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Subject: Re: Question on handling managed IRQs when hotplugging CPUs
To:     Hannes Reinecke <hare@suse.de>, Keith Busch <keith.busch@intel.com>
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CC:     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: <5c9b33c7-ae48-6ae8-b9d9-12ab7c486001@huawei.com>
Date:   Tue, 5 Feb 2019 15:27:40 +0000
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On 05/02/2019 15:15, Hannes Reinecke wrote:
> On 2/5/19 4:09 PM, 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.
>>
> No, not as such.
> But each IO sent to the HW requires a unique identifcation (ie a valid
> tag). And as the tagspace is managed by block-mq (minus management
> commands, but I'm working on that currently) we can easily figure out if
> the device is busy by checking for an empty tag map.

That sounds like a reasonable starting solution.

Thanks,
John

>
> Should be doable for most modern HBAs.
>
> Cheers,
>
> Hannes