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Subject: Re: Question on handling managed IRQs when hotplugging CPUs
To:     Keith Busch <keith.busch@intel.com>
References: <f6f6e031-8b79-439d-c2af-8d3e76f30710@huawei.com>
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CC:     "tglx@linutronix.de" <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>
From:   John Garry <john.garry@huawei.com>
Message-ID: <3fe63dab-0791-f476-69c4-9866b70e8520@huawei.com>
Date:   Wed, 30 Jan 2019 10:38:18 +0000
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On 29/01/2019 17:20, Keith Busch wrote:
> On Tue, Jan 29, 2019 at 05:12:40PM +0000, John Garry wrote:
>> On 29/01/2019 15:44, Keith Busch wrote:
>>>
>>> Hm, we used to freeze the queues with CPUHP_BLK_MQ_PREPARE callback,
>>> which would reap all outstanding commands before the CPU and IRQ are
>>> taken offline. That was removed with commit 4b855ad37194f ("blk-mq:
>>> Create hctx for each present CPU"). It sounds like we should bring
>>> something like that back, but make more fine grain to the per-cpu context.
>>>
>>
>> Seems reasonable. But we would need it to deal with drivers where they only
>> expose a single queue to BLK MQ, but use many queues internally. I think
>> megaraid sas does this, for example.
>>
>> I would also be slightly concerned with commands being issued from the
>> driver unknown to blk mq, like SCSI TMF.
>
> I don't think either of those descriptions sound like good candidates
> for using managed IRQ affinities.

I wouldn't say that this behaviour is obvious to the developer. I can't 
see anything in Documentation/PCI/MSI-HOWTO.txt

It also seems that this policy to rely on upper layer to flush+freeze 
queues would cause issues if managed IRQs are used by drivers in other 
subsystems. Networks controllers may have multiple queues and 
unsoliciated interrupts.

Thanks,
John

>
> .
>