Re: [Lsf] [LSF/MM TOPIC] block-mq issues with FC

[Sagi Grimberg <sagi@grimberg.me>]

Hey Willy,

>   - Interrupt steering needs to be controlled by block-mq instead of
>     the driver.  It's pointless to have each driver implement its own
>     policies on interrupt steering, irqbalanced remains a source of
>     end-user frustration, and block-mq can change the queue<->cpu mapping
>     without the driver's knowledge.

I honestly don't think that block-mq is the right place to
*assign* interrupt steering. Not all HW devices are dedicated
to storage, take RDMA for example, a RNIC is shared by block
storage, networking and even user-space workloads so obviously
block-mq can't understand how a user wants to steer interrupts.

I think that block-mq needs to ask the device driver:
"what is the optimal queue index for cpu X?" and use it
while *someone* will be responsible for optimum interrupt
steering (can be the driver itself or user-space).

 From some discussions I had with HCH I think he intends to
use the cpu reverse-mapping API to try and do what's described
above (if I'm not mistaken).