On Sun, Apr 07, 2019 at 12:51:23AM -0700, Christoph Hellwig wrote: > On Fri, Apr 05, 2019 at 05:36:32PM -0600, Keith Busch wrote: > > On Fri, Apr 5, 2019 at 5:04 PM Jens Axboe <axboe@xxxxxxxxx> wrote: > > > Looking at current peak testing, I've got around 1.2% in queue enter > > > and exit. It's definitely not free, hence my question. Probably safe > > > to assume that we'll double that cycle counter, per IO. > > > > Okay, that's not negligible at all. I don't know of a faster reference > > than the percpu_ref, but that much overhead would have to rule out > > having a per hctx counter. > > Can we just replace queue_enter/exit with the per-hctx reference > entirely? I don't think that we can readily do that. We still need to protect a request_queue access prior to selecting the hctx.
