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SubjectRe: [PATCH] lockdep: Optimize the memory usage of circular queue
On Mon, Sep 28, 2020 at 10:51:04AM +0200, Peter Zijlstra wrote:
> On Thu, Sep 17, 2020 at 04:01:50PM +0800, Boqun Feng wrote:
>
> > __cq_init(cq);
> > __cq_enqueue(cq, source_entry);
> >
> > + 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
>
> > @@ -1698,29 +1685,68 @@ static enum bfs_result __bfs(struct lock_list *source_entry,
> >
> > /* If nothing left, we skip */
> > if (!dep)
> > + goto next;
> >
> > /* If there are only -(*R)-> left, set that for the next step */
> > + lock->only_xr = !(dep & (DEP_SN_MASK | DEP_EN_MASK));
> > + }
> >
> > + /*
> > + * 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;
> >
> > 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);
>
> This confused me for a while. I think it might be a little clearer if we
> put this inside the previous block.
>
> Alternatively, we could try and write it something like so:
>
> /*
> * Step 4: if not match, expand the path by adding the
> * afterwards or backwards dependencis in the search
> */
> first = true;
> head = get_dep_list(lock, offset);
> list_for_each_entry_rcu(entry, head, entry) {
> visit_lock_entry(entry, lock);
>
> if (!first)
> continue;
>
> /*
> * Only enqueue the first entry of the list,
> * we'll iterate it's siblings at the next
> * label.
> */
> first = false;
> if (__cq_enqueue(cq, entry))
> return BFS_EQUEUEFULL;
>
> cq_depth = __cq_get_elem_count(cq);
> if (max_bfs_queue_depth < cq_depth)
> max_bfs_queue_depth = cq_depth;
> }
>
> Hmm?
>

Better than mine ;-)

> > +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;
> > + }
>
> I think that if we hide this in a __bfs_next() helper, the iteration
> becomes nicer.
>
> > }
> > -exit:
> > +
> > + return BFS_RNOMATCH;
> > }
>
> How's this?
>

I think your version is better and should be functionally identical to
mine, also FWIW, I tested with a lockdep boot selftests, everything
worked fine.

Regards,
Boqun

> ---
> Subject: lockdep: Optimize the memory usage of circular queue
> From: Boqun Feng <boqun.feng@gmail.com>
> Date: Thu, 17 Sep 2020 16:01:50 +0800
>
> From: Boqun Feng <boqun.feng@gmail.com>
>
> 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>
> Reported-by: syzbot+62ebe501c1ce9a91f68c@syzkaller.appspotmail.com
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: https://lkml.kernel.org/r/20200917080210.108095-1-boqun.feng@gmail.com
> ---
> kernel/locking/lockdep.c | 101 ++++++++++++++++++++++++++++-------------------
> 1 file changed, 61 insertions(+), 40 deletions(-)
>
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -1606,6 +1606,15 @@ static inline void bfs_init_rootb(struct
> lock->only_xr = (hlock->read != 0);
> }
>
> +static inline struct lock_list *__bfs_next(struct lock_list *lock, int offset)
> +{
> + if (!lock || !lock->parent)
> + return NULL;
> +
> + return list_next_or_null_rcu(get_dep_list(lock->parent, offset),
> + &lock->entry, struct lock_list, entry);
> +}
> +
> /*
> * Breadth-First Search to find a strong path in the dependency graph.
> *
> @@ -1639,36 +1648,25 @@ static enum bfs_result __bfs(struct lock
> struct lock_list **target_entry,
> int offset)
> {
> + struct circular_queue *cq = &lock_cq;
> + struct lock_list *lock = NULL;
> struct lock_list *entry;
> - struct lock_list *lock;
> struct list_head *head;
> - struct circular_queue *cq = &lock_cq;
> - enum bfs_result ret = BFS_RNOMATCH;
> + unsigned int cq_depth;
> + bool first;
>
> 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 = __bfs_next(lock, offset)) || (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
> @@ -1680,13 +1678,13 @@ static enum bfs_result __bfs(struct lock
> 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
> @@ -1701,26 +1699,49 @@ static enum bfs_result __bfs(struct lock
> continue;
>
> /* 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));
> + }
> +
> + /*
> + * 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
> + *
> + */
> + first = true;
> + head = get_dep_list(lock, offset);
> + list_for_each_entry_rcu(entry, head, entry) {
> visit_lock_entry(entry, lock);
> - if (match(entry, data)) {
> - *target_entry = entry;
> - ret = BFS_RMATCH;
> - goto exit;
> - }
> -
> - if (__cq_enqueue(cq, entry)) {
> - ret = BFS_EQUEUEFULL;
> - goto exit;
> - }
> +
> + /*
> + * Note we only enqueue the first of the list into the
> + * queue, because we can always find a sibling
> + * dependency from one (see _bfs_next()), as a result
> + * the space of queue is saved.
> + */
> + if (!first)
> + continue;
> +
> + first = false;
> +
> + if (__cq_enqueue(cq, entry))
> + return BFS_EQUEUEFULL;
> +
> cq_depth = __cq_get_elem_count(cq);
> if (max_bfs_queue_depth < cq_depth)
> max_bfs_queue_depth = cq_depth;
> }
> }
> -exit:
> - return ret;
> +
> + return BFS_RNOMATCH;
> }
>
> static inline enum bfs_result

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