From: John Garry <john.garry@huawei.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Keith Busch <keith.busch@intel.com>,
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>,
"linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>
Subject: Re: Question on handling managed IRQs when hotplugging CPUs
Date: Thu, 31 Jan 2019 17:48:02 +0000 [thread overview]
Message-ID: <86d5028d-44ab-3696-f7fe-828d7655faa9@huawei.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1901301338170.5537@nanos.tec.linutronix.de>
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
>
> .
>
next prev parent reply other threads:[~2019-01-31 17:48 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-29 11:25 Question on handling managed IRQs when hotplugging CPUs John Garry
2019-01-29 11:54 ` Hannes Reinecke
2019-01-29 12:01 ` Thomas Gleixner
2019-01-29 15:27 ` John Garry
2019-01-29 16:27 ` Thomas Gleixner
2019-01-29 17:23 ` John Garry
2019-01-29 15:44 ` Keith Busch
2019-01-29 17:12 ` John Garry
2019-01-29 17:20 ` Keith Busch
2019-01-30 10:38 ` John Garry
2019-01-30 12:43 ` Thomas Gleixner
2019-01-31 17:48 ` John Garry [this message]
2019-02-01 15:56 ` Hannes Reinecke
2019-02-01 21:57 ` Thomas Gleixner
2019-02-04 7:12 ` Hannes Reinecke
2019-02-05 13:24 ` John Garry
2019-02-05 14:52 ` Keith Busch
2019-02-05 15:09 ` John Garry
2019-02-05 15:11 ` Keith Busch
2019-02-05 15:15 ` Hannes Reinecke
2019-02-05 15:27 ` John Garry
2019-02-05 18:23 ` Christoph Hellwig
2019-02-06 9:21 ` John Garry
2019-02-06 13:34 ` Benjamin Block
2019-02-05 15:10 ` Hannes Reinecke
2019-02-05 15:16 ` Keith Busch
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