From mboxrd@z Thu Jan 1 00:00:00 1970 From: shan.hai@oracle.com (Shan Hai) Date: Thu, 3 Jan 2019 12:36:42 +0800 Subject: [PATCH V2 3/3] nvme pci: introduce module parameter of 'default_queues' In-Reply-To: <20190103033131.GI25044@ming.t460p> References: <20181229032650.27256-1-ming.lei@redhat.com> <20181229032650.27256-4-ming.lei@redhat.com> <20190101054735.GB17588@ming.t460p> <20190103021237.GA25044@ming.t460p> <4d8f963e-df7d-b2d9-3bf8-4852dfe6808e@oracle.com> <9d1a0052-85c9-9cbd-f824-7812eceb11bf@oracle.com> <20190103033131.GI25044@ming.t460p> Message-ID: 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 wrote: >>>>> > >>>>> > On Mon, Dec 31, 2018@03:24:55PM -0600, Bjorn Helgaas wrote: >>>>> > > On Fri, Dec 28, 2018@9:27 PM Ming Lei wrote: >>>>> > > > >>>>> > > > On big system with lots of CPU cores, it is easy to >>>>> consume up irq >>>>> > > > vectors by assigning defaut queue with >>>>> num_possible_cpus() irq vectors. >>>>> > > > Meantime it is often not necessary to allocate so many >>>>> vectors for >>>>> > > > reaching NVMe's top performance under that situation. >>>>> > > >>>>> > > s/defaut/default/ >>>>> > > >>>>> > > > This patch introduces module parameter of 'default_queues' to try >>>>> > > > to address this issue reported by Shan Hai. >>>>> > > >>>>> > > Is there a URL to this report by Shan? >>>>> > >>>>> > http://lists.infradead.org/pipermail/linux-nvme/2018-December/021863.html >>>>> > http://lists.infradead.org/pipermail/linux-nvme/2018-December/021862.html >>>>> > >>>>> > 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.) >>>>> >>>>> >>>>>>> >>>>>>> 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 >