Re: [PATCH 2/2] nvme-pci: allow unmanaged interrupts

Ming Lei
2024-05-13T08:40:13Z

On Mon, May 13, 2024 at 09:33:27AM +0200, Benjamin Meier wrote:
> > From: Christoph Hellwig <hch@lst.de>
> >
> > So let them argue why. I'd rather have a really, really, really
> > good argument for this crap, and I'd like to hear it from the horses
> > mouth.
>
> I reached out to Keith to explore the possibility of manually defining
> which cores handle NVMe interrupts.
>
> The application which we develop and maintain (in the company I work)
> has very high requirements regarding latency. We have some isolated cores

Are these isolated cores controlled by kernel command line `isolcpus=`?

> and we run our application on those.
>
> Our system is using kernel 5.4 which unfortunately does not support
> "isolcpus=managed_irq". Actually, we did not even know about that
> option, because we are focussed on kernel 5.4. It solves part
> of our problem, but being able to specify where exactly interrupts
> are running is still superior in our opinion.
>
> E.g. assume the number of house-keeping cores is small, because we
> want to have full control over the system. In our case we have threads
> of different priorities where some get an exclusive core. Some other threads
> share a core (or a group of cores) with other threads. Now we are still
> happy to assign some interrupts to some of the cores which we consider as
> "medium-priority". Due to the small number of non-isolated cores, it can

So these "medium-priority" cores belong to isolated cpu list, you still expect
NVMe interrupts can be handled on these cpu cores, do I understand correctly?

If yes, I think your case still can be covered with 'isolcpus=managed_irq' which
needn't to be same with cpu cores specified from `isolcpus=`, such as
excluding medium-priority cores from 'isolcpus=managed_irq', and
meantime include them in plain `isolcpus=`.

> be tricky to assign all interrupts to those without a performance-penalty.
>
> Given these requirements, manually specifying interrupt/core assignments
> would offer greater flexibility and control over system performance.
> Moreover, the proposed code changes appear minimal and have no
> impact on existing functionalities.

Looks your main concern is performance, but as Keith mentioned, the proposed
change may degrade nvme perf too:

https://lore.kernel.org/linux-nvme/Zj6745UDnwX1BteO@kbusch-mbp.dhcp.thefacebook.com/

thanks,
Ming