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From:   Yuyang Du <duyuyang@gmail.com>
To:     peterz@infradead.org, will.deacon@arm.com, mingo@kernel.org
Cc:     bvanassche@acm.org, ming.lei@redhat.com, frederic@kernel.org,
        tglx@linutronix.de, linux-kernel@vger.kernel.org,
        Yuyang Du <duyuyang@gmail.com>
Subject: [PATCH v2 20/23] locking/lockdep: Check redundant dependency only when CONFIG_LOCKDEP_SMALL
Date:   Mon,  6 May 2019 16:19:36 +0800
Message-Id: <20190506081939.74287-21-duyuyang@gmail.com>
In-Reply-To: <20190506081939.74287-1-duyuyang@gmail.com>
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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>
---
 kernel/locking/lockdep.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 2502ea4..9d2728c 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1734,6 +1734,7 @@ unsigned long lockdep_count_backward_deps(struct lock_class *class)
 	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
@@ -1763,6 +1764,7 @@ unsigned long lockdep_count_backward_deps(struct lock_class *class)
 
 	return ret;
 }
+#endif
 
 #ifdef CONFIG_TRACE_IRQFLAGS
 
@@ -2423,12 +2425,14 @@ static inline void inc_chains(void)
 		}
 	}
 
+#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;
-- 
1.8.3.1