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SubjectRe: [PATCH] lockdep: Optimize the memory usage of circular queue
Ping ;-)

Regards,
Boqun

On Thu, Sep 17, 2020 at 04:01:50PM +0800, Boqun Feng wrote:
> Qian Cai reported a BFS_EQUEUEFULL warning [1] after read recursive
> deadlock detection merged into tip tree recently. Unlike the previous
> lockep graph searching, which iterate every lock class (every node in
> the graph) exactly once, the graph searching for read recurisve deadlock
> detection needs to iterate every lock dependency (every edge in the
> graph) once, as a result, the maximum memory cost of the circular queue
> changes from O(V), where V is the number of lock classes (nodes or
> vertices) in the graph, to O(E), where E is the number of lock
> dependencies (edges), because every lock class or dependency gets
> enqueued once in the BFS. Therefore we hit the BFS_EQUEUEFULL case.
>
> However, actually we don't need to enqueue all dependencies for the BFS,
> because every time we enqueue a dependency, we almostly enqueue all
> other dependencies in the same dependency list ("almostly" is because
> we currently check before enqueue, so if a dependency doesn't pass the
> check stage we won't enqueue it, however, we can always do in reverse
> ordering), based on this, we can only enqueue the first dependency from
> a dependency list and every time we want to fetch a new dependency to
> work, we can either:
>
> 1) fetch the dependency next to the current dependency in the
> dependency list
> or
> 2) if the dependency in 1) doesn't exist, fetch the dependency from
> the queue.
>
> With this approach, the "max bfs queue depth" for a x86_64_defconfig +
> lockdep and selftest config kernel can get descreased from:
>
> max bfs queue depth: 201
>
> to (after apply this patch)
>
> max bfs queue depth: 61
>
> While I'm at it, clean up the code logic a little (e.g. directly return
> other than set a "ret" value and goto the "exit" label).
>
> [1]: https://lore.kernel.org/lkml/17343f6f7f2438fc376125384133c5ba70c2a681.camel@redhat.com/
>
> Reported-by: Qian Cai <cai@redhat.com>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
> kernel/locking/lockdep.c | 108 ++++++++++++++++++++++++---------------
> 1 file changed, 67 insertions(+), 41 deletions(-)
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index cccf4bc759c6..761c2327e9cf 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -1640,35 +1640,22 @@ static enum bfs_result __bfs(struct lock_list *source_entry,
> int offset)
> {
> struct lock_list *entry;
> - struct lock_list *lock;
> + struct lock_list *lock = NULL;
> struct list_head *head;
> struct circular_queue *cq = &lock_cq;
> - enum bfs_result ret = BFS_RNOMATCH;
>
> lockdep_assert_locked();
>
> - if (match(source_entry, data)) {
> - *target_entry = source_entry;
> - ret = BFS_RMATCH;
> - goto exit;
> - }
> -
> - head = get_dep_list(source_entry, offset);
> - if (list_empty(head))
> - goto exit;
> -
> __cq_init(cq);
> __cq_enqueue(cq, source_entry);
>
> - while ((lock = __cq_dequeue(cq))) {
> - bool prev_only_xr;
> -
> - if (!lock->class) {
> - ret = BFS_EINVALIDNODE;
> - goto exit;
> - }
> + while (lock || (lock = __cq_dequeue(cq))) {
> + if (!lock->class)
> + return BFS_EINVALIDNODE;
>
> /*
> + * Step 1: check whether we already finish on this one.
> + *
> * If we have visited all the dependencies from this @lock to
> * others (iow, if we have visited all lock_list entries in
> * @lock->class->locks_{after,before}) we skip, otherwise go
> @@ -1676,17 +1663,17 @@ static enum bfs_result __bfs(struct lock_list *source_entry,
> * list accessed.
> */
> if (lock_accessed(lock))
> - continue;
> + goto next;
> else
> mark_lock_accessed(lock);
>
> - head = get_dep_list(lock, offset);
> -
> - prev_only_xr = lock->only_xr;
> -
> - list_for_each_entry_rcu(entry, head, entry) {
> - unsigned int cq_depth;
> - u8 dep = entry->dep;
> + /*
> + * Step 2: check whether prev dependency and this form a strong
> + * dependency path.
> + */
> + if (lock->parent) { /* Parent exists, check prev dependency */
> + u8 dep = lock->dep;
> + bool prev_only_xr = lock->parent->only_xr;
>
> /*
> * Mask out all -(S*)-> if we only have *R in previous
> @@ -1698,29 +1685,68 @@ static enum bfs_result __bfs(struct lock_list *source_entry,
>
> /* If nothing left, we skip */
> if (!dep)
> - continue;
> + goto next;
>
> /* If there are only -(*R)-> left, set that for the next step */
> - entry->only_xr = !(dep & (DEP_SN_MASK | DEP_EN_MASK));
> + lock->only_xr = !(dep & (DEP_SN_MASK | DEP_EN_MASK));
> + }
>
> - visit_lock_entry(entry, lock);
> - if (match(entry, data)) {
> - *target_entry = entry;
> - ret = BFS_RMATCH;
> - goto exit;
> - }
> + /*
> + * Step 3: we haven't visited this and there is a strong
> + * dependency path to this, so check with @match.
> + */
> + if (match(lock, data)) {
> + *target_entry = lock;
> + return BFS_RMATCH;
> + }
> +
> + /*
> + * Step 4: if not match, expand the path by adding the
> + * afterwards or backwards dependencis in the search
> + *
> + * Note we only enqueue the first of the list into the queue,
> + * because we can always find a sibling dependency from one
> + * (see label 'next'), as a result the space of queue is saved.
> + */
> + head = get_dep_list(lock, offset);
> + entry = list_first_or_null_rcu(head, struct lock_list, entry);
> + if (entry) {
> + unsigned int cq_depth;
> +
> + if (__cq_enqueue(cq, entry))
> + return BFS_EQUEUEFULL;
>
> - if (__cq_enqueue(cq, entry)) {
> - ret = BFS_EQUEUEFULL;
> - goto exit;
> - }
> cq_depth = __cq_get_elem_count(cq);
> if (max_bfs_queue_depth < cq_depth)
> max_bfs_queue_depth = cq_depth;
> }
> +
> + /*
> + * Update the ->parent, so when @entry is iterated, we know the
> + * previous dependency.
> + */
> + list_for_each_entry_rcu(entry, head, entry)
> + visit_lock_entry(entry, lock);
> +next:
> + /*
> + * Step 5: fetch the next dependency to process.
> + *
> + * If there is a previous dependency, we fetch the sibling
> + * dependency in the dep list of previous dependency.
> + *
> + * Otherwise set @lock to NULL to fetch the next entry from
> + * queue.
> + */
> + if (lock->parent) {
> + head = get_dep_list(lock->parent, offset);
> + lock = list_next_or_null_rcu(head, &lock->entry,
> + struct lock_list, entry);
> + } else {
> + lock = NULL;
> + }
> }
> -exit:
> - return ret;
> +
> + return BFS_RNOMATCH;
> }
>
> static inline enum bfs_result
> --
> 2.28.0
>

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