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

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Date:   Mon, 6 Feb 2017 12:03:05 -0500
From:   Keith Busch <keith.busch@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Joe Korty <joe.korty@...r.com>, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: 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.