On 2023/6/28 12:13, Ming Lei wrote:
> On Tue, Jun 27, 2023 at 08:08:52PM +0800, chengming.zhou@xxxxxxxxx wrote:
>> From: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
>>
>> The flush state machine use a double list to link all inflight
>> flush_data requests, to avoid issuing separate post-flushes for
>> these flush_data requests which shared PREFLUSH.
>>
>> So we can't reuse rq->queuelist, this is why we need rq->flush.list
>>
>> In preparation of the next patch that reuse rq->queuelist for flush
>> state machine, we change the double linked list to a u64 counter,
>> which count all inflight flush_data requests.
>>
>> This is ok since we only need to know if there is any inflight
>> flush_data request, so a u64 counter is good. The only problem I can
>> think of is that u64 counter may overflow, which should be unlikely happen.
>
> It won't overflow, q->nr_requests is 'unsigned long', which should have
> been limited to one more reasonable value, such as 2 * BLK_MQ_MAX_DEPTH, so
> u16 should be big enough in theory.
Ah, right. q->nr_requests is 'unsigned long' and q->queue_depth is 'unsigned int',
so 'unsigned long' counter here won't overflow.
Should I change it to smaller 'unsigned short' or just leave it as 'unsigned long' ?
(Now the size of struct blk_flush_queue is exactly 64 bytes)
Thanks.
>
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
>> ---
>> block/blk-flush.c | 9 +++++----
>> block/blk.h | 5 ++---
>> 2 files changed, 7 insertions(+), 7 deletions(-)
>>
>> diff --git a/block/blk-flush.c b/block/blk-flush.c
>> index dba392cf22be..bb7adfc2a5da 100644
>> --- a/block/blk-flush.c
>> +++ b/block/blk-flush.c
>> @@ -187,7 +187,8 @@ static void blk_flush_complete_seq(struct request *rq,
>> break;
>>
>> case REQ_FSEQ_DATA:
>> - list_move_tail(&rq->flush.list, &fq->flush_data_in_flight);
>> + list_del_init(&rq->flush.list);
>> + fq->flush_data_in_flight++;
>> spin_lock(&q->requeue_lock);
>> list_add_tail(&rq->queuelist, &q->flush_list);
>> spin_unlock(&q->requeue_lock);
>> @@ -299,7 +300,7 @@ static void blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq,
>> return;
>>
>> /* C2 and C3 */
>> - if (!list_empty(&fq->flush_data_in_flight) &&
>> + if (fq->flush_data_in_flight &&
>> time_before(jiffies,
>> fq->flush_pending_since + FLUSH_PENDING_TIMEOUT))
>> return;
>> @@ -374,6 +375,7 @@ static enum rq_end_io_ret mq_flush_data_end_io(struct request *rq,
>> * the comment in flush_end_io().
>> */
>> spin_lock_irqsave(&fq->mq_flush_lock, flags);
>> + fq->flush_data_in_flight--;
>> blk_flush_complete_seq(rq, fq, REQ_FSEQ_DATA, error);
>> spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
>>
>> @@ -445,7 +447,7 @@ bool blk_insert_flush(struct request *rq)
>> blk_rq_init_flush(rq);
>> rq->flush.seq |= REQ_FSEQ_POSTFLUSH;
>> spin_lock_irq(&fq->mq_flush_lock);
>> - list_move_tail(&rq->flush.list, &fq->flush_data_in_flight);
>> + fq->flush_data_in_flight++;
>> spin_unlock_irq(&fq->mq_flush_lock);
>> return false;
>> default:
>> @@ -496,7 +498,6 @@ struct blk_flush_queue *blk_alloc_flush_queue(int node, int cmd_size,
>>
>> INIT_LIST_HEAD(&fq->flush_queue[0]);
>> INIT_LIST_HEAD(&fq->flush_queue[1]);
>> - INIT_LIST_HEAD(&fq->flush_data_in_flight);
>>
>> return fq;
>>
>> diff --git a/block/blk.h b/block/blk.h
>> index 608c5dcc516b..686712e13835 100644
>> --- a/block/blk.h
>> +++ b/block/blk.h
>> @@ -15,15 +15,14 @@ struct elevator_type;
>> extern struct dentry *blk_debugfs_root;
>>
>> struct blk_flush_queue {
>> + spinlock_t mq_flush_lock;
>> unsigned int flush_pending_idx:1;
>> unsigned int flush_running_idx:1;
>> blk_status_t rq_status;
>> unsigned long flush_pending_since;
>> struct list_head flush_queue[2];
>> - struct list_head flush_data_in_flight;
>> + unsigned long flush_data_in_flight;
>> struct request *flush_rq;
>> -
>> - spinlock_t mq_flush_lock;
>> };
>
> The part of replacing inflight data rq list with counter looks fine.
>
> Thanks,
> Ming
>
