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SubjectRe: 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.


> .

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