Re: [PATCH 1/2] block: make rq sector size accessible for block stats

[Pavel Begunkov <asml.silence@gmail.com>]

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On 27/05/2019 14:39, Hou Tao wrote:
> Hi,
> 
> On 2019/5/25 19:46, Pavel Begunkov wrote:
>> That's the same bug I posted a patch about 3+ weeks ago. No answer yet,
>> however.
>> https://www.spinics.net/lists/linux-block/msg40049.html
>>
> Sorry for missing that.
Anyway I haven't fixed it properly there.

> 
>> I think putting time sampling at __blk_mq_end_request() won't give
>> sufficient precision for more sophisticated hybrid polling, because path
>> __blk_mq_complete_request() -> __blk_mq_end_request() adds a lot of
>> overhead (redirect rq to another cpu, dma unmap, etc). That's the case
>> for mentioned patchset
>> https://www.spinics.net/lists/linux-block/msg40044.html
>>
>> It works OK for 50-80us requests (e.g. SSD read), but is not good enough
>> for 20us and less (e.g. SSD write). I think it would be better to save
>> the time in advance, and use it later.
>>
> Yes, getting the end time from blk_mq_complete_request() will be more accurate.
What bothers me, if we sample time there, could we use it in throttling?
I'm not sure what it exactly expects.

> 
>> By the way, it seems, that __blk_mq_end_request() is really better place
>> to record stats, as not all drivers use __blk_mq_complete_request(),
>> even though they don't implement blk_poll().
>>
> And __blk_mq_complete_request() may be invoked multiple times (although no possible
> for nvme device), but __blk_mq_end_request() is only invoked once.
Doubt it, that would be rather strange to *complete* a request multiple
times. Need to check, but so far I only seen drivers either using it
once or calling blk_mq_end_request() directly.

> 
> Regards,
> Tao
> 
>>
>> On 21/05/2019 10:59, Hou Tao wrote:
>>> Currently rq->data_len will be decreased by partial completion or
>>> zeroed by completion, so when blk_stat_add() is invoked, data_len
>>> will be zero and there will never be samples in poll_cb because
>>> blk_mq_poll_stats_bkt() will return -1 if data_len is zero.
>>>
>>> We could move blk_stat_add() back to __blk_mq_complete_request(),
>>> but that would make the effort of trying to call ktime_get_ns()
>>> once in vain. Instead we can reuse throtl_size field, and use
>>> it for both block stats and block throttle, and adjust the
>>> logic in blk_mq_poll_stats_bkt() accordingly.
>>>
>>> Fixes: 4bc6339a583c ("block: move blk_stat_add() to __blk_mq_end_request()")
>>> Signed-off-by: Hou Tao <houtao1@huawei.com>
>>> ---
>>>  block/blk-mq.c         | 11 +++++------
>>>  block/blk-throttle.c   |  3 ++-
>>>  include/linux/blkdev.h | 15 ++++++++++++---
>>>  3 files changed, 19 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>> index 08a6248d8536..4d1462172f0f 100644
>>> --- a/block/blk-mq.c
>>> +++ b/block/blk-mq.c
>>> @@ -44,12 +44,12 @@ static void blk_mq_poll_stats_fn(struct blk_stat_callback *cb);
>>>  
>>>  static int blk_mq_poll_stats_bkt(const struct request *rq)
>>>  {
>>> -	int ddir, bytes, bucket;
>>> +	int ddir, sectors, bucket;
>>>  
>>>  	ddir = rq_data_dir(rq);
>>> -	bytes = blk_rq_bytes(rq);
>>> +	sectors = blk_rq_stats_sectors(rq);
>>>  
>>> -	bucket = ddir + 2*(ilog2(bytes) - 9);
>>> +	bucket = ddir + 2 * ilog2(sectors);
>>>  
>>>  	if (bucket < 0)
>>>  		return -1;
>>> @@ -329,6 +329,7 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
>>>  	else
>>>  		rq->start_time_ns = 0;
>>>  	rq->io_start_time_ns = 0;
>>> +	rq->stats_sectors = 0;
>>>  	rq->nr_phys_segments = 0;
>>>  #if defined(CONFIG_BLK_DEV_INTEGRITY)
>>>  	rq->nr_integrity_segments = 0;
>>> @@ -678,9 +679,7 @@ void blk_mq_start_request(struct request *rq)
>>>  
>>>  	if (test_bit(QUEUE_FLAG_STATS, &q->queue_flags)) {
>>>  		rq->io_start_time_ns = ktime_get_ns();
>>> -#ifdef CONFIG_BLK_DEV_THROTTLING_LOW
>>> -		rq->throtl_size = blk_rq_sectors(rq);
>>> -#endif
>>> +		rq->stats_sectors = blk_rq_sectors(rq);
>>>  		rq->rq_flags |= RQF_STATS;
>>>  		rq_qos_issue(q, rq);
>>>  	}
>>> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
>>> index 1b97a73d2fb1..88459a4ac704 100644
>>> --- a/block/blk-throttle.c
>>> +++ b/block/blk-throttle.c
>>> @@ -2249,7 +2249,8 @@ void blk_throtl_stat_add(struct request *rq, u64 time_ns)
>>>  	struct request_queue *q = rq->q;
>>>  	struct throtl_data *td = q->td;
>>>  
>>> -	throtl_track_latency(td, rq->throtl_size, req_op(rq), time_ns >> 10);
>>> +	throtl_track_latency(td, blk_rq_stats_sectors(rq), req_op(rq),
>>> +			     time_ns >> 10);
>>>  }
>>>  
>>>  void blk_throtl_bio_endio(struct bio *bio)
>>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>>> index 1aafeb923e7b..68a0841d3554 100644
>>> --- a/include/linux/blkdev.h
>>> +++ b/include/linux/blkdev.h
>>> @@ -202,9 +202,12 @@ struct request {
>>>  #ifdef CONFIG_BLK_WBT
>>>  	unsigned short wbt_flags;
>>>  #endif
>>> -#ifdef CONFIG_BLK_DEV_THROTTLING_LOW
>>> -	unsigned short throtl_size;
>>> -#endif
>>> +	/*
>>> +	 * rq sectors used for blk stats. It has the same value
>>> +	 * with blk_rq_sectors(rq), except that it never be zeroed
>>> +	 * by completion.
>>> +	 */
>>> +	unsigned short stats_sectors;
>>>  
>>>  	/*
>>>  	 * Number of scatter-gather DMA addr+len pairs after
>>> @@ -892,6 +895,7 @@ static inline struct request_queue *bdev_get_queue(struct block_device *bdev)
>>>   * blk_rq_err_bytes()		: bytes left till the next error boundary
>>>   * blk_rq_sectors()		: sectors left in the entire request
>>>   * blk_rq_cur_sectors()		: sectors left in the current segment
>>> + * blk_rq_stats_sectors()	: sectors of the entire request used for stats
>>>   */
>>>  static inline sector_t blk_rq_pos(const struct request *rq)
>>>  {
>>> @@ -920,6 +924,11 @@ static inline unsigned int blk_rq_cur_sectors(const struct request *rq)
>>>  	return blk_rq_cur_bytes(rq) >> SECTOR_SHIFT;
>>>  }
>>>  
>>> +static inline unsigned int blk_rq_stats_sectors(const struct request *rq)
>>> +{
>>> +	return rq->stats_sectors;
>>> +}
>>> +
>>>  #ifdef CONFIG_BLK_DEV_ZONED
>>>  static inline unsigned int blk_rq_zone_no(struct request *rq)
>>>  {
>>>
>>
> 

-- 
Yours sincerely,
Pavel Begunkov


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