Ming,

Ming Lei [off-list ref] writes:

On Fri, Jan 10, 2020 at 08:43:14PM +0100, Thomas Gleixner wrote:

quoted

Ming Lei [off-list ref] writes:

quoted

That is why I try to exclude isolated CPUs from interrupt effective affinity,
turns out the approach is simple and doable.

Yes, it's doable. But it still is inconsistent behaviour. Assume the
following configuration:

  8 CPUs CPU0,1 assigned for housekeeping

With 8 queues the proposed change does nothing because each queue is
mapped to exactly one CPU.

That is expected behavior for this RT case, given userspace won't submit
IO from isolated CPUs.

What is _this_ RT case? We really don't implement policy for a specific
use case. If the kernel implements a policy then it has to be generally
useful and practical.

quoted

With 4 queues you get the following:

 CPU0,1       queue 0
 CPU2,3       queue 1
 CPU4,5       queue 2
 CPU6,7       queue 3

No effect on the isolated CPUs either.

With 2 queues you get the following:

 CPU0,1,2,3   queue 0
 CPU4,5,6,7   queue 1

So here the isolated CPUs 2 and 3 get the isolation, but 4-7
not. That's perhaps intended, but definitely not documented.

That is intentional change, given no IO will be submitted from 4-7
most of times in RT case, so it is fine to select effective CPU from
isolated CPUs in this case. As peter mentioned, IO may just be submitted
from isolated CPUs during booting. Once the system is setup, no IO
comes from isolated CPUs, then no interrupt is delivered to isolated
CPUs, then meet RT's requirement.

Again. This is a specific usecase. Is this generally applicable?

We can document this change somewhere.

Yes, this needs to be documented very clearly with that command line
parameter.

quoted

So you really need to make your mind up and describe what the intended
effect of this is and why you think that the result is correct.

In short, if there is at least one housekeeping available in the
interrupt's affinity, we choose effective CPU from housekeeping CPUs.
Otherwise, keep the current behavior wrt. selecting effective CPU.

With this approach, no interrupts can be delivered to isolated CPUs
if no IOs are submitted from these CPUs.

Please let us know if it addresses your concerns.

Mostly. See above.

Thanks,

        tglx