Re: [PATCH] bcache: dont reset bio opf in bch_data_insert_start

[Coly Li]

On 1/26/21 12:32 PM, Dongsheng Yang wrote:
> 
> 在 2021/1/25 星期一 下午 12:53, Coly Li 写道:
>> On 1/25/21 12:29 PM, Dongsheng Yang wrote:
>>> commit ad0d9e76(bcache: use bio op accessors) makes the bi_opf
>>> modified by bio_set_op_attrs(). But there is a logical
>>> problem in this commit:
>>>
>>>                  trace_bcache_cache_insert(k);
>>>                  bch_keylist_push(&op->insert_keys);
>>>
>>> -               n->bi_rw |= REQ_WRITE;
>>> +               bio_set_op_attrs(n, REQ_OP_WRITE, 0);
>>>                  bch_submit_bbio(n, op->c, k, 0);
>>>          } while (n != bio);
>>>
>>> The old code add REQ_WRITE into bio n and keep other flags; the
>>> new code set REQ_OP_WRITE to bi_opf, but reset all other flags.
>>>
>>> This problem is discoverd in our performance testing:
>>> (1) start a fio with 1M x 128depth for read in /dev/nvme0n1p1
>>> (2) start a fio with 1M x 128depth for write in /dev/escache0 (cache
>>> device is /dev/nvme0n1p2)
>>>
>>> We found the BW of reading is 2000+M/s, but the BW of writing is
>>> 0-100M/s. After some debugging, we found the problem is io submit in
>>> writting is very slow.
>>>
>>> bch_data_insert_start() insert a bio to /dev/nvme0n1p1, but as
>>> cached_dev submit stack bio will be added into current->bio_list, and
>>> return.Then __submit_bio_noacct() will submit the new bio in bio_list
>>> into /dev/nvme0n1p1. This operation would be slow in
>>> blk_mq_submit_bio() -> rq_qos_throttle(q, bio);
>>>
>>> The rq_qos_throttle() will call wbt_should_throttle(),
>>> static inline bool wbt_should_throttle(struct rq_wb *rwb, struct bio
>>> *bio)
>>> {
>>>          switch (bio_op(bio)) {
>>>          case REQ_OP_WRITE:
>>>                  /*
>>>                   * Don't throttle WRITE_ODIRECT
>>>                   */
>>>                  if ((bio->bi_opf & (REQ_SYNC | REQ_IDLE)) ==
>>>                      (REQ_SYNC | REQ_IDLE))
>>>                          return false;
>>> ... ...
>>> }
>>>
>>> As the bio_set_op_attrs() reset the (REQ_SYNC | REQ_IDLE), so this write
>>> bio will be considered as non-direct write.
>>>
>>> After this fix, bio to nvme will flaged as (REQ_SYNC | REQ_IDLE),
>>> then fio for writing will get about 1000M/s bandwidth.
>>>
>>> Fixes: ad0d9e76a4124708dddd00c04fc4b56fc86c02d6
>> It should be,
>> Fixes: ad0d9e76a412 ("bcache: use bio op accessors")
>>
>>> Signed-off-by: Dongsheng Yang<dongsheng.yang@xxxxxxxxxxxx>
>> Please CC the fixed patch author  Mike Christie<mchristi@xxxxxxxxxx>.
> 
> 
> Hi Coly,
> 
>     Should I send a V2 for commit message update?
> 
> Or you can help to fix it when you take it from maillist?
> 

Yes, please fix it in v2 version. And let's wait for response from Mike,
maybe he has better suggestion to fix.

Thanks.

Coly Li