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Date:   Mon, 3 Jun 2019 06:20:14 -0700
From:   tip-bot for Yuyang Du <tipbot@zytor.com>
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To:     linux-tip-commits@vger.kernel.org
Subject: [tip:locking/core] locking/lockdep: Check redundant dependency only
 when CONFIG_LOCKDEP_SMALL
Git-Commit-ID: 68e9dc29f8f42c79d2a3755223ed910ce36b4ae2
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Commit-ID:  68e9dc29f8f42c79d2a3755223ed910ce36b4ae2
Gitweb:     https://git.kernel.org/tip/68e9dc29f8f42c79d2a3755223ed910ce36b4ae2
Author:     Yuyang Du <duyuyang@gmail.com>
AuthorDate: Mon, 6 May 2019 16:19:36 +0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 3 Jun 2019 11:55:50 +0200

locking/lockdep: Check redundant dependency only when CONFIG_LOCKDEP_SMALL

As Peter has put it all sound and complete for the cause, I simply quote:

"It (check_redundant) was added for cross-release (which has since been
reverted) which would generate a lot of redundant links (IIRC) but
having it makes the reports more convoluted -- basically, if we had an
A-B-C relation, then A-C will not be added to the graph because it is
already covered. This then means any report will include B, even though
a shorter cycle might have been possible."

This would increase the number of direct dependencies. For a simple workload
(make clean; reboot; make vmlinux -j8), the data looks like this:

 CONFIG_LOCKDEP_SMALL: direct dependencies:                  6926

!CONFIG_LOCKDEP_SMALL: direct dependencies:                  9052    (+30.7%)

Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Yuyang Du <duyuyang@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bvanassche@acm.org
Cc: frederic@kernel.org
Cc: ming.lei@redhat.com
Cc: will.deacon@arm.com
Link: https://lkml.kernel.org/r/20190506081939.74287-21-duyuyang@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/locking/lockdep.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 30a1c0e32573..63b82921698d 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1739,6 +1739,7 @@ check_noncircular(struct held_lock *src, struct held_lock *target,
 	return ret;
 }
 
+#ifdef CONFIG_LOCKDEP_SMALL
 /*
  * Check that the dependency graph starting at <src> can lead to
  * <target> or not. If it can, <src> -> <target> dependency is already
@@ -1768,6 +1769,7 @@ check_redundant(struct held_lock *src, struct held_lock *target)
 
 	return ret;
 }
+#endif
 
 #ifdef CONFIG_TRACE_IRQFLAGS
 
@@ -2428,12 +2430,14 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
 		}
 	}
 
+#ifdef CONFIG_LOCKDEP_SMALL
 	/*
 	 * Is the <prev> -> <next> link redundant?
 	 */
 	ret = check_redundant(prev, next);
 	if (ret != 1)
 		return ret;
+#endif
 
 	if (!trace->nr_entries && !save_trace(trace))
 		return 0;