c | 9 +-
3 files changed, 239 insertions(+), 15 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index a63976c75253..70288bb2331d 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1071,13 +1071,15 @@ static inline void check_data_structures(void) { }

#endif /* CONFIG_DEBUG_LOCKDEP */

+static void init_chain_block_buckets(void);
+
/*
* Initialize the lock_classes[] array elements, the free_lock_classes list
* and also the delayed_free structure.
*/
static void init_data_structures_once(void)
{
- static bool ds_initialized, rcu_head_initialized;
+ static bool __read_mostly ds_initialized, rcu_head_initialized;
int i;

if (likely(rcu_head_initialized))
@@ -1101,6 +1103,7 @@ static void init_data_structures_once(void)
INIT_LIST_HEAD(&lock_classes[i].locks_after);
INIT_LIST_HEAD(&lock_classes[i].locks_before);
}
+ init_chain_block_buckets();
}

static inline struct hlist_head *keyhashentry(const struct lock_class_key *key)
@@ -2627,7 +2630,221 @@ struct lock_chain lock_chains[MAX_LOCKDEP_CHAINS];
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 */
+
+/*
+ * 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 (bucket-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
+ */
+#define MAX_CHAIN_BUCKETS 10
+#define CHAIN_BLK_FLAG (1U << 15)
+#define CHAIN_BLK_LIST_END 0xFFFFU
+
+static int chain_block_buckets[MAX_CHAIN_BUCKETS];
+
+static inline int size_to_bucket(int size)
+{
+ if (size > MAX_CHAIN_BUCKETS)
+ return 0;
+
+ return size - 1;
+}
+
+/*
+ * Iterate all the chain blocks in a bucket.
+ * The loop body has to set the next parameter.
+ */
+#define for_each_chain_block(bucket, prev, curr, next) \
+ for ((prev) = bucket - MAX_CHAIN_BUCKETS, \
+ (curr) = chain_block_buckets[bucket]; \
+ (curr) >= 0; \
+ (prev) = (curr), (curr) = (next))
+
+/*
+ * next block or -1
+ */
+static inline int chain_block_next(int offset)
+{
+ int next = chain_hlocks[offset];
+
+ WARN_ON_ONCE(!(next & CHAIN_BLK_FLAG));
+
+ if (next == CHAIN_BLK_LIST_END)
+ return -1;
+
+ next &= ~CHAIN_BLK_FLAG;
+ next <<= 16;
+ next |= chain_hlocks[offset + 1];
+
+ return next;
+}
+
+/*
+ * bucket-0 only
+ */
+static inline int chain_block_size(int offset)
+{
+ return (chain_hlocks[offset + 2] << 16) | chain_hlocks[offset + 3];
+}
+
+static inline void init_chain_block(int offset, int next, int bucket, int size)
+{
+ chain_hlocks[offset] = (next >> 16) | CHAIN_BLK_FLAG;
+ chain_hlocks[offset + 1] = (u16)next;
+
+ if (bucket == 0) {
+ chain_hlocks[offset + 2] = size >> 16;
+ chain_hlocks[offset + 3] = (u16)size;
+ }
+}
+
+static inline void add_chain_block(int offset, int size)
+{
+ int bucket = size_to_bucket(size);
+ int next = chain_block_buckets[bucket];
+
+ if (unlikely(size < 2))
+ return; // XXX leak!
+
+ init_chain_block(offset, next, bucket, size);
+ chain_block_buckets[bucket] = offset;
+ nr_free_chain_hlocks += size;
+
+ if (bucket == 0)
+ nr_large_chain_blocks++;
+}
+
+/*
+ * When offset < 0, the bucket number is encoded in it.
+ */
+static inline void del_chain_block(int offset, int size, int next)
+{
+ int bucket = size_to_bucket(size);
+
+ nr_free_chain_hlocks -= size;
+ if (offset < 0)
+ chain_block_buckets[offset + MAX_CHAIN_BUCKETS] = next;
+ else
+ init_chain_block(offset, next, bucket, size);
+
+ if (bucket == 0)
+ nr_large_chain_blocks--;
+}
+
+static void init_chain_block_buckets(void)
+{
+ int i;
+
+ for (i = 0; i < MAX_CHAIN_BUCKETS; i++)
+ chain_block_buckets[i] = -1;
+
+ add_chain_block(0, ARRAY_SIZE(chain_hlocks));
+}
+
+/*
+ * Return offset of a chain block of the right size or -1 if not found.
+ *
+ * Fairly simple worst-fit allocator with the addition of a number of size
+ * specific free lists.
+ */
+static int alloc_chain_hlocks(int req)
+{
+ int max_prev, max_curr, max_next, max_size = INT_MIN;
+ int prev, curr, next, size;
+ int bucket;
+
+ /*
+ * We rely on the MSB to act as an escape bit to denote freelist
+ * pointers. Make sure this bit isn't set in 'normal' class_idx usage.
+ */
+ BUILD_BUG_ON((MAX_LOCKDEP_KEYS-1) & CHAIN_BLK_FLAG);
+
+ init_data_structures_once();
+
+ if (!nr_free_chain_hlocks)
+ return -1;
+
+ /*
+ * We require a minimum of 2 (u16) entries to encode a freelist
+ * 'pointer'.
+ */
+ req = max(req, 2);
+ bucket = size_to_bucket(req);
+
+ if (bucket != 0) {
+ curr = chain_block_buckets[bucket];
+ if (curr < 0)
+ goto search;
+
+ del_chain_block(bucket - MAX_CHAIN_BUCKETS, req,
+ chain_block_next(curr));
+ return curr;
+ }
+
+search:
+ /*
+ * linear search in the 'dump' bucket; look for an exact match or the
+ * largest block.
+ */
+ for_each_chain_block(0, prev, curr, next) {
+ size = chain_block_size(curr);
+ next = chain_block_next(curr);
+
+ if (size == req) {
+ del_chain_block(prev, req, next);
+ return curr;
+ }
+
+ if (size > max_size) {
+ max_prev = prev;
+ max_curr = curr;
+ max_next = next;
+ max_size = size;
+ }
+ }
+
+ if (likely(max_size > req)) {
+ del_chain_block(max_prev, max_size, max_next);
+ add_chain_block(max_curr + req, max_size - req);
+ return max_curr;
+ }
+
+ /*
+ * Last resort, split a block in a larger sized bucket.
+ */
+ for (size = MAX_CHAIN_BUCKETS; size > req; size--) {
+ bucket = size_to_bucket(size);
+ curr = chain_block_buckets[bucket];
+ if (curr < 0)
+ continue;
+
+ del_chain_block(bucket - MAX_CHAIN_BUCKETS, size,
+ chain_block_next(curr));
+ add_chain_block(curr + req, size - req);
+ return curr;
+ }
+
+ return -1;
+}
+
+static inline void free_chain_hlocks(int base, int size)
+{
+ add_chain_block(base, max(size, 2));
+}

struct lock_class *lock_chain_get_class(struct lock_chain *chain, int i)
{
@@ -2828,15 +3045,8 @@ static inline int add_chain_cache(struct task_struct *curr,
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 {
+ j = alloc_chain_hlocks(chain->depth);
+ if (j < 0) {
if (!debug_locks_off_graph_unlock())
return 0;

@@ -2845,6 +3055,13 @@ static inline int add_chain_cache(struct task_struct *curr,
return 0;
}

+ chain->base = j;
+ 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;
hlist_add_head_rcu(&chain->entry, hash_head);
debug_atomic_inc(chain_lookup_misses);
inc_chains(chain->irq_context);
@@ -2991,6 +3208,8 @@ static inline int validate_chain(struct task_struct *curr,
{
return 1;
}
+
+static void init_chain_block_buckets(void) { }
#endif /* CONFIG_PROVE_LOCKING */

/*
@@ -4788,6 +5007,7 @@ 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);
dec_chains(chain->irq_context);
diff --git a/kernel/locking/lockdep_internals.h b/kernel/locking/lockdep_internals.h
index af722ceeda33..5040e6729c05 100644
--- a/kernel/locking/lockdep_internals.h
+++ b/kernel/locking/lockdep_internals.h
@@ -140,7 +140,8 @@ extern unsigned long nr_stack_trace_entries;
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 92fe0742453c..14932ea50317 100644
--- a/kernel/locking/lockdep_proc.c
+++ b/kernel/locking/lockdep_proc.c
@@ -137,7 +137,7 @@ static int lc_show(struct seq_file *m, void *v)
};

if (v == SEQ_START_TOKEN) {
- if (nr_chain_hlocks > MAX_LOCKDEP_CHAIN_HLOCKS)
+ if (!nr_free_chain_hlocks)
seq_printf(m, "(buggered) ");
seq_printf(m, "all lock chains:\n");
return 0;
@@ -278,8 +278,9 @@ static int lockdep_stats_show(struct seq_file *m, void *v)
#ifdef CONFIG_PROVE_LOCKING
seq_printf(m, " dependency chains: %11lu [max: %lu]\n",
lock_chain_count(), MAX_LOCKDEP_CHAINS);
- seq_printf(m, " dependency chain hlocks: %11u [max: %lu]\n",
- nr_chain_hlocks, MAX_LOCKDEP_CHAIN_HLOCKS);
+ seq_printf(m, " dependency chain hlocks: %11lu [max: %lu]\n",
+ MAX_LOCKDEP_CHAIN_HLOCKS - nr_free_chain_hlocks,
+ MAX_LOCKDEP_CHAIN_HLOCKS);
#endif

#ifdef CONFIG_TRACE_IRQFLAGS
@@ -352,6 +353,8 @@ static int lockdep_stats_show(struct seq_file *m, void *v)
#ifdef CONFIG_PROVE_LOCKING
seq_printf(m, " zapped lock chains: %11lu\n",
nr_zapped_lock_chains);
+ seq_printf(m, " large chain blocks: %11u\n",
+ nr_large_chain_blocks);
#endif
return 0;
}
--
2.18.1
\
 
 \ /
  Last update: 2020-02-03 17:43    [W:0.135 / U:3.364 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site