Re: Revert "dm: always call blk_queue_split() in dm_process_bio()"

[Mike Snitzer <snitzer@redhat.com>]

[cc'ing dm-devel and linux-block because this is upstream concern too]

On Wed, Sep 09 2020 at  1:00pm -0400,
Vijayendra Suman <vijayendra.suman@oracle.com> wrote:

>    Hello Mike,
> 
>    While Running pgbench tool with  5.4.17 kernel build
> 
>    Following performance degrade is found out
> 
>    buffer read/write metric : -17.2%
>    cache read/write metric : -18.7%
>    disk read/write metric : -19%
> 
>    buffer
>    number of transactions actually processed: 840972
>    latency average = 24.013 ms
>    tps = 4664.153934 (including connections establishing)
>    tps = 4664.421492 (excluding connections establishing)
> 
>    cache
>    number of transactions actually processed: 551345
>    latency average = 36.949 ms
>    tps = 3031.223905 (including connections establishing)
>    tps = 3031.402581 (excluding connections establishing)
> 
>    After revert of Commit
>    2892100bc85ae446088cebe0c00ba9b194c0ac9d ( Revert "dm: always call
>    blk_queue_split() in dm_process_bio()")

I assume 2892100bc85ae446088cebe0c00ba9b194c0ac9d is 5.4-stable's
backport of upstream commit 120c9257f5f19e5d1e87efcbb5531b7cd81b7d74 ?

>    Performance is Counter measurement
> 
>    buffer ->
>    number of transactions actually processed: 1135735
>    latency average = 17.799 ms
>    tps = 6292.586749 (including connections establishing)
>    tps = 6292.875089 (excluding connections establishing)
> 
>    cache ->
>    number of transactions actually processed: 648177
>    latency average = 31.217 ms
>    tps = 3587.755975 (including connections establishing)
>    tps = 3587.966359 (excluding connections establishing)
> 
>    Following is your commit
> 
>    diff --git a/drivers/md/dm.c b/drivers/md/dm.c
>    index cf71a2277d60..1e6e0c970e19 100644
>    --- a/drivers/md/dm.c
>    +++ b/drivers/md/dm.c
>    @@ -1760,8 +1760,9 @@ static blk_qc_t dm_process_bio(struct mapped_device
>    *md,
>             * won't be imposed.
>             */
>            if (current->bio_list) {
>    -               blk_queue_split(md->queue, &bio);
>    -               if (!is_abnormal_io(bio))
>    +               if (is_abnormal_io(bio))
>    +                       blk_queue_split(md->queue, &bio);
>    +               else
>                            dm_queue_split(md, ti, &bio);
>            }
> 
>    Could you have a look if it is safe to revert this commit.

No, it really isn't a good idea given what was documented in the commit
header for commit 120c9257f5f19e5d1e87efcbb5531b7cd81b7d74 -- the
excessive splitting is not conducive to performance either.

So I think we need to identify _why_ reverting this commit is causing
such a performance improvement.  Why is calling blk_queue_split() before
dm_queue_split() benefiting your pgbench workload?

Thanks,
Mike