On Wed, 21 Oct 2020 22:25:48 +0200 Thomas Gleixner wrote: > On Tue, Oct 20 2020 at 20:07, Thomas Gleixner wrote: > > On Tue, Oct 20 2020 at 12:18, Nitesh Narayan Lal wrote: > >> However, IMHO we would still need a logic to prevent the devices from > >> creating excess vectors. > > > > Managed interrupts are preventing exactly that by pinning the interrupts > > and queues to one or a set of CPUs, which prevents vector exhaustion on > > CPU hotplug. > > > > Non-managed, yes that is and always was a problem. One of the reasons > > why managed interrupts exist. > > But why is this only a problem for isolation? The very same problem > exists vs. CPU hotplug and therefore hibernation. > > On x86 we have at max. 204 vectors available for device interrupts per > CPU. So assumed the only device interrupt in use is networking then any > machine which has more than 204 network interrupts (queues, aux ...) > active will prevent the machine from hibernation. > > Aside of that it's silly to have multiple queues targeted at a single > CPU in case of hotplug. And that's not a theoretical problem. Some > power management schemes shut down sockets when the utilization of a > system is low enough, e.g. outside of working hours. > > The whole point of multi-queue is to have locality so that traffic from > a CPU goes through the CPU local queue. What's the point of having two > or more queues on a CPU in case of hotplug? > > The right answer to this is to utilize managed interrupts and have > according logic in your network driver to handle CPU hotplug. When a CPU > goes down, then the queue which is associated to that CPU is quiesced > and the interrupt core shuts down the relevant interrupt instead of > moving it to an online CPU (which causes the whole vector exhaustion > problem on x86). When the CPU comes online again, then the interrupt is > reenabled in the core and the driver reactivates the queue. I think Mellanox folks made some forays into managed irqs, but I don't remember/can't find the details now. For networking the locality / queue per core does not always work, since the incoming traffic is usually spread based on a hash. Many applications perform better when network processing is done on a small subset of CPUs, and application doesn't get interrupted every 100us. So we do need extra user control here. We have a bit of a uAPI problem since people had grown to depend on IRQ == queue == NAPI to configure their systems. "The right way" out would be a proper API which allows associating queues with CPUs rather than IRQs, then we can use managed IRQs and solve many other problems. Such new API has been in the works / discussions for a while now. (Magnus keep me honest here, if you disagree the queue API solves this.)
