Messages in this thread | | | Date | Thu, 24 Sep 2020 23:12:26 +0800 | From | Boqun Feng <> | Subject | Re: [PATCH] lockdep: Optimize the memory usage of circular queue |
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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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