Messages in this thread | | | Date | Wed, 15 Jul 2015 14:48:00 -0400 | From | Matthew Wilcox <> | Subject | Re: [Ksummit-discuss] [TECH TOPIC] IRQ affinity |
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On Wed, Jul 15, 2015 at 11:25:55AM -0600, Jens Axboe wrote: > On 07/15/2015 11:19 AM, Keith Busch wrote: > >On Wed, 15 Jul 2015, Bart Van Assche wrote: > >>* With blk-mq and scsi-mq optimal performance can only be achieved if > >> the relationship between MSI-X vector and NUMA node does not change > >> over time. This is necessary to allow a blk-mq/scsi-mq driver to > >> ensure that interrupts are processed on the same NUMA node as the > >> node on which the data structures for a communication channel have > >> been allocated. However, today there is no API that allows > >> blk-mq/scsi-mq drivers and irqbalanced to exchange information > >> about the relationship between MSI-X vector ranges and NUMA nodes. > > > >We could have low-level drivers provide blk-mq the controller's irq > >associated with a particular h/w context, and the block layer can provide > >the context's cpumask to irqbalance with the smp affinity hint. > > > >The nvme driver already uses the hwctx cpumask to set hints, but this > >doesn't seems like it should be a driver responsibility. It currently > >doesn't work correctly anyway with hot-cpu since blk-mq could rebalance > >the h/w contexts without syncing with the low-level driver. > > > >If we can add this to blk-mq, one additional case to consider is if the > >same interrupt vector is used with multiple h/w contexts. Blk-mq's cpu > >assignment needs to be aware of this to prevent sharing a vector across > >NUMA nodes. > > Exactly. I may have promised to do just that at the last LSF/MM conference, > just haven't done it yet. The point is to share the mask, I'd ideally like > to take it all the way where the driver just asks for a number of vecs > through a nice API that takes care of all this. Lots of duplicated code in > drivers for this these days, and it's a mess.
Yes. I think the fundamental problem is that our MSI-X API is so funky. We have this incredibly flexible scheme where each MSI-X vector could have its own interrupt handler, but that's not what drivers want. They want to say "Give me eight MSI-X vectors spread across the CPUs, and use this interrupt handler for all of them". That is, instead of the current scheme where each MSI-X vector gets its own Linux interrupt, we should have one interrupt handler (of the per-cpu interrupt type), which shows up with N bits set in its CPU mask.
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