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Subject: Re: Question on handling managed IRQs when hotplugging CPUs
To:     Keith Busch <keith.busch@intel.com>
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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>
From:   John Garry <john.garry@huawei.com>
Message-ID: <d4a04c13-b7db-84df-b1d3-90022905c084@huawei.com>
Date:   Tue, 5 Feb 2019 15:09:28 +0000
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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.

Thanks,
John

>
> .
>