Hi Ingo,
I am seeing a problem on the latest -rt where lockdep completely overwhelms
the system to the point that it grinds to a halt on large (8-way+) systems.
The problem seems to be that the class->locks_before and locks_after grow
unbounded (I have observed over 1M+ entries in them) so a lock_acquire call
can take over 10 seconds to finish resolving. Related to this seems to be
that lockdep appears to see a chain-hash miss over and over for what I would
assume should be an established graph (for instance, in
double_lock_balance() in an rt_overload condition). Turning off
PROVE_LOCKING (statically, or by setting debug_locks=0 dynamically restores
the system to normal behavior.
I took some time tonight to study lockdep (it is quite an impressive body of
code!), and came up with the following "fix". It does improve things
significantly by addressing what I believe is the issue with the
cache-misses (though it would appear there are still a few more issues
there that need addressing as some boots are still very lethargic). I use
the term "fix" loosely since I am not confident that I fully understand the
intention of your logic here so I can't say for sure if it was really
broken, or if I have made it worse ;)
Could you comment on what I have done here, or offer any advice on what to
look for elsewhere? I based the patch on pure linux-2.6.git since I see the
same issue (by visual inspection, that is) there as well.
Thanks in advance!
-Greg
------
LOCKDEP: fix mismatched lockdep_depth/curr_chain_hash
It is possible for the current->curr_chain_key to become inconsistent with the
current index if the chain fails to validate. The end result is that future
lock_acquire() operations may inadvertently fail to find a hit in the cache
resulting in a new node being added to the graph for every acquire.
Signed-off-by: Gregory Haskins <[email protected]>
---
kernel/lockdep.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 734da57..efb0d7e 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -2450,11 +2450,11 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
chain_head = 1;
}
chain_key = iterate_chain_key(chain_key, id);
- curr->curr_chain_key = chain_key;
if (!validate_chain(curr, lock, hlock, chain_head))
return 0;
+ curr->curr_chain_key = chain_key;
curr->lockdep_depth++;
check_chain_key(curr);
#ifdef CONFIG_DEBUG_LOCKDEP
-
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