Re: [PATCH 1/4] block: bio: introduce helpers to get the 1st and last bvec

[Ming Lei]

On Tue, Feb 16, 2016 at 4:06 AM, Sagi Grimberg <sagig@xxxxxxxxxxxxxxxxxx> wrote:
>
>> Cc Kent and Keith.
>>
>> Follows another version which should be more efficient.
>> Kent and Keith, I appreciate much if you may give a review on it.
>>
>> diff --git a/include/linux/bio.h b/include/linux/bio.h
>> index 56d2db8..ef45fec 100644
>> --- a/include/linux/bio.h
>> +++ b/include/linux/bio.h
>> @@ -278,11 +278,21 @@ static inline void bio_get_first_bvec(struct bio
>> *bio, struct bio_vec *bv)
>>    */
>>   static inline void bio_get_last_bvec(struct bio *bio, struct bio_vec
>> *bv)
>>   {
>> -       struct bvec_iter iter;
>> +       struct bvec_iter iter = bio->bi_iter;
>> +       int idx;
>> +
>> +       bio_advance_iter(bio, &iter, iter.bi_size);
>> +
>> +       WARN_ON(!iter.bi_idx && !iter.bi_bvec_done);
>> +
>> +       if (!iter.bi_bvec_done)
>> +               idx = iter.bi_idx - 1;
>> +       else    /* in the middle of bvec */
>> +               idx = iter.bi_idx;
>>
>> -       bio_for_each_segment(*bv, bio, iter)
>> -               if (bv->bv_len == iter.bi_size)
>> -                       break;
>> +       *bv = bio->bi_io_vec[idx];
>> +       if (iter.bi_bvec_done)
>> +               bv->bv_len = iter.bi_bvec_done;
>>   }
>>
>>   /*
>>
>
> This looks good too.
>
>>
>>>
>>> However, given that it's a regression bug fix I'm not sure it's the best
>>> idea to add logic here.
>>
>>
>> But the issue is obviously in bio_will_gap(), isn't it?
>>
>> Simply reverting 52cc6eead9095(block: blk-merge: fast-clone bio when
>> splitting rw bios)
>> still might cause performance regression too.
>
>
> That's correct. I assume that the bio splitting code affects
> specific I/O pattern (gappy), however bio_will_gap is also tested

I don't understand why bio splitting affects specific I/O pattern, could you
explain a bit?

>From commit b54ffb73c(block: remove bio_get_nr_vecs()), the upper
layer(fs, dm, dio,...) creates bio with its max size, and splitting should
be triggered easily.

> for bio merges (even if the bios won't merge eventually). This means

As I mentioned, bio_will_gap() is only called for non-splitted bio.

> that each merge check will invoke bio_advance_iter() which is something
> I'd like to avoid...

One idea is to use original way to compute the last bvec for non-cloned
bio, and use the approach in this patch for cloned bio(often splitted bio).
I will take this way in v1 if no one objects.

thanks,
Ming