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

[Jens Axboe <axboe@kernel.dk>]

On 6/11/25 11:22 PM, Christoph Hellwig wrote:
> On Wed, Jun 11, 2025 at 11:53:07AM -0600, 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.
> 
> Maybe byte the bullet and just make the request lists doubly linked?
> Unlike the bio memory usage for request should not be quite as
> critical.  Right now in my config the las cacheline in struct request
> only has a single 8 byte field anyway, so in practive we won't even
> bloat it.

The space isn't a concern, as you found as well. It's the fact that
doubly linked lists suck in terms of needing to touch both prev
and next for removal.

-- 
Jens Axboe