[PATCH] block: ensure that struct blk_mq_alloc_data is fully initialized

[PATCH] block: ensure that struct blk_mq_alloc_data is fully initialized

[Jens Axboe]

On x86, rep stos will be emitted to clear the the blk_mq_alloc_data
struct, as not all members are being initialied. Depending on the
type of CPU, this is a noticeable slowdown compared to just ensuring
that the struct is fully initialized when setup.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

diff --git a/block/blk-mq.c b/block/blk-mq.c
index c2697db59109..9fb43b09a401 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -584,9 +584,13 @@ static struct request *blk_mq_rq_cache_fill(struct request_queue *q,
 	struct blk_mq_alloc_data data = {
 		.q		= q,
 		.flags		= flags,
+		.shallow_depth	= 0,
 		.cmd_flags	= opf,
+		.rq_flags	= 0,
 		.nr_tags	= plug->nr_ios,
 		.cached_rqs	= &plug->cached_rqs,
+		.ctx		= NULL,
+		.hctx		= NULL
 	};
 	struct request *rq;
 
@@ -646,8 +650,13 @@ struct request *blk_mq_alloc_request(struct request_queue *q, blk_opf_t opf,
 		struct blk_mq_alloc_data data = {
 			.q		= q,
 			.flags		= flags,
+			.shallow_depth	= 0,
 			.cmd_flags	= opf,
+			.rq_flags	= 0,
 			.nr_tags	= 1,
+			.cached_rqs	= NULL,
+			.ctx		= NULL,
+			.hctx		= NULL
 		};
 		int ret;
 
@@ -675,8 +684,13 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
 	struct blk_mq_alloc_data data = {
 		.q		= q,
 		.flags		= flags,
+		.shallow_depth	= 0,
 		.cmd_flags	= opf,
+		.rq_flags	= 0,
 		.nr_tags	= 1,
+		.cached_rqs	= NULL,
+		.ctx		= NULL,
+		.hctx		= NULL
 	};
 	u64 alloc_time_ns = 0;
 	struct request *rq;
@@ -2969,8 +2983,14 @@ static struct request *blk_mq_get_new_requests(struct request_queue *q,
 {
 	struct blk_mq_alloc_data data = {
 		.q		= q,
-		.nr_tags	= 1,
+		.flags		= 0,
+		.shallow_depth	= 0,
 		.cmd_flags	= bio->bi_opf,
+		.rq_flags	= 0,
+		.nr_tags	= 1,
+		.cached_rqs	= NULL,
+		.ctx		= NULL,
+		.hctx		= NULL
 	};
 	struct request *rq;
 
-- 
Jens Axboe

Re: [PATCH] block: ensure that struct blk_mq_alloc_data is fully initialized

[Chaitanya Kulkarni]

On 4/15/25 07:51, Jens Axboe wrote:
> On x86, rep stos will be emitted to clear the the blk_mq_alloc_data
> struct, as not all members are being initialied. Depending on the
> type of CPU, this is a noticeable slowdown compared to just ensuring
> that the struct is fully initialized when setup.
>
> Signed-off-by: Jens Axboe<axboe@kernel.dk>

Looks good.

Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>

-ck

Re: [PATCH] block: ensure that struct blk_mq_alloc_data is fully initialized

[Bart Van Assche]

On 4/15/25 7:51 AM, Jens Axboe wrote:
> On x86, rep stos will be emitted to clear the the blk_mq_alloc_data
> struct, as not all members are being initialied.

"Partial initialization" never happens in the C language when
initializing a data structure. If a data structure is initialized,
members that have not been specified are initialized to zero (the 
compiler is not required to initialize padding bytes). In other words,
the description of this patch needs to be improved.

Thanks,

Bart.

Re: [PATCH] block: ensure that struct blk_mq_alloc_data is fully initialized

[Jens Axboe]

On 4/15/25 9:50 AM, Bart Van Assche wrote:
> On 4/15/25 7:51 AM, Jens Axboe wrote:
>> On x86, rep stos will be emitted to clear the the blk_mq_alloc_data
>> struct, as not all members are being initialied.
> 
> "Partial initialization" never happens in the C language when
> initializing a data structure. If a data structure is initialized,
> members that have not been specified are initialized to zero (the
> compiler is not required to initialize padding bytes). In other words,
> the description of this patch needs to be improved.

How is the description inaccurate? As not all members are being
explicitly initialized, rep stos is emitted to do so.

-- 
Jens Axboe

Re: [PATCH] block: ensure that struct blk_mq_alloc_data is fully initialized

[Bart Van Assche]

On 4/15/25 8:59 AM, Jens Axboe wrote:
> On 4/15/25 9:50 AM, Bart Van Assche wrote:
>> On 4/15/25 7:51 AM, Jens Axboe wrote:
>>> On x86, rep stos will be emitted to clear the the blk_mq_alloc_data
>>> struct, as not all members are being initialied.
>>
>> "Partial initialization" never happens in the C language when
>> initializing a data structure. If a data structure is initialized,
>> members that have not been specified are initialized to zero (the
>> compiler is not required to initialize padding bytes). In other words,
>> the description of this patch needs to be improved.
> 
> How is the description inaccurate? As not all members are being
> explicitly initialized, rep stos is emitted to do so.

Hi Jens,

I think we agree if the word "explicit" would be added to the patch
description.

Thanks,

Bart.

Re: [PATCH] block: ensure that struct blk_mq_alloc_data is fully initialized

[Jens Axboe]

On 4/15/25 10:13 AM, Bart Van Assche wrote:
> On 4/15/25 8:59 AM, Jens Axboe wrote:
>> On 4/15/25 9:50 AM, Bart Van Assche wrote:
>>> On 4/15/25 7:51 AM, Jens Axboe wrote:
>>>> On x86, rep stos will be emitted to clear the the blk_mq_alloc_data
>>>> struct, as not all members are being initialied.
>>>
>>> "Partial initialization" never happens in the C language when
>>> initializing a data structure. If a data structure is initialized,
>>> members that have not been specified are initialized to zero (the
>>> compiler is not required to initialize padding bytes). In other words,
>>> the description of this patch needs to be improved.
>>
>> How is the description inaccurate? As not all members are being
>> explicitly initialized, rep stos is emitted to do so.
> 
> Hi Jens,
> 
> I think we agree if the word "explicit" would be added to the patch
> description.

Sure

-- 
Jens Axboe