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

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Message-ID: <alpine.DEB.2.21.1902012255550.8200@nanos.tec.linutronix.de>
Date:   Fri, 1 Feb 2019 22:57:59 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Hannes Reinecke <hare@...e.de>
cc:     John Garry <john.garry@...wei.com>,
        Keith Busch <keith.busch@...el.com>,
        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 Fri, 1 Feb 2019, Hannes Reinecke wrote:
> Thing is, if we have _managed_ CPU hotplug (ie if the hardware provides some
> means of quiescing the CPU before hotplug) then the whole thing is trivial;
> disable SQ and wait for all outstanding commands to complete.
> Then trivially all requests are completed and the issue is resolved.
> Even with todays infrastructure.
> 
> And I'm not sure if we can handle surprise CPU hotplug at all, given all the
> possible race conditions.
> But then I might be wrong.

The kernel would completely fall apart when a CPU would vanish by surprise,
i.e. uncontrolled by the kernel. Then the SCSI driver exploding would be
the least of our problems.

Thanks,

	tglx