Re: [PATCH v2 1/6] locking/lockdep: Track number of zapped classes

[Bart Van Assche]

On 1/13/20 6:58 AM, Waiman Long wrote:
> On 1/13/20 9:55 AM, Peter Zijlstra wrote:
> > On Mon, Dec 16, 2019 at 10:15:12AM -0500, Waiman Long wrote:
> > > The whole point of the lockdep dynamic key patch is to allow unused
> > > locks to be removed from the lockdep data buffers so that existing
> > > buffer space can be reused. However, there is no way to find out how
> > > many unused locks are zapped and so we don't know if the zapping process
> > > is working properly.
> > >
> > > Add a new nr_zapped_classes variable to track that and show it in
> > > /proc/lockdep_stats if it is non-zero.
> > >
> > > diff --git a/kernel/locking/lockdep_proc.c b/kernel/locking/lockdep_proc.c
> > > index dadb7b7fba37..d98d349bb648 100644
> > > --- a/kernel/locking/lockdep_proc.c
> > > +++ b/kernel/locking/lockdep_proc.c
> > > @@ -336,6 +336,15 @@ static int lockdep_stats_show(struct seq_file *m, void *v)
> > >   	seq_printf(m, " debug_locks:                   %11u\n",
> > >   			debug_locks);
> > >   
> > > +	/*
> > > +	 * Zappped classes and lockdep data buffers reuse statistics.
> > > +	 */
> > > +	if (!nr_zapped_classes)
> > > +		return 0;
> > > +
> > > +	seq_puts(m, "\n");
> > > +	seq_printf(m, " zapped classes:                %11lu\n",
> > > +			nr_zapped_classes);
> > >   	return 0;
> > >   }
> > Why is that conditional?
> Because I thought zapping class doesn't occur that often. Apparently,
> class zapping happens when the system first boots up. I guess that
> conditional check isn't needed. I can remove it in the next version.

Zapping a lock class happens every time a lock key is unregistered. That 
can happen any time as one can see in the following grep output:

$ git grep -lw 'lockdep_unregister_key'
drivers/net/bonding/bond_main.c:4446: 
lockdep_unregister_key(&bond->stats_lock_key);
drivers/net/team/team.c:1676:	lockdep_unregister_key(&team->team_lock_key);
drivers/net/team/team.c:1984:		lockdep_unregister_key(&team->team_lock_key);
include/linux/lockdep.h:294:extern void lockdep_unregister_key(struct 
lock_class_key *key);
include/linux/lockdep.h:464:static inline void 
lockdep_unregister_key(struct lock_class_key *key)
kernel/locking/lockdep.c:5166:void lockdep_unregister_key(struct 
lock_class_key *key)
kernel/locking/lockdep.c:5201:EXPORT_SYMBOL_GPL(lockdep_unregister_key);
kernel/workqueue.c:3456:	lockdep_unregister_key(&wq->key);
net/core/dev.c:9172:	lockdep_unregister_key(&dev->qdisc_tx_busylock_key);
net/core/dev.c:9173:	lockdep_unregister_key(&dev->qdisc_running_key);
net/core/dev.c:9174:	lockdep_unregister_key(&dev->qdisc_xmit_lock_key);
net/core/dev.c:9175:	lockdep_unregister_key(&dev->addr_list_lock_key);
net/core/dev.c:9183:	lockdep_unregister_key(&dev->qdisc_xmit_lock_key);
net/core/dev.c:9184:	lockdep_unregister_key(&dev->addr_list_lock_key);
tools/lib/lockdep/include/liblockdep/common.h:48:void 
lockdep_unregister_key(struct lock_class_key *key);
tools/lib/lockdep/include/liblockdep/mutex.h:58: 
lockdep_unregister_key(&lock->key);

Bart.