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Date:   Tue, 29 Jan 2019 10:20:59 -0700
From:   Keith Busch <keith.busch@intel.com>
To:     John Garry <john.garry@huawei.com>
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>
Subject: Re: Question on handling managed IRQs when hotplugging CPUs
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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.