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Re: [PATCH 1/6] genirq: allow assigning affinity to present but not online CPUs

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  Re: [PATCH 1/6] genirq: allow assigning affinity to present but         not online CPUs Christoph Hellwig <hch@lst.de> - 2017-02-05 17:50 +0100
    Re: [PATCH 1/6] genirq: allow assigning affinity to present but not  online CPUs Keith Busch <keith.busch@intel.com> - 2017-02-06 18:00 +0100
      Re: [PATCH 1/6] genirq: allow assigning affinity to present but         not online CPUs Christoph Hellwig <hch@lst.de> - 2017-02-06 18:10 +0100

#1573932 — Re: [PATCH 1/6] genirq: allow assigning affinity to present but not online CPUs

Hi Joe,

On Fri, Feb 03, 2017 at 08:58:09PM -0500, Joe Korty wrote:
> IIRC, some years ago I ran across a customer system where
> the #cpus_present was twice as big as #cpus_possible.
> 
> Hyperthreading was turned off in the BIOS so it was not
> entirely out of line for the extra cpus to be declared
> present, even though none of them would ever be available
> for use.

This sounds like a system we should quirk around instead of optimizing
for it.  Unless I totally misunderstand the idea behind cpu_possible
and cpu_present.

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#1574991 — Re: [PATCH 1/6] genirq: allow assigning affinity to present but not online CPUs

On Sun, Feb 05, 2017 at 05:40:23PM +0100, Christoph Hellwig wrote:
> Hi Joe,
> 
> On Fri, Feb 03, 2017 at 08:58:09PM -0500, Joe Korty wrote:
> > IIRC, some years ago I ran across a customer system where
> > the #cpus_present was twice as big as #cpus_possible.
> > 
> > Hyperthreading was turned off in the BIOS so it was not
> > entirely out of line for the extra cpus to be declared
> > present, even though none of them would ever be available
> > for use.
> 
> This sounds like a system we should quirk around instead of optimizing
> for it.  Unless I totally misunderstand the idea behind cpu_possible
> and cpu_present.

Can we use the online CPUs and create a new hot-cpu notifier to the nvme
driver to free/reallocate as needed? We were doing that before blk-mq. Now
blk-mq can change the number hardware contexts on a live queue, so we
can reintroduce that behavior to nvme and only allocate what we need.

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#1575006

On Mon, Feb 06, 2017 at 12:03:05PM -0500, Keith Busch wrote:
> Can we use the online CPUs and create a new hot-cpu notifier to the nvme
> driver to free/reallocate as needed? We were doing that before blk-mq. Now
> blk-mq can change the number hardware contexts on a live queue, so we
> can reintroduce that behavior to nvme and only allocate what we need.

That could be a next step, but given how badly I keep messing up core
IRQ bits we'll probably want to wait for two rewrites of those bits
from Thomas before moving on :)

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