linux-kernel - Re: Question on handling managed IRQs when hotplugging CPUs

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Message-ID: <20190205145244.GB28023@localhost.localdomain>
Date:   Tue, 5 Feb 2019 07:52:44 -0700
From:   Keith Busch <keith.busch@...el.com>
To:     John Garry <john.garry@...wei.com>
Cc:     Hannes Reinecke <hare@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@....de>,
        Marc Zyngier <marc.zyngier@....com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        Peter Zijlstra <peterz@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Linuxarm <linuxarm@...wei.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Hannes Reinecke <hare@...e.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Subject: Re: Question on handling managed IRQs when hotplugging CPUs

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.