Matthew Wilcox <willy@xxxxxxxxxxxxx> writes: > On Fri, Aug 22, 2025 at 08:05:50AM -0700, Darrick J. Wong wrote: >> On Fri, Aug 22, 2025 at 04:26:06PM +0800, Fengnan Chang wrote: >> > 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@xxxxxxxxx/ -ritesh
