Re: [PATCH] block: use plug request list tail for one-shot backmerge attempt

["Mohamed Abuelfotoh, Hazem" <abuehaze@amazon.com>]

On 11/06/2025 18:53, Jens Axboe wrote:
> Yes we can't revert it, and honestly I would not want to even if that
> was an option. If the multi-queue case is particularly important, you
> could just do something ala the below - keep scanning until you a merge
> _could_ have happened but didn't. Ideally we'd want to iterate the plug
> list backwards and then we could keep the same single shot logic, where
> you only attempt one request that has a matching queue. And obviously we
> could just doubly link the requests, there's space in the request
> linkage code to do that. But that'd add overhead in general, I think
> it's better to shove a bit of that overhead to the multi-queue case.
> 
> I suspect the below would do the trick, however.
> 
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 70d704615be5..4313301f131c 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -1008,6 +1008,8 @@ bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
>          rq_list_for_each(&plug->mq_list, rq) {
>                  if (rq->q != q)
>                          continue;
> +               if (blk_try_merge(rq, bio) == ELEVATOR_NO_MERGE)
> +                       continue;
>                  if (blk_attempt_bio_merge(q, rq, bio, nr_segs, false) ==
>                      BIO_MERGE_OK)
>                          return true;
> 
> --
> Jens Axboe

Sorry for my delayed reply here as I was on business trip for the last 
couple of weeks. I have done some testing on 6 SSDs aggregated as raid0 
to simulate the multi-queue case but I haven't seen measurable impact 
from that change at least on the random write test case. Looks like the 
patch has been queued to 6.15 & 6.12 stable without this change so I 
assume we are dropping it?

Kernel         |  fio (B.W MiB/sec)  | I/O size (iostat)
-------------- +---------------------+--------------------
6.15.2         |   639               |  4KiB
6.15.2+patchv1 |   648               |  4KiB
6.15.2+patchv2 |   665               |  4KiB
--------------+----------------------+--------------------

Hazem