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Date:   Tue, 5 Feb 2019 08:11:15 -0700
From:   Keith Busch <keith.busch@intel.com>
To:     John Garry <john.garry@huawei.com>
Cc:     Hannes Reinecke <hare@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Christoph Hellwig <hch@lst.de>,
        Marc Zyngier <marc.zyngier@arm.com>,
        "axboe@kernel.dk" <axboe@kernel.dk>,
        Peter Zijlstra <peterz@infradead.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Linuxarm <linuxarm@huawei.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Hannes Reinecke <hare@suse.com>,
        "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>
Subject: Re: Question on handling managed IRQs when hotplugging CPUs
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On Tue, Feb 05, 2019 at 03:09:28PM +0000, John Garry wrote:
> On 05/02/2019 14:52, Keith Busch wrote:
> > On Tue, Feb 05, 2019 at 05:24:11AM -0800, John Garry wrote:
> > > On 04/02/2019 07:12, Hannes Reinecke wrote:
> > > 
> > > Hi Hannes,
> > > 
> > > > 
> > > > So, as the user then has to wait for the system to declars 'ready for
> > > > CPU remove', why can't we just disable the SQ and wait for all I/O to
> > > > complete?
> > > > We can make it more fine-grained by just waiting on all outstanding I/O
> > > > on that SQ to complete, but waiting for all I/O should be good as an
> > > > initial try.
> > > > With that we wouldn't need to fiddle with driver internals, and could
> > > > make it pretty generic.
> > > 
> > > I don't fully understand this idea - specifically, at which layer would
> > > we be waiting for all the IO to complete?
> > 
> > Whichever layer dispatched the IO to a CPU specific context should
> > be the one to wait for its completion. That should be blk-mq for most
> > block drivers.
> 
> For SCSI devices, unfortunately not all IO sent to the HW originates from
> blk-mq or any other single entity.

Then they'll need to register their own CPU notifiers and handle the
ones they dispatched.