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

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Date:   Wed, 30 Jan 2019 10:38:18 +0000
From:   John Garry <john.garry@...wei.com>
To:     Keith Busch <keith.busch@...el.com>
CC:     "tglx@...utronix.de" <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>
Subject: Re: Question on handling managed IRQs when hotplugging CPUs

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

>
> .
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