linux-kernel - Re: [PATCH 8/8] writeback: throttle buffered writeback

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Date:	Sat, 23 Apr 2016 15:37:07 -0600
From:	Jens Axboe <axboe@...com>
To:	xiakaixu <xiakaixu@...wei.com>
CC:	<linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-block@...r.kernel.org>, <jack@...e.cz>,
	<dchinner@...hat.com>, "miaoxie (A)" <miaoxie@...wei.com>,
	Bintian <bintian.wang@...wei.com>, Huxinwei <huxinwei@...wei.com>
Subject: Re: [PATCH 8/8] writeback: throttle buffered writeback

On 04/23/2016 02:21 AM, xiakaixu wrote:
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index 40b57bf4852c..d941f69dfb4b 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -39,6 +39,7 @@
>>
>>   #include "blk.h"
>>   #include "blk-mq.h"
>> +#include "blk-wb.h"
>>
>>   EXPORT_TRACEPOINT_SYMBOL_GPL(block_bio_remap);
>>   EXPORT_TRACEPOINT_SYMBOL_GPL(block_rq_remap);
>> @@ -880,6 +881,7 @@ blk_init_allocated_queue(struct request_queue *q, request_fn_proc *rfn,
>>
>>   fail:
>>   	blk_free_flush_queue(q->fq);
>> +	blk_wb_exit(q);
>>   	return NULL;
>>   }
>>   EXPORT_SYMBOL(blk_init_allocated_queue);
>> @@ -1395,6 +1397,7 @@ void blk_requeue_request(struct request_queue *q, struct request *rq)
>>   	blk_delete_timer(rq);
>>   	blk_clear_rq_complete(rq);
>>   	trace_block_rq_requeue(q, rq);
>> +	blk_wb_requeue(q->rq_wb, rq);
>>
>>   	if (rq->cmd_flags & REQ_QUEUED)
>>   		blk_queue_end_tag(q, rq);
>> @@ -1485,6 +1488,8 @@ void __blk_put_request(struct request_queue *q, struct request *req)
>>   	/* this is a bio leak */
>>   	WARN_ON(req->bio != NULL);
>>
>> +	blk_wb_done(q->rq_wb, req);
>> +
>>   	/*
>>   	 * Request may not have originated from ll_rw_blk. if not,
>>   	 * it didn't come out of our reserved rq pools
>> @@ -1714,6 +1719,7 @@ static blk_qc_t blk_queue_bio(struct request_queue *q, struct bio *bio)
>>   	int el_ret, rw_flags, where = ELEVATOR_INSERT_SORT;
>>   	struct request *req;
>>   	unsigned int request_count = 0;
>> +	bool wb_acct;
>>
>>   	/*
>>   	 * low level driver can indicate that it wants pages above a
>> @@ -1766,6 +1772,8 @@ static blk_qc_t blk_queue_bio(struct request_queue *q, struct bio *bio)
>>   	}
>>
>>   get_rq:
>> +	wb_acct = blk_wb_wait(q->rq_wb, bio, q->queue_lock);
>> +
>>   	/*
>>   	 * This sync check and mask will be re-done in init_request_from_bio(),
>>   	 * but we need to set it earlier to expose the sync flag to the
>> @@ -1781,11 +1789,16 @@ get_rq:
>>   	 */
>>   	req = get_request(q, rw_flags, bio, GFP_NOIO);
>>   	if (IS_ERR(req)) {
>> +		if (wb_acct)
>> +			__blk_wb_done(q->rq_wb);
>>   		bio->bi_error = PTR_ERR(req);
>>   		bio_endio(bio);
>>   		goto out_unlock;
>>   	}
>>
>> +	if (wb_acct)
>> +		req->cmd_flags |= REQ_BUF_INFLIGHT;
>> +
>>   	/*
>>   	 * After dropping the lock and possibly sleeping here, our request
>>   	 * may now be mergeable after it had proven unmergeable (above).
>> @@ -2515,6 +2528,7 @@ void blk_start_request(struct request *req)
>>   	blk_dequeue_request(req);
>>
>>   	req->issue_time = ktime_to_ns(ktime_get());
>> +	blk_wb_issue(req->q->rq_wb, req);
>>
>>   	/*
>>   	 * We are now handing the request to the hardware, initialize
>> @@ -2751,6 +2765,7 @@ void blk_finish_request(struct request *req, int error)
>>   		blk_unprep_request(req);
>>
>>   	blk_account_io_done(req);
>> +	blk_wb_done(req->q->rq_wb, req);
>
> Hi Jens,
>
> Seems the function blk_wb_done() will be executed twice even if the end_io
> callback is set.
> Maybe the same thing would happen in blk-mq.c.

Yeah, that was a mistake, the current version has it fixed. It was 
inadvertently added when I discovered that the flush request didn't work 
properly. Now it just duplicates the call inside the check for if it has 
an ->end_io() defined, since we don't use the normal path for that.

-- 
Jens Axboe