Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: Kernel-managed IRQ affinity (cont) | Date | Tue, 14 Jan 2020 14:45:00 +0100 |
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Ming,
Ming Lei <ming.lei@redhat.com> writes: > On Fri, Jan 10, 2020 at 08:43:14PM +0100, Thomas Gleixner wrote: >> Ming Lei <ming.lei@redhat.com> writes: >> > 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.
>> 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.
>> 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
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