Re: [PATCH v10 6/9] copy-on-read: skip non-guest reads if no copy needed

[Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>]

On 07.10.2020 22:28, Vladimir Sementsov-Ogievskiy wrote:
> 07.10.2020 22:01, Andrey Shinkevich wrote:
>>
>> On 07.10.2020 13:06, Vladimir Sementsov-Ogievskiy wrote:
>>> 29.09.2020 15:38, Andrey Shinkevich wrote:
>>>> If the flag BDRV_REQ_PREFETCH was set, pass it further to the
>>>> COR-driver to skip unneeded reading. It can be taken into account for
>>>> the COR-algorithms optimization. That check is being made during the
>>>> block stream job by the moment.
>>>>
>>>> Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
>>>> ---
>>>>   block/copy-on-read.c | 14 ++++++++++----
>>>>   block/io.c           |  2 +-
>>>>   2 files changed, 11 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/block/copy-on-read.c b/block/copy-on-read.c
>>>> index f53f7e0..5389dca 100644
>>>> --- a/block/copy-on-read.c
>>>> +++ b/block/copy-on-read.c
>>>> @@ -145,10 +145,16 @@ static int coroutine_fn 
>>>> cor_co_preadv_part(BlockDriverState *bs,
>>>>               }
>>>>           }
>>>> -        ret = bdrv_co_preadv_part(bs->file, offset, n, qiov, 
>>>> qiov_offset,
>>>> -                                  local_flags);
>>>> -        if (ret < 0) {
>>>> -            return ret;
>>>> +        if ((flags & BDRV_REQ_PREFETCH) &
>>>
>>> BDRV_REQ_PREFETCH is documented to be only used with 
>>> BDRV_REQ_COPY_ON_READ. But here
>>> BDRV_REQ_COPY_ON_READ appears intermediately. We should change 
>>> documentation in block.h
>>> in a separate patch (and probably code in bdrv_aligned_preadv())
>>>
>>
>> OK, we will come here without the BDRV_REQ_PREFETCH flag set.
> 
> flag BDRV_REQ_PREFETCH should be set in stream job. Where should it be 
> handled, I don't follow?
> 

If we leave block/io.c unchanged in this patch, what I'm agreeing with, 
we'll come to the COR-driver with the hardcoded flags = 0 :

#4  0x000055a22bb480cf in cor_co_preadv_part (bs=0x55a22d593710, 
offset=0, bytes=524288, qiov=0x0, qiov_offset=0, flags=0) at 
../block/copy-on-read.c:149
#5  0x000055a22badcb1d in bdrv_driver_preadv (bs=0x55a22d593710, 
offset=0, bytes=524288, qiov=0x0, qiov_offset=0, flags=0) at 
../block/io.c:1129
#6  0x000055a22baddc81 in bdrv_aligned_preadv (child=0x55a22d814780, 
req=0x7f8c1abffce0, offset=0, bytes=524288, align=1, qiov=0x0, 
qiov_offset=0, flags=512) at ../block/io.c:1515
#7  0x000055a22bade59a in bdrv_co_preadv_part (child=0x55a22d814780, 
offset=0, bytes=524288, qiov=0x0, qiov_offset=0, 
flags=BDRV_REQ_PREFETCH) at ../block/io.c:1757
#8  0x000055a22bade3d2 in bdrv_co_preadv (child=0x55a22d814780, 
offset=0, bytes=524288, qiov=0x0, flags=BDRV_REQ_PREFETCH) at 
../block/io.c:1715
#9  0x000055a22baf5d09 in blk_do_preadv (blk=0x55a22d818c00, offset=0, 
bytes=524288, qiov=0x0, flags=BDRV_REQ_PREFETCH) at 
../block/block-backend.c:1211
#10 0x000055a22baf5d61 in blk_co_preadv (blk=0x55a22d818c00, offset=0, 
bytes=524288, qiov=0x0, flags=BDRV_REQ_PREFETCH) at 
../block/block-backend.c:1223
#11 0x000055a22bab4eba in stream_populate (blk=0x55a22d818c00, offset=0, 
bytes=524288) at ../block/stream.c:50
#12 0x000055a22bab52c2 in stream_run (job=0x55a22d810a20, 
errp=0x55a22d810aa0) at ../block/stream.c:162
#13 0x000055a22bab79f0 in job_co_entry (opaque=0x55a22d810a20) at 
../job.c:908

So, the only way for the COR-filter driver to differ between guests 
reads and the stream job is to check the qiov pointer for NULL and reset 
the flags as appropriate. This is what I am going to do in the next version.

Andrey

>> To differ between guest reads and the stream job ones, we would set it 
>> here by checking for the qiov NULL pointer:
>>
>>
>> diff --git a/block/copy-on-read.c b/block/copy-on-read.c
>> index 4e3b1c5..df2c2ab 100644
>> --- a/block/copy-on-read.c
>> +++ b/block/copy-on-read.c
>> @@ -144,6 +144,9 @@ static int coroutine_fn 
>> cor_co_preadv_part(BlockDriverState *bs,
>>                                             n, &n);
>>               if (ret) {
>>                   local_flags |= BDRV_REQ_COPY_ON_READ;
>> +                if (!qiov) {
>> +                    local_flags |= BDRV_REQ_PREFETCH;
> 
> if qiov is NULL, this means that flags must include BDRV_REQ_PREFETCH. 
> local_flags should inherit flags I think.
> 
>> +                }
>>               }
>>           }
>>
>> Andrey
>>
>>>> +            !(local_flags & BDRV_REQ_COPY_ON_READ)) {
>>>> +            /* Skip non-guest reads if no copy needed */
>>>> +        } else {
>>>> +
>>>
>>> extra new-line ?
>>>
>>>> +            ret = bdrv_co_preadv_part(bs->file, offset, n, qiov, 
>>>> qiov_offset,
>>>> +                                      local_flags);
>>>> +            if (ret < 0) {
>>>> +                return ret;
>>>> +            }
>>>>           }
>>>>           offset += n;
>>>> diff --git a/block/io.c b/block/io.c
>>>> index 11df188..62b75a5 100644
>>>> --- a/block/io.c
>>>> +++ b/block/io.c
>>>> @@ -1388,7 +1388,7 @@ static int coroutine_fn 
>>>> bdrv_co_do_copy_on_readv(BdrvChild *child,
>>>>               qemu_iovec_init_buf(&local_qiov, bounce_buffer, pnum);
>>>>               ret = bdrv_driver_preadv(bs, cluster_offset, pnum,
>>>> -                                     &local_qiov, 0, 0);
>>>> +                                     &local_qiov, 0, flags & 
>>>> BDRV_REQ_PREFETCH);
>>>
>>> Why? In this place we want to read. We'll write back the data a few 
>>> lines below. What will we write,
>>> if underlying driver decide to do nothing because of BDRV_REQ_PREFETCH?
>>>
>>
>> See my comment above please.
> 
> Anyway, BDRV_REQ_PREFETCH here is wrong. You should not pass any qiov, 
> if you set BDRV_REQ_PREFETCH flag.
> 
> If we come to bdrv_co_do_copy_on_readv, it means that we have 
> COPY_ON_READ flag. And therefore, we will handle
> PREFETCH and COPY_ON_READ here in generic layer. And therefore, we 
> shouldn't pass them to driver.
> 
> On the contrary, if we have PREFETCH flag in bdrv_co_aligned_preadv, but 
> don't have COPY_ON_READ in the same time,
> this means that we must pass PREFETCH flag to the driver if it supports 
> it. And do nothing if driver
> doesn't support PREFETCH. That's how I see it.
> 
>>
>>>>               if (ret < 0) {
>>>>                   goto err;
>>>>               }
>>>>
>>>
>>>
> 
>