[PATCH 0/3] some improvements for lockdep

[PATCH 0/3] some improvements for lockdep

[Xiongwei Song]

Unlikely the checks of return values of graph walk will improve the
performance to some degree, patch 1 and patch 2 are for this.

The patch 3 will print a warning after counting lock deps when hitting
bfs errors.

Xiongwei Song (3):
  locking/lockdep: unlikely bfs_error function
  locking/lockdep: unlikely conditons about BFS_RMATCH
  locking/lockdep: print possible warning after counting deps

 kernel/locking/lockdep.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

-- 
2.30.2

[PATCH 1/3] locking/lockdep: unlikely bfs_error() inside

[Xiongwei Song]

From: Xiongwei Song <sxwjean@gmail.com>

The error from graph walk is small probability event, and there are some
bfs_error calls during lockdep detection, so unlikely bfs_error inside
can improve performance a little bit.

Suggested-by: Waiman Long <longman@redhat.com>
Signed-off-by: Xiongwei Song <sxwjean@gmail.com>
---
 kernel/locking/lockdep.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 7641bd407239..a8a66a2a9bc1 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -1540,7 +1540,7 @@ enum bfs_result {
  */
 static inline bool bfs_error(enum bfs_result res)
 {
-	return res < 0;
+	return unlikely(res < 0);
 }
 
 /*
@@ -2089,7 +2089,7 @@ check_path(struct held_lock *target, struct lock_list *src_entry,
 
 	ret = __bfs_forwards(src_entry, target, match, skip, target_entry);
 
-	if (unlikely(bfs_error(ret)))
+	if (bfs_error(ret))
 		print_bfs_bug(ret);
 
 	return ret;
@@ -2936,7 +2936,7 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
 	 * in the graph whose neighbours are to be checked.
 	 */
 	ret = check_noncircular(next, prev, trace);
-	if (unlikely(bfs_error(ret) || ret == BFS_RMATCH))
+	if (bfs_error(ret) || unlikely(ret == BFS_RMATCH))
 		return 0;
 
 	if (!check_irq_usage(curr, prev, next))
-- 
2.30.2

[PATCH 2/3] locking/lockdep: unlikely conditons about BFS_RMATCH

[Xiongwei Song]

From: Xiongwei Song <sxwjean@gmail.com>

The probability that graph walk will return BFS_RMATCH is slim, so unlikey
conditons about BFS_RMATCH can improve performance a little bit.

Signed-off-by: Xiongwei Song <sxwjean@gmail.com>
---
 kernel/locking/lockdep.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index a8a66a2a9bc1..cb94097014d8 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -2750,7 +2750,7 @@ check_redundant(struct held_lock *src, struct held_lock *target)
 	 */
 	ret = check_path(target, &src_entry, hlock_equal, usage_skip, &target_entry);
 
-	if (ret == BFS_RMATCH)
+	if (unlikely(ret == BFS_RMATCH))
 		debug_atomic_inc(nr_redundant);
 
 	return ret;
@@ -2992,7 +2992,7 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
 	ret = check_redundant(prev, next);
 	if (bfs_error(ret))
 		return 0;
-	else if (ret == BFS_RMATCH)
+	else if (unlikely(ret == BFS_RMATCH))
 		return 2;
 
 	if (!*trace) {
-- 
2.30.2

[PATCH 3/3] locking/lockdep: print possible warning after counting deps

[Xiongwei Song]

From: Xiongwei Song <sxwjean@gmail.com>

The graph walk might hit error when counting dependencies. Once the
return value is negative, print a warning to reminder users.

Suggested-by: Waiman Long <longman@redhat.com>
Signed-off-by: Xiongwei Song <sxwjean@gmail.com>
---
 kernel/locking/lockdep.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index cb94097014d8..cfe0f4374594 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -2028,8 +2028,12 @@ static unsigned long __lockdep_count_forward_deps(struct lock_list *this)
 {
 	unsigned long  count = 0;
 	struct lock_list *target_entry;
+	enum bfs_result ret;
+
+	ret = __bfs_forwards(this, (void *)&count, noop_count, NULL, &target_entry);
 
-	__bfs_forwards(this, (void *)&count, noop_count, NULL, &target_entry);
+	if (bfs_error(ret))
+		print_bfs_bug(ret);
 
 	return count;
 }
@@ -2053,8 +2057,12 @@ static unsigned long __lockdep_count_backward_deps(struct lock_list *this)
 {
 	unsigned long  count = 0;
 	struct lock_list *target_entry;
+	enum bfs_result ret;
+
+	ret = __bfs_backwards(this, (void *)&count, noop_count, NULL, &target_entry);
 
-	__bfs_backwards(this, (void *)&count, noop_count, NULL, &target_entry);
+	if (bfs_error(ret))
+		print_bfs_bug(ret);
 
 	return count;
 }
-- 
2.30.2

Re: [PATCH 3/3] locking/lockdep: print possible warning after counting deps

[Boqun Feng]

Hi,

On Thu, Jun 17, 2021 at 10:28:28PM +0800, Xiongwei Song wrote:
> From: Xiongwei Song <sxwjean@gmail.com>
> 
> The graph walk might hit error when counting dependencies. Once the
> return value is negative, print a warning to reminder users.
> 

Thanks for the improvement, but please see below:

> Suggested-by: Waiman Long <longman@redhat.com>
> Signed-off-by: Xiongwei Song <sxwjean@gmail.com>
> ---
>  kernel/locking/lockdep.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index cb94097014d8..cfe0f4374594 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -2028,8 +2028,12 @@ static unsigned long __lockdep_count_forward_deps(struct lock_list *this)
>  {
>  	unsigned long  count = 0;
>  	struct lock_list *target_entry;
> +	enum bfs_result ret;
> +
> +	ret = __bfs_forwards(this, (void *)&count, noop_count, NULL, &target_entry);
>  
> -	__bfs_forwards(this, (void *)&count, noop_count, NULL, &target_entry);
> +	if (bfs_error(ret))
> +		print_bfs_bug(ret);

Here print_bfs_bug() will eventually call debug_locks_off_graph_unlock()
to release the graph lock, and the caller (lockdep_count_fowards_deps())
will also call graph_unlock() afterwards, and that means we unlock
*twice* if a BFS error happens... although in that case, lockdep should
stop working so messing up with the graph lock may not hurt anything,
but still, I don't think we want to do that.

So probably you can open-code __lockdep_count_forward_deps() into
lockdep_count_forwards_deps(), and call print_bfs_bug() or
graph_unlock() accordingly. The body of __lockdep_count_forward_deps()
is really small, so I think it's OK to open-code it into its caller.

Regards,
Boqun

>  
>  	return count;
>  }
> @@ -2053,8 +2057,12 @@ static unsigned long __lockdep_count_backward_deps(struct lock_list *this)
>  {
>  	unsigned long  count = 0;
>  	struct lock_list *target_entry;
> +	enum bfs_result ret;
> +
> +	ret = __bfs_backwards(this, (void *)&count, noop_count, NULL, &target_entry);
>  
> -	__bfs_backwards(this, (void *)&count, noop_count, NULL, &target_entry);
> +	if (bfs_error(ret))
> +		print_bfs_bug(ret);
>  
>  	return count;
>  }
> -- 
> 2.30.2
> 

Re: [PATCH 3/3] locking/lockdep: print possible warning after counting deps

[Xiongwei Song]

On Thu, Jun 17, 2021 at 11:13 PM Boqun Feng <boqun.feng@gmail.com> wrote:
>
> Hi,
>
> On Thu, Jun 17, 2021 at 10:28:28PM +0800, Xiongwei Song wrote:
> > From: Xiongwei Song <sxwjean@gmail.com>
> >
> > The graph walk might hit error when counting dependencies. Once the
> > return value is negative, print a warning to reminder users.
> >
>
> Thanks for the improvement, but please see below:
>
> > Suggested-by: Waiman Long <longman@redhat.com>
> > Signed-off-by: Xiongwei Song <sxwjean@gmail.com>
> > ---
> >  kernel/locking/lockdep.c | 12 ++++++++++--
> >  1 file changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > index cb94097014d8..cfe0f4374594 100644
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -2028,8 +2028,12 @@ static unsigned long __lockdep_count_forward_deps(struct lock_list *this)
> >  {
> >       unsigned long  count = 0;
> >       struct lock_list *target_entry;
> > +     enum bfs_result ret;
> > +
> > +     ret = __bfs_forwards(this, (void *)&count, noop_count, NULL, &target_entry);
> >
> > -     __bfs_forwards(this, (void *)&count, noop_count, NULL, &target_entry);
> > +     if (bfs_error(ret))
> > +             print_bfs_bug(ret);
>
> Here print_bfs_bug() will eventually call debug_locks_off_graph_unlock()
> to release the graph lock, and the caller (lockdep_count_fowards_deps())
> will also call graph_unlock() afterwards, and that means we unlock
> *twice* if a BFS error happens... although in that case, lockdep should
> stop working so messing up with the graph lock may not hurt anything,
> but still, I don't think we want to do that.
>
> So probably you can open-code __lockdep_count_forward_deps() into
> lockdep_count_forwards_deps(), and call print_bfs_bug() or
> graph_unlock() accordingly. The body of __lockdep_count_forward_deps()
> is really small, so I think it's OK to open-code it into its caller.

Thank you so much for the detailed comments. Let me improve and update
the patch.

Regards,
Xiongwei

>
> Regards,
> Boqun
>
> >
> >       return count;
> >  }
> > @@ -2053,8 +2057,12 @@ static unsigned long __lockdep_count_backward_deps(struct lock_list *this)
> >  {
> >       unsigned long  count = 0;
> >       struct lock_list *target_entry;
> > +     enum bfs_result ret;
> > +
> > +     ret = __bfs_backwards(this, (void *)&count, noop_count, NULL, &target_entry);
> >
> > -     __bfs_backwards(this, (void *)&count, noop_count, NULL, &target_entry);
> > +     if (bfs_error(ret))
> > +             print_bfs_bug(ret);
> >
> >       return count;
> >  }
> > --
> > 2.30.2
> >