On Wed, Jun 20, 2018 at 11:08:05AM +0200, Christoph Hellwig wrote: > On Mon, Jun 18, 2018 at 11:32:06AM -0600, Keith Busch wrote: > > The default mapping of a cpu to a hardware context is often generally > > applicable, however a user may know of a more appropriate mapping for > > their specific access usage. > > > > This patch allows a user to define their own policy by making the mq hctx > > cpu_list writable. The usage allows a user to append a comma separated > > and/or range list of CPUs to a given hctx's tag set mapping to reassign > > what hctx a cpu may map. > > > > While the writable attribute exists under a specific request_queue, the > > settings will affect all request queues sharing the same tagset. > > > > The user defined setting is lost if the block device is removed and > > re-added, or if the driver re-runs the queue mapping. > > We can't do this without driver opt-in. Managed interrupt rely on > the fact that we can't generate more interrupts once all cpus mapped > to the interrupt line have been offlined. > > So what exactly is the use case? What drivers do you care about?
This patch came at a customer request for NVMe. The controllers have 1:1 queues to CPUs, so currently a submission on CPU A will interrupt CPU A. The user really wants their application to run in CPU A and have the interrupt run in CPU B. We can't change the IRQ affinity, so I thought changing the submission affinity would be less intrusive. I think you're saying this will break if CPU B is offlined. I hadn't considered that, so it doesn't sound like this will work.
