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

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Date:   Thu, 31 Jan 2019 17:48:02 +0000
From:   John Garry <john.garry@...wei.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     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 30/01/2019 12:43, Thomas Gleixner wrote:
> On Wed, 30 Jan 2019, John Garry wrote:
>> 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.
>
> It's doesn't matter which part is managing flush/freeze of queues as long
> as something (either common subsystem code, upper layers or the driver
> itself) does it.
>
> So for the megaraid SAS example the BLK MQ layer obviously can't do
> anything because it only sees a single request queue. But the driver could,
> if the the hardware supports it. tell the device to stop queueing
> completions on the completion queue which is associated with a particular
> CPU (or set of CPUs) during offline and then wait for the on flight stuff
> to be finished. If the hardware does not allow that, then managed
> interrupts can't work for it.
>

A rough audit of current SCSI drivers tells that these set 
PCI_IRQ_AFFINITY in some path but don't set Scsi_host.nr_hw_queues at all:
aacraid, be2iscsi, csiostor, megaraid, mpt3sas

I don't know specific driver details, like changing completion queue.

Thanks,
John

> Thanks,
>
> 	tglx
>
> .
>