On Fri, Aug 22, 2025 at 09:37:32PM +0530, Ritesh Harjani wrote: > Matthew Wilcox <willy@xxxxxxxxxxxxx> writes: > > On Fri, Aug 22, 2025 at 08:05:50AM -0700, Darrick J. Wong wrote: > >> 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. So why does the flag now exist at all? Why not use the cache unconditionally?
