[PATCH V2 3/3] nvme pci: introduce module parameter of 'default_queues'

Shan Hai
2019-01-03T04:36:42Z

On 2019/1/3 ??11:31, Ming Lei wrote:
> On Thu, Jan 03, 2019@11:11:07AM +0800, Shan Hai wrote:
>>
>>
>> On 2019/1/3 ??10:52, Shan Hai wrote:
>>>
>>>
>>> On 2019/1/3 ??10:12, Ming Lei wrote:
>>>> On Wed, Jan 02, 2019@02:11:22PM -0600, Bjorn Helgaas wrote:
>>>>> [Sorry about the quote corruption below. I'm responding with gmail in
>>>>> plain text mode, but seems like it corrupted some of the quoting when
>>>>> saving as a draft]
>>>>>
>>>>> On Mon, Dec 31, 2018@11:47 PM Ming Lei <ming.lei@redhat.com> wrote:
>>>>> &gt;
>>>>> &gt; On Mon, Dec 31, 2018@03:24:55PM -0600, Bjorn Helgaas wrote:
>>>>> &gt; &gt; On Fri, Dec 28, 2018@9:27 PM Ming Lei <ming.lei@redhat.com> wrote:
>>>>> &gt; &gt; &gt;
>>>>> &gt; &gt; &gt; On big system with lots of CPU cores, it is easy to
>>>>> consume up irq
>>>>> &gt; &gt; &gt; vectors by assigning defaut queue with
>>>>> num_possible_cpus() irq vectors.
>>>>> &gt; &gt; &gt; Meantime it is often not necessary to allocate so many
>>>>> vectors for
>>>>> &gt; &gt; &gt; reaching NVMe's top performance under that situation.
>>>>> &gt; &gt;
>>>>> &gt; &gt; s/defaut/default/
>>>>> &gt; &gt;
>>>>> &gt; &gt; &gt; This patch introduces module parameter of 'default_queues' to try
>>>>> &gt; &gt; &gt; to address this issue reported by Shan Hai.
>>>>> &gt; &gt;
>>>>> &gt; &gt; Is there a URL to this report by Shan?
>>>>> &gt;
>>>>> &gt; http://lists.infradead.org/pipermail/linux-nvme/2018-December/021863.html
>>>>> &gt; http://lists.infradead.org/pipermail/linux-nvme/2018-December/021862.html
>>>>> &gt;
>>>>> &gt; http://lists.infradead.org/pipermail/linux-nvme/2018-December/021872.html
>>>>>
>>>>> It'd be good to include this. I think the first is the interesting
>>>>> one. It'd be nicer to have an https://lore.kernel.org/... URL, but it
>>>>> doesn't look like lore hosts linux-nvme yet. (Is anybody working on
>>>>> that? I have some archives I could contribute, but other folks
>>>>> probably have more.)
>>>>>
>>>>> </ming.lei at redhat.com></ming.lei at redhat.com>
>>>>>>>
>>>>>>> Is there some way you can figure this out automatically instead of
>>>>>>> forcing the user to use a module parameter?
>>>>>>
>>>>>> Not yet, otherwise, I won't post this patch out.
>>>>>>
>>>>>>> If not, can you provide some guidance in the changelog for how a user
>>>>>>> is supposed to figure out when it's needed and what the value should
>>>>>>> be? If you add the parameter, I assume that will eventually have to
>>>>>>> be mentioned in a release note, and it would be nice to have something
>>>>>>> to start from.
>>>>>>
>>>>>> Ok, that is a good suggestion, how about documenting it via the
>>>>>> following words:
>>>>>>
>>>>>> Number of IRQ vectors is system-wide resource, and usually it is big enough
>>>>>> for each device. However, we allocate num_possible_cpus() + 1 irq vectors for
>>>>>> each NVMe PCI controller. In case that system has lots of CPU cores, or there
>>>>>> are more than one NVMe controller, IRQ vectors can be consumed up
>>>>>> easily by NVMe. When this issue is triggered, please try to pass smaller
>>>>>> default queues via the module parameter of 'default_queues', usually
>>>>>> it have to be >= number of NUMA nodes, meantime it needs be big enough
>>>>>> to reach NVMe's top performance, which is often less than num_possible_cpus()
>>>>>> + 1.
>>>>>
>>>>> You say "when this issue is triggered." How does the user know when
>>>>> this issue triggered?
>>>>
>>>> Any PCI IRQ vector allocation fails.
>>>>
>>>>>
>>>>> The failure in Shan's email (021863.html) is a pretty ugly hotplug
>>>>> failure and it would take me personally a long time to connect it with
>>>>> an IRQ exhaustion issue and even longer to dig out this module
>>>>> parameter to work around it. I suppose if we run out of IRQ numbers,
>>>>> NVMe itself might work fine, but some other random driver might be
>>>>> broken?
>>>>
>>>> Yeah, seems that is true in Shan's report.
>>>>
>>>> However, Shan mentioned that the issue is only triggered in case of
>>>> CPU hotplug, especially "The allocation is caused by IRQ migration of
>>>> non-managed interrupts from dying to online CPUs."
>>>>
>>>> I still don't understand why new IRQ vector allocation is involved
>>>> under CPU hotplug since Shan mentioned that no IRQ exhaustion issue
>>>> during booting.
>>>>
>>>
>>> Yes, the bug can be reproduced easily by CPU-hotplug.
>>> We have to separate the PCI IRQ and CPU IRQ vectors first of all. We know that
>>> the MSI-X permits up to 2048 interrupts allocation per device, but the CPU,
>>> X86 as an example, could provide maximum 255 interrupt vectors, and the sad fact
>>> is that these vectors are not all available for peripheral devices.
>>>
>>> So even though the controllers are luxury in PCI IRQ space and have got
>>> thousands of vectors to use but the heavy lifting is done by the precious CPU
>>> irq vectors.
>>>
>>> The CPU-hotplug causes IRQ vectors exhaustion problem because the interrupt
>>> handlers of the controllers will be migrated from dying cpu to the online cpu
>>> as long as the driver's irq affinity is not managed by the kernel, the drivers
>>> smp_affinity of which can be set by procfs interface belong to this class.
>>>
>>> And the irq migration does not do irq free/realloc stuff, so the irqs of a
>>> controller will be migrated to the target CPU cores according to its irq
>>> affinity hint value and will consume a irq vector on the target core.
>>>
>>> If we try to offline 360 cores out of total 384 cores on a NUMA system attached
>>> with 6 NVMe and 6 NICs we are out of luck and observe a kernel panic due to the
>>> failure of I/O.
>>>
>>
>> Put it simply we ran out of CPU irq vectors on CPU-hotplug rather than MSI-X
>> vectors, adding this knob to the NVMe driver is for let it to be a good citizen
>> considering the drivers out there irqs of which are still not managed by the
>> kernel and be migrated between CPU cores on hot-plugging.
>
> Yeah, look we all think this way might address this issue sort of.
>
> But in reality, it can be hard to use this kind of workaround, given
> people may not conclude easily this kind of failure should be addressed
> by reducing 'nvme.default_queues'. At least, we should provide hint to
> user about this solution when the failure is triggered, as mentioned by
> Bjorn.
>
>>
>> If all driver's irq affinities are managed by the kernel I guess we will not
>> be bitten by this bug, but we are not so lucky till today.
>
> I am still not sure why changing affinities may introduce extra irq
> vector allocation.
>

Below is a simple math to illustrate the problem:

CPU = 384, NVMe = 6, NIC = 6
2 * 6 * 384 local irq vectors are assigned to the controllers irqs

offline 364 cpu, 6 * 364 NIC irqs are migrated to 20 remaining online CPUs,
while the irqs of the NVMe controllers are not, which means extra 6 * 364
local irq vectors of 20 online CPUs need to be assigned to these migrated
interrupt handlers.

Probably I should use assign instead of allocate to avoid the misunderstanding.

Thanks
Shan Hai
>
> Thanks,
> Ming
>
> _______________________________________________
> Linux-nvme mailing list
> Linux-nvme at lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-nvme
>