Re: [PATCHv3] blk-mq: pop cached request if it is usable

[Ming Lei <tom.leiming@gmail.com>]

On Thu, May 21, 2026 at 07:44:50PM -0600, Keith Busch wrote:
> On Fri, May 22, 2026 at 07:33:39AM +0800, Ming Lei wrote:
> > 
> > BTW, as mentioned in v2, the request may be added back in case of merge,
> > but seems not a big deal given blk_mq_free_plug_rqs() doesn't free requests
> > in batch.
> 
> We could introduce a special goto label for the merge case to push it

It can be done simply by replacing the added `blk_mq_free_request` with moving
it back to plug list.

> back to the cached requests, but it's not clear that it's worth it. The
> cached requests are preallocated to match what is about to be dispatched
> (up to a limit), so if a merge happens, there may not be another
> subsequent bio submissions to use it, so it would just get freed as an
> unused request later anyway. And if we block on the split bio allocation
> such that it frees the cached rq list, I think we've already lost the
> optimization battle, so no need to complicate things.

I mean plain sequential IO workload, in which batch allocation is taken, and
just the 1st request is used, all others are freed one by one, which may
be improved to batch release too in theory.


Thanks,
Ming