On 10/21/2020 5:02 PM, Jakub Kicinski wrote: > 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. > I remember looking into this a few years ago, and not getting very far either. > 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. > I think we (Intel) hit some of the same issues you mention. I know I personally would like to see something that lets a lot of the current driver-specific policy be moved out. I think it should be possible to significantly simplify the abstraction used by the drivers. > 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.) >
