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Subject: Re: [PATCH] locking/lockdep: unlikely bfs error check
From:   Xiongwei Song <sxwjean@me.com>
In-Reply-To: <a341e1f1-39a5-dca3-9454-8eabe085928b@redhat.com>
Date:   Wed, 16 Jun 2021 22:59:16 +0800
Cc:     peterz@infradead.org, mingo@redhat.com, will@kernel.org,
        boqun.feng@gmail.com, linux-kernel@vger.kernel.org,
        Xiongwei Song <sxwjean@gmail.com>
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Message-Id: <BF7CB29B-0B34-4462-B793-732C1EF9DC40@me.com>
References: <20210616144210.278662-1-sxwjean@me.com>
 <a341e1f1-39a5-dca3-9454-8eabe085928b@redhat.com>
To:     Waiman Long <llong@redhat.com>
Precedence: bulk


> On Jun 16, 2021, at 10:48 PM, Waiman Long <llong@redhat.com> wrote:
>=20
> On 6/16/21 10:42 AM, Xiongwei Song wrote:
>> From: Xiongwei Song <sxwjean@gmail.com>
>>=20
>> The error from graph walk is small probability event, so unlikely
>> bfs_error can improve performance a little bit.
>>=20
>> Signed-off-by: Xiongwei Song <sxwjean@gmail.com>
>> ---
>>  kernel/locking/lockdep.c | 12 ++++++------
>>  1 file changed, 6 insertions(+), 6 deletions(-)
>>=20
>> 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 =3D __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 =3D 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 =3D =
original_mask(target_entry1->class->usage_mask);
>>    	ret =3D 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 =3D check_redundant(prev, next);
>> -	if (bfs_error(ret))
>> +	if (unlikely(bfs_error(ret)))
>>  		return 0;
>>  	else if (ret =3D=3D 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 =3D 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 =3D find_usage_backwards(&root, usage_mask, &target_entry);
>> -	if (bfs_error(ret)) {
>> +	if (unlikely(bfs_error(ret))) {
>>  		print_bfs_bug(ret);
>>  		return 0;
>>  	}
>=20
> I think it is better to put the unlikely() directly into the =
bfs_error() inline function instead of sprinkling it all over the place.

Sounds good. Thank you for the suggestion. I will update the patch.

Regards,
Xiongwei

>=20
> Cheers,
> Longman
>=20