Messages in this thread |  | | Date | Thu, 19 Dec 2019 09:32:14 -0500 | From | Peter Xu <> | Subject | Re: Kernel-managed IRQ affinity (cont) |
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On Thu, Dec 19, 2019 at 04:28:19PM +0800, Ming Lei wrote: > Hi Peter,
Hi, Ming,
> > On Mon, Dec 16, 2019 at 02:57:12PM -0500, Peter Xu wrote: > > Hi, Thomas, > > > > (Sorry I must have lost the discussion during an email migration, so > > I'll start with a new one) > > > > This is a continued discussion of previous one on kernel managed IRQ > > affinity [1]. I think at that time the conclusion is that we don't > > have a usage scenario to change current policy [2]. However recently > > I noticed that it is probably a very fundamental requirement for some > > real-time scenarios, even when there's no multi-queue involved. > > > > In my test case, it was a very common realtime guest with 10 vcpus, > > 0-1 are housekeeping vcpus, 2-9 are realtime vcpus. The guest has one > > virtio-blk device as boot disk. With a distribution very close to > > latest upstream, we can observe high spikes, probably due to the IRQs. > > > > To guarantee realtime responsiveness, we need to make sure the IRQs > > will be managable, say, when I run a real-time workload on vcpu9, we > > should be able to move all the IRQs from vcpu9 to the other vcpus > > (most probably vcpu0 and vcpu1). However with the kernel managed IRQs > > we can't echo to /proc/irq/N/smp_affinity. Here, vcpu9 gets IRQ 38 > > from the virtio-blk device: > > > > # cat /proc/interrupts | grep -w 38 > > 38: 0 0 0 0 0 0 0 0 0 15206 PCI-MSI 2621441-edge virtio2-req.0 > > # cat /proc/irq/38/smp_affinity > > 3ff > > # cat /proc/irq/38/effective_affinity > > 200 > > > > Meanwhile, I don't think there's anything special for VMs, so this > > issue should exist even for hosts as long as the IRQ is managed in the > > same way here as the virtio-blk device. > > > > As Ming has mentioned in previous discussions [3], I think it would be > > at least good if the kernel IRQ system can respect "irqaffinity=" when > > assigning IRQs to the cores. Currently it's not. What would you > > suggest in this case? Do you think this is a valid user scenario? > > > > Thanks, > > > > [1] https://lkml.org/lkml/2019/3/18/15 > > [2] https://lkml.org/lkml/2019/3/25/562 > > [3] https://lkml.org/lkml/2019/3/25/308 > > The following patch supposes to implementation the requirement for you, > can you test it by passing 'isolcpus=managed_irq,X-Y'?
I really appreciate your patch! I'll keep this version, while before I start to test it...
> > With this kind of change, you can't run any IO from any isolated > CPU core, otherwise, unpredictable error may be triggered, either oops or > IO hang.
... I'm not sure whether this can be acceptable for a production environment.
In our case, the IRQ should come from virtio-blk which is the root disk, so I assume even the RT core could use it at least when loading the executable into RAM. So...
> > Another conservative approach is to only select effective CPU from > non-isolated cpus, however, the assigned CPUs may not be balanced among > interrupt vectors. But it is safer, since the system still works even if > someone submits IO from any isolated cpu core.
... this one seems to be more appealing at least to me.
Thanks,
-- Peter Xu
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