Commit 68e9dc29 authored May 06, 2019 by Yuyang Du Committed by Ingo Molnar Jun 03, 2019

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>

parent 8c2c2b44

kernel/locking/lockdep.c

+4 −0

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

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