Matthew Wilcox [off-list ref] writes:

On Fri, Aug 22, 2025 at 08:05:50AM -0700, Darrick J. Wong wrote:

quoted

On Fri, Aug 22, 2025 at 04:26:06PM +0800, Fengnan Chang wrote:

quoted

When use io_uring with direct IO, we could use per-cpu bio cache
from bio_alloc_bioset, So pass IOCB_ALLOC_CACHE flag to alloc
bio helper.
 
+	if (iter->flags & IOMAP_ALLOC_CACHE)
+		bio_opf |= REQ_ALLOC_CACHE;

Is there a reason /not/ to use the per-cpu bio cache unconditionally?

AIUI it's not safe because completions might happen on a different CPU
from the submission.

At max the bio de-queued from cpu X can be returned to cpu Y cache, this
shouldn't be unsafe right? e.g. bio_put_percpu_cache(). 
Not optimal for performance though.

Also even for io-uring the IRQ completions (non-polling requests) can
get routed to a different cpu then the submitting cpu, correct?
Then the completions (bio completion processing) are handled via IPIs on
the submtting cpu or based on the cache topology, right?

At least, there's nowhere that sets REQ_ALLOC_CACHE unconditionally.

This could do with some better documentation ..

Agreed. Looking at the history this got added for polling mode first but
later got enabled for even irq driven io-uring rw requests [1]. So it
make sense to understand if this can be added unconditionally for DIO
requests or not.

[1]: https://lore.kernel.org/io-uring/aab3521d49fd6c1ff6ea194c9e63d05565efc103.1666347703.git.asml.silence@gmail.com/ (local)

-ritesh