Hi Thomas,

On Tue, Jan 14, 2020 at 02:45:00PM +0100, Thomas Gleixner wrote:

Ming,

Ming Lei [off-list ref] writes:

quoted

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.

Maybe the word of 'RT case' isn't accurate, I thought isolated CPUs is only
used for realtime cases, at least that is Peter's usage, maybe I was
wrong.

But it can be generic for all isolated CPUs cases, in which users
don't want managed interrupts to disturb the isolated CPU cores.

quoted

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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?

As mentioned above, it can be applied for all isolated CPUs, when users
don't want managed interrupts to disturb these CPU cores.

quoted

We can document this change somewhere.

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

OK, will do that in formal post.

Thanks, 
Ming