On Tue, 29 Jan 2019, John Garry wrote: > On 29/01/2019 12:01, Thomas Gleixner wrote: > > If the last CPU which is associated to a queue (and the corresponding > > interrupt) goes offline, then the subsytem/driver code has to make sure > > that: > > > > 1) No more requests can be queued on that queue > > > > 2) All outstanding of that queue have been completed or redirected > > (don't know if that's possible at all) to some other queue. > > This may not be possible. For the HW I deal with, we have symmetrical delivery > and completion queues, and a command delivered on DQx will always complete on > CQx. Each completion queue has a dedicated IRQ. So you can stop queueing on DQx and wait for all outstanding ones to come in on CQx, right? > > That has to be done in that order obviously. Whether any of the > > subsystems/drivers actually implements this, I can't tell. > > Going back to c5cb83bb337c25, it seems to me that the change was made with the > idea that we can maintain the affinity for the IRQ as we're shutting it down > as no interrupts should occur. > > However I don't see why we can't instead keep the IRQ up and set the affinity > to all online CPUs in offline path, and restore the original affinity in > online path. The reason we set the queue affinity to specific CPUs is for > performance, but I would not say that this matters for handling residual IRQs. Oh yes it does. The problem is especially on x86, that if you have a large number of queues and you take a large number of CPUs offline, then you run into vector space exhaustion on the remaining online CPUs. In the worst case a single CPU on x86 has only 186 vectors available for device interrupts. So just take a quad socket machine with 144 CPUs and two multiqueue devices with a queue per cpu. ---> FAIL It probably fails already with one device because there are lots of other devices which have regular interrupt which cannot be shut down. Thanks, tglx
