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Re: [PATCH] lockdep: reduce stack_trace usage

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On Mon, May 03, 2010 at 02:11:29PM +0200, Peter Zijlstra wrote:
> OK, I like the idea, but I'm a little confused as to why you pull
> save_trace() up two functions,

I also want to avoid adding redundant trace in case of trylock.
On my machine, I catched about 150 trylock dependences.

> from what I can see we can now end up
> saving a trace where we previously would not have done one (the whole
> recursive lock mess.

Oh, Yes, this patch will not give result as expected. Thank you for pointing
it. I will respin it in V2.

> 
> So please respin this with save_trace() in check_prev_add() right before
> the first add_lock_to_list().

No problem. 
BTW, in case of trylock dependence, I will let check_prevs_add() carry a flag
to check_prev_add(). Thus, we can save more redundant trace. How do you
think about it?

Thanks,
Yong

> 
> > Signed-off-by: Yong Zhang <yong.zhang0@xxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> > ---
> >  kernel/lockdep.c |   20 ++++++++++++--------
> >  1 files changed, 12 insertions(+), 8 deletions(-)
> > 
> > diff --git a/kernel/lockdep.c b/kernel/lockdep.c
> > index 2594e1c..097d5fb 100644
> > --- a/kernel/lockdep.c
> > +++ b/kernel/lockdep.c
> > @@ -818,7 +818,8 @@ static struct lock_list *alloc_list_entry(void)
> >   * Add a new dependency to the head of the list:
> >   */
> >  static int add_lock_to_list(struct lock_class *class, struct lock_class *this,
> > -			    struct list_head *head, unsigned long ip, int distance)
> > +			    struct list_head *head, unsigned long ip,
> > +			    int distance, struct stack_trace *trace)
> >  {
> >  	struct lock_list *entry;
> >  	/*
> > @@ -829,11 +830,9 @@ static int add_lock_to_list(struct lock_class *class, struct lock_class *this,
> >  	if (!entry)
> >  		return 0;
> >  
> > -	if (!save_trace(&entry->trace))
> > -		return 0;
> > -
> >  	entry->class = this;
> >  	entry->distance = distance;
> > +	entry->trace = *trace;
> >  	/*
> >  	 * Since we never remove from the dependency list, the list can
> >  	 * be walked lockless by other CPUs, it's only allocation
> > @@ -1635,7 +1634,7 @@ check_deadlock(struct task_struct *curr, struct held_lock *next,
> >   */
> >  static int
> >  check_prev_add(struct task_struct *curr, struct held_lock *prev,
> > -	       struct held_lock *next, int distance)
> > +	       struct held_lock *next, int distance, struct stack_trace *trace)
> >  {
> >  	struct lock_list *entry;
> >  	int ret;
> > @@ -1694,14 +1693,14 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
> >  	 */
> >  	ret = add_lock_to_list(hlock_class(prev), hlock_class(next),
> >  			       &hlock_class(prev)->locks_after,
> > -			       next->acquire_ip, distance);
> > +			       next->acquire_ip, distance, trace);
> >  
> >  	if (!ret)
> >  		return 0;
> >  
> >  	ret = add_lock_to_list(hlock_class(next), hlock_class(prev),
> >  			       &hlock_class(next)->locks_before,
> > -			       next->acquire_ip, distance);
> > +			       next->acquire_ip, distance, trace);
> >  	if (!ret)
> >  		return 0;
> >  
> > @@ -1732,6 +1731,7 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
> >  {
> >  	int depth = curr->lockdep_depth;
> >  	struct held_lock *hlock;
> > +	struct stack_trace trace;
> >  
> >  	/*
> >  	 * Debugging checks.
> > @@ -1748,6 +1748,9 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
> >  			curr->held_locks[depth-1].irq_context)
> >  		goto out_bug;
> >  
> > +	if (!save_trace(&trace))
> > +		return 0;
> > +
> >  	for (;;) {
> >  		int distance = curr->lockdep_depth - depth + 1;
> >  		hlock = curr->held_locks + depth-1;
> > @@ -1756,7 +1759,8 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
> >  		 * added:
> >  		 */
> >  		if (hlock->read != 2) {
> > -			if (!check_prev_add(curr, hlock, next, distance))
> > +			if (!check_prev_add(curr, hlock, next,
> > +						distance, &trace))
> >  				return 0;
> >  			/*
> >  			 * Stop after the first non-trylock entry,
--
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