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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH] locking/lockdep: unlikely bfs error check
To:     Xiongwei Song <sxwjean@me.com>, peterz@infradead.org,
        mingo@redhat.com, will@kernel.org, boqun.feng@gmail.com
Cc:     linux-kernel@vger.kernel.org, Xiongwei Song <sxwjean@gmail.com>
References: <20210616144210.278662-1-sxwjean@me.com>
Message-ID: <a341e1f1-39a5-dca3-9454-8eabe085928b@redhat.com>
Date:   Wed, 16 Jun 2021 10:48:56 -0400
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On 6/16/21 10:42 AM, Xiongwei Song wrote:
> From: Xiongwei Song <sxwjean@gmail.com>
>
> The error from graph walk is small probability event, so unlikely
> bfs_error can improve performance a little bit.
>
> Signed-off-by: Xiongwei Song <sxwjean@gmail.com>
> ---
>   kernel/locking/lockdep.c | 12 ++++++------
>   1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 074fd6418c20..af8c9203cd3e 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -2646,7 +2646,7 @@ static int check_irq_usage(struct task_struct *curr, struct held_lock *prev,
>   	bfs_init_rootb(&this, prev);
>   
>   	ret = __bfs_backwards(&this, &usage_mask, usage_accumulate, usage_skip, NULL);
> -	if (bfs_error(ret)) {
> +	if (unlikely(bfs_error(ret))) {
>   		print_bfs_bug(ret);
>   		return 0;
>   	}
> @@ -2664,7 +2664,7 @@ static int check_irq_usage(struct task_struct *curr, struct held_lock *prev,
>   	bfs_init_root(&that, next);
>   
>   	ret = find_usage_forwards(&that, forward_mask, &target_entry1);
> -	if (bfs_error(ret)) {
> +	if (unlikely(bfs_error(ret))) {
>   		print_bfs_bug(ret);
>   		return 0;
>   	}
> @@ -2679,7 +2679,7 @@ static int check_irq_usage(struct task_struct *curr, struct held_lock *prev,
>   	backward_mask = original_mask(target_entry1->class->usage_mask);
>   
>   	ret = find_usage_backwards(&this, backward_mask, &target_entry);
> -	if (bfs_error(ret)) {
> +	if (unlikely(bfs_error(ret))) {
>   		print_bfs_bug(ret);
>   		return 0;
>   	}
> @@ -2998,7 +2998,7 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
>   	 * Is the <prev> -> <next> link redundant?
>   	 */
>   	ret = check_redundant(prev, next);
> -	if (bfs_error(ret))
> +	if (unlikely(bfs_error(ret)))
>   		return 0;
>   	else if (ret == BFS_RMATCH)
>   		return 2;
> @@ -3911,7 +3911,7 @@ check_usage_forwards(struct task_struct *curr, struct held_lock *this,
>   
>   	bfs_init_root(&root, this);
>   	ret = find_usage_forwards(&root, usage_mask, &target_entry);
> -	if (bfs_error(ret)) {
> +	if (unlikely(bfs_error(ret))) {
>   		print_bfs_bug(ret);
>   		return 0;
>   	}
> @@ -3946,7 +3946,7 @@ check_usage_backwards(struct task_struct *curr, struct held_lock *this,
>   
>   	bfs_init_rootb(&root, this);
>   	ret = find_usage_backwards(&root, usage_mask, &target_entry);
> -	if (bfs_error(ret)) {
> +	if (unlikely(bfs_error(ret))) {
>   		print_bfs_bug(ret);
>   		return 0;
>   	}

I think it is better to put the unlikely() directly into the bfs_error() 
inline function instead of sprinkling it all over the place.

Cheers,
Longman