Commit ae813308 authored Mar 03, 2017 by Peter Zijlstra Committed by Ingo Molnar Aug 10, 2017

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>

parent d92a8cfc

kernel/locking/lockdep.c

+27 −0

Original line number	Diff line number	Diff line
		@@ -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;

kernel/locking/lockdep_internals.h

+2 −0

Original line number	Diff line number	Diff line
		@@ -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;

kernel/locking/lockdep_proc.c

+4 −0

Original line number	Diff line number	Diff line
		@@ -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",

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