Matthew Wilcox <willy@xxxxxxxxxxxxx> writes: > 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? I am hoping the author of this patch or folks with io-uring expertise (which added the per-cpu bio cache in the first place) could answer this better. i.e. Now that per-cpu bio cache is being used by io-uring rw requests for both polled and non-polled I/O. Does that mean, we can kill IOCB_ALLOC_CACHE check from iomap dio path completely and use per-cpu bio cache unconditionally by passing REQ_ALLOC_CACHE flag? That means all DIO requests via iomap can now use this per-cpu bio cache and not just the one initiated via io-uring path. Or are there still restrictions in using this per-cpu bio cache, which limits it to be only used via io-uring path? If yes, what are they? And can this be documented somewhere? -ritesh
