On Mon, Oct 26, 2020 at 08:00:39PM +0100, Thomas Gleixner wrote: > On Mon, Oct 26 2020 at 14:30, Marcelo Tosatti wrote: > > On Fri, Oct 23, 2020 at 11:00:52PM +0200, Thomas Gleixner wrote: > >> So without information from the driver which tells what the best number > >> of interrupts is with a reduced number of CPUs, this cutoff will cause > >> more problems than it solves. Regressions guaranteed. > > > > One might want to move from one interrupt per isolated app core > > to zero, or vice versa. It seems that "best number of interrupts > > is with reduced number of CPUs" information, is therefore in userspace, > > not in driver... > > How does userspace know about the driver internals? Number of management > interrupts, optimal number of interrupts per queue? > > >> Managed interrupts base their interrupt allocation and spreading on > >> information which is handed in by the individual driver and not on crude > >> assumptions. They are not imposing restrictions on the use case. > >> > >> It's perfectly fine for isolated work to save a data set to disk after > >> computation has finished and that just works with the per-cpu I/O queue > >> which is otherwise completely silent. > > > > Userspace could only change the mask of interrupts which are not > > triggered by requests from the local CPU (admin, error, mgmt, etc), > > to avoid the vector exhaustion problem. > > > > However, there is no explicit way for userspace to know that, as far as > > i know. > > > > 130: 34845 0 0 0 0 0 0 0 IR-PCI-MSI 33554433-edge nvme0q1 > > 131: 0 27062 0 0 0 0 0 0 IR-PCI-MSI 33554434-edge nvme0q2 > > 132: 0 0 24393 0 0 0 0 0 IR-PCI-MSI 33554435-edge nvme0q3 > > 133: 0 0 0 24313 0 0 0 0 IR-PCI-MSI 33554436-edge nvme0q4 > > 134: 0 0 0 0 20608 0 0 0 IR-PCI-MSI 33554437-edge nvme0q5 > > 135: 0 0 0 0 0 22163 0 0 IR-PCI-MSI 33554438-edge nvme0q6 > > 136: 0 0 0 0 0 0 23020 0 IR-PCI-MSI 33554439-edge nvme0q7 > > 137: 0 0 0 0 0 0 0 24285 IR-PCI-MSI 33554440-edge nvme0q8 > > > > Can that be retrieved from PCI-MSI information, or drivers > > have to inform this? > > The driver should use a different name for the admin queues. Works for me. Sounds more like a heuristic which can break, so documenting this as an "interface" seems appropriate.
