[PATCH v2 20/23] locking/lockdep: Check redundant dependency only when CONFIG_LOCKDEP_SMALL

From: Yuyang Du
Date: Mon May 06 2019 - 04:21:33 EST

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@xxxxxxxxxxxxx>
Signed-off-by: Yuyang Du <duyuyang@xxxxxxxxx>
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;

* 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;


@@ -2423,12 +2425,14 @@ static inline void inc_chains(void)

* Is the <prev> -> <next> link redundant?
ret = check_redundant(prev, next);
if (ret != 1)
return ret;

if (!trace->nr_entries && !save_trace(trace))
return 0;