diff mbox series

[v2,1/4] blk-mq: use percpu csd to remote complete instead of per-rq csd

Message ID 20230629110359.1111832-2-chengming.zhou@linux.dev (mailing list archive)
State New, archived
Headers show
Series blk-mq: optimize the size of struct request | expand

Commit Message

Chengming Zhou June 29, 2023, 11:03 a.m. UTC 
From: Chengming Zhou <zhouchengming@bytedance.com>

If request need to be completed remotely, we insert it into percpu llist,
and smp_call_function_single_async() if llist is empty previously.

We don't need to use per-rq csd, percpu csd is enough. And the size of
struct request is decreased by 24 bytes.

This way is cleaner, and looks correct, given block softirq is guaranteed to be
scheduled to consume the list if one new request is added to this percpu list,
either smp_call_function_single_async() returns -EBUSY or 0.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
---
v2:
 - Change to use call_single_data_t, which avoid to use 2 cache lines
   for 1 csd, as suggested by Ming Lei.
 - Improve the commit log, the explanation is copied from Ming Lei.
---
 block/blk-mq.c         | 12 ++++++++----
 include/linux/blk-mq.h |  5 +----
 2 files changed, 9 insertions(+), 8 deletions(-)

Comments

Ming Lei July 5, 2023, 3:01 p.m. UTC | #1 
On Thu, Jun 29, 2023 at 07:03:56PM +0800, chengming.zhou@linux.dev wrote:
> From: Chengming Zhou <zhouchengming@bytedance.com>
> 
> If request need to be completed remotely, we insert it into percpu llist,
> and smp_call_function_single_async() if llist is empty previously.
> 
> We don't need to use per-rq csd, percpu csd is enough. And the size of
> struct request is decreased by 24 bytes.
> 
> This way is cleaner, and looks correct, given block softirq is guaranteed to be
> scheduled to consume the list if one new request is added to this percpu list,
> either smp_call_function_single_async() returns -EBUSY or 0.
> 
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> ---
> v2:
>  - Change to use call_single_data_t, which avoid to use 2 cache lines
>    for 1 csd, as suggested by Ming Lei.
>  - Improve the commit log, the explanation is copied from Ming Lei.

Reviewed-by: Ming Lei <ming.lei@redhat.com>

Thanks,
Ming
Christoph Hellwig July 6, 2023, 1:07 p.m. UTC | #2 
On Thu, Jun 29, 2023 at 07:03:56PM +0800, chengming.zhou@linux.dev wrote:
> From: Chengming Zhou <zhouchengming@bytedance.com>
> 
> If request need to be completed remotely, we insert it into percpu llist,
> and smp_call_function_single_async() if llist is empty previously.
> 
> We don't need to use per-rq csd, percpu csd is enough. And the size of
> struct request is decreased by 24 bytes.
> 
> This way is cleaner, and looks correct, given block softirq is guaranteed to be
> scheduled to consume the list if one new request is added to this percpu list,
> either smp_call_function_single_async() returns -EBUSY or 0.

Please trim your commit logs to 73 characters per line so that they
are readable in git log output.

>  static void blk_mq_request_bypass_insert(struct request *rq,
> @@ -1156,13 +1157,13 @@ static void blk_mq_complete_send_ipi(struct request *rq)
>  {
>  	struct llist_head *list;
>  	unsigned int cpu;
> +	call_single_data_t *csd;
>  
>  	cpu = rq->mq_ctx->cpu;
>  	list = &per_cpu(blk_cpu_done, cpu);
> -	if (llist_add(&rq->ipi_list, list)) {
> -		INIT_CSD(&rq->csd, __blk_mq_complete_request_remote, rq);
> -		smp_call_function_single_async(cpu, &rq->csd);
> -	}
> +	csd = &per_cpu(blk_cpu_csd, cpu);
> +	if (llist_add(&rq->ipi_list, list))
> +		smp_call_function_single_async(cpu, csd);
>  }

No need for the list and csd variables here as they are only used
once.

But I think this code has a rpboem when it is preemptd between
the llist_add and smp_call_function_single_async.  We either need a
get_cpu/put_cpu around them, or instroduce a structure with the list
and csd, and then you can use one pointer from per_cpu and still ensure
the list and csd are for the same CPU.
Chengming Zhou July 6, 2023, 2:23 p.m. UTC | #3 
On 2023/7/6 21:07, Christoph Hellwig wrote:
> On Thu, Jun 29, 2023 at 07:03:56PM +0800, chengming.zhou@linux.dev wrote:
>> From: Chengming Zhou <zhouchengming@bytedance.com>
>>
>> If request need to be completed remotely, we insert it into percpu llist,
>> and smp_call_function_single_async() if llist is empty previously.
>>
>> We don't need to use per-rq csd, percpu csd is enough. And the size of
>> struct request is decreased by 24 bytes.
>>
>> This way is cleaner, and looks correct, given block softirq is guaranteed to be
>> scheduled to consume the list if one new request is added to this percpu list,
>> either smp_call_function_single_async() returns -EBUSY or 0.
> 
> Please trim your commit logs to 73 characters per line so that they
> are readable in git log output.

Ok, will fix in the next version.

> 
>>  static void blk_mq_request_bypass_insert(struct request *rq,
>> @@ -1156,13 +1157,13 @@ static void blk_mq_complete_send_ipi(struct request *rq)
>>  {
>>  	struct llist_head *list;
>>  	unsigned int cpu;
>> +	call_single_data_t *csd;
>>  
>>  	cpu = rq->mq_ctx->cpu;
>>  	list = &per_cpu(blk_cpu_done, cpu);
>> -	if (llist_add(&rq->ipi_list, list)) {
>> -		INIT_CSD(&rq->csd, __blk_mq_complete_request_remote, rq);
>> -		smp_call_function_single_async(cpu, &rq->csd);
>> -	}
>> +	csd = &per_cpu(blk_cpu_csd, cpu);
>> +	if (llist_add(&rq->ipi_list, list))
>> +		smp_call_function_single_async(cpu, csd);
>>  }
> 
> No need for the list and csd variables here as they are only used
> once.

Yes, should I change like below? Looks like much long code. :-)

if (llist_add(&rq->ipi_list, &per_cpu(blk_cpu_done, cpu)))
	smp_call_function_single_async(cpu, &per_cpu(blk_cpu_csd, cpu));

> 
> But I think this code has a rpboem when it is preemptd between
> the llist_add and smp_call_function_single_async.  We either need a
> get_cpu/put_cpu around them, or instroduce a structure with the list
> and csd, and then you can use one pointer from per_cpu and still ensure
> the list and csd are for the same CPU.
> 

cpu = rq->mq_ctx->cpu; So it's certainly the same CPU, right?

Thanks!
Christoph Hellwig July 6, 2023, 2:39 p.m. UTC | #4 
On Thu, Jul 06, 2023 at 10:23:49PM +0800, Chengming Zhou wrote:
> Yes, should I change like below? Looks like much long code. :-)
> 
> if (llist_add(&rq->ipi_list, &per_cpu(blk_cpu_done, cpu)))
> 	smp_call_function_single_async(cpu, &per_cpu(blk_cpu_csd, cpu));

Doesn't look bad too me.

>
> 
> > 
> > But I think this code has a rpboem when it is preemptd between
> > the llist_add and smp_call_function_single_async.  We either need a
> > get_cpu/put_cpu around them, or instroduce a structure with the list
> > and csd, and then you can use one pointer from per_cpu and still ensure
> > the list and csd are for the same CPU.
> > 
> 
> cpu = rq->mq_ctx->cpu; So it's certainly the same CPU, right?

You're right of couse - cpu is the submitting cpu and not the current
one and thus not affected by preemption.  Sorry for the noise.
diff mbox series

Patch

diff --git a/block/blk-mq.c b/block/blk-mq.c
index decb6ab2d508..e52200edd2b1 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -43,6 +43,7 @@ 
 #include "blk-ioprio.h"
 
 static DEFINE_PER_CPU(struct llist_head, blk_cpu_done);
+static DEFINE_PER_CPU(call_single_data_t, blk_cpu_csd);
 
 static void blk_mq_insert_request(struct request *rq, blk_insert_t flags);
 static void blk_mq_request_bypass_insert(struct request *rq,
@@ -1156,13 +1157,13 @@  static void blk_mq_complete_send_ipi(struct request *rq)
 {
 	struct llist_head *list;
 	unsigned int cpu;
+	call_single_data_t *csd;
 
 	cpu = rq->mq_ctx->cpu;
 	list = &per_cpu(blk_cpu_done, cpu);
-	if (llist_add(&rq->ipi_list, list)) {
-		INIT_CSD(&rq->csd, __blk_mq_complete_request_remote, rq);
-		smp_call_function_single_async(cpu, &rq->csd);
-	}
+	csd = &per_cpu(blk_cpu_csd, cpu);
+	if (llist_add(&rq->ipi_list, list))
+		smp_call_function_single_async(cpu, csd);
 }
 
 static void blk_mq_raise_softirq(struct request *rq)
@@ -4796,6 +4797,9 @@  static int __init blk_mq_init(void)
 
 	for_each_possible_cpu(i)
 		init_llist_head(&per_cpu(blk_cpu_done, i));
+	for_each_possible_cpu(i)
+		INIT_CSD(&per_cpu(blk_cpu_csd, i),
+			 __blk_mq_complete_request_remote, NULL);
 	open_softirq(BLOCK_SOFTIRQ, blk_done_softirq);
 
 	cpuhp_setup_state_nocalls(CPUHP_BLOCK_SOFTIRQ_DEAD,
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index f401067ac03a..070551197c0e 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -182,10 +182,7 @@  struct request {
 		rq_end_io_fn		*saved_end_io;
 	} flush;
 
-	union {
-		struct __call_single_data csd;
-		u64 fifo_time;
-	};
+	u64 fifo_time;
 
 	/*
 	 * completion callback.