[tip:locking/core] locking/lockdep: Avoid creating redundant links

tip-bot for Peter Zijlstra
2017-08-10T12:21:46Z

Commit-ID: ae813308f4630642d2c1c87553929ce95f29f9ef
Gitweb: http://git.kernel.org/tip/ae813308f4630642d2c1c87553929ce95f29f9ef
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Fri, 3 Mar 2017 10:13:38 +0100
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 10 Aug 2017 12:29:04 +0200

locking/lockdep: Avoid creating redundant links

Two boots + a make defconfig, the first didn't have the redundant bit
in, the second did:

lock-classes: 1168 1169 [max: 8191]
direct dependencies: 7688 5812 [max: 32768]
indirect dependencies: 25492 25937
all direct dependencies: 220113 217512
dependency chains: 9005 9008 [max: 65536]
dependency chain hlocks: 34450 34366 [max: 327680]
in-hardirq chains: 55 51
in-softirq chains: 371 378
in-process chains: 8579 8579
stack-trace entries: 108073 88474 [max: 524288]
combined max dependencies: 178738560 169094640

max locking depth: 15 15
max bfs queue depth: 320 329

cyclic checks: 9123 9190

redundant checks: 5046
redundant links: 1828

find-mask forwards checks: 2564 2599
find-mask backwards checks: 39521 39789

So it saves nearly 2k links and a fair chunk of stack-trace entries, but
as expected, makes no real difference on the indirect dependencies.

At the same time, you see the max BFS depth increase, which is also
expected, although it could easily be boot variance -- these numbers are
not entirely stable between boots.

The down side is that the cycles in the graph become larger and thus
the reports harder to read.

XXX: do we want this as a CONFIG variable, implied by LOCKDEP_SMALL?

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Byungchul Park <byungchul.park@lge.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Nikolay Borisov <nborisov@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: akpm@linux-foundation.org
Cc: boqun.feng@gmail.com
Cc: iamjoonsoo.kim@lge.com
Cc: kernel-team@lge.com
Cc: kirill@shutemov.name
Cc: npiggin@gmail.com
Cc: walken@google.com
Link: http://lkml.kernel.org/r/20170303091338.GH6536@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
kernel/locking/lockdep.c | 27 +++++++++++++++++++++++++++
kernel/locking/lockdep_internals.h | 2 ++
kernel/locking/lockdep_proc.c | 4 ++++
3 files changed, 33 insertions(+)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 986f2fa7..b2dd313 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1307,6 +1307,19 @@ check_noncircular(struct lock_list *root, struct lock_class *target,
return result;
}

+static noinline int
+check_redundant(struct lock_list *root, struct lock_class *target,
+ struct lock_list **target_entry)
+{
+ int result;
+
+ debug_atomic_inc(nr_redundant_checks);
+
+ result = __bfs_forwards(root, target, class_equal, target_entry);
+
+ return result;
+}
+
#if defined(CONFIG_TRACE_IRQFLAGS) && defined(CONFIG_PROVE_LOCKING)
/*
* Forwards and backwards subgraph searching, for the purposes of
@@ -1872,6 +1885,20 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
}
}

+ /*
+ * Is the <prev> -> <next> link redundant?
+ */
+ this.class = hlock_class(prev);
+ this.parent = NULL;
+ ret = check_redundant(&this, hlock_class(next), &target_entry);
+ if (!ret) {
+ debug_atomic_inc(nr_redundant);
+ return 2;
+ }
+ if (ret < 0)
+ return print_bfs_bug(ret);
+
+
if (!*stack_saved) {
if (!save_trace(&trace))
return 0;
diff --git a/kernel/locking/lockdep_internals.h b/kernel/locking/lockdep_internals.h
index c08fbd2..1da4669 100644
--- a/kernel/locking/lockdep_internals.h
+++ b/kernel/locking/lockdep_internals.h
@@ -143,6 +143,8 @@ struct lockdep_stats {
int redundant_softirqs_on;
int redundant_softirqs_off;
int nr_unused_locks;
+ int nr_redundant_checks;
+ int nr_redundant;
int nr_cyclic_checks;
int nr_cyclic_check_recursions;
int nr_find_usage_forwards_checks;
diff --git a/kernel/locking/lockdep_proc.c b/kernel/locking/lockdep_proc.c
index 6d1fcc7..68d9e26 100644
--- a/kernel/locking/lockdep_proc.c
+++ b/kernel/locking/lockdep_proc.c
@@ -201,6 +201,10 @@ static void lockdep_stats_debug_show(struct seq_file *m)
debug_atomic_read(chain_lookup_hits));
seq_printf(m, " cyclic checks: %11llu\n",
debug_atomic_read(nr_cyclic_checks));
+ seq_printf(m, " redundant checks: %11llu\n",
+ debug_atomic_read(nr_redundant_checks));
+ seq_printf(m, " redundant links: %11llu\n",
+ debug_atomic_read(nr_redundant));
seq_printf(m, " find-mask forwards checks: %11llu\n",
debug_atomic_read(nr_find_usage_forwards_checks));
seq_printf(m, " find-mask backwards checks: %11llu\n",