Hi Kashyap, On Mon, Jan 15, 2018 at 05:42:05PM +0530, Kashyap Desai wrote: > Hi All - > > We have seen cpu lock up issue from fields if system has greater (more > than 96) logical cpu count. > SAS3.0 controller (Invader series) supports at max 96 msix vector and > SAS3.5 product (Ventura) supports at max 128 msix vectors. > > This may be a generic issue (if PCI device support completion on multiple > reply queues). Let me explain it w.r.t to mpt3sas supported h/w just to > simplify the problem and possible changes to handle such issues. IT HBA > (mpt3sas) supports multiple reply queues in completion path. Driver > creates MSI-x vectors for controller as "min of ( FW supported Reply > queue, Logical CPUs)". If submitter is not interrupted via completion on > same CPU, there is a loop in the IO path. This behavior can cause > hard/soft CPU lockups, IO timeout, system sluggish etc. As I mentioned in another thread, this issue may be solved by SCSI_MQ via mapping reply queue into hctx of blk_mq, together with QUEUE_FLAG_SAME_FORCE, especially you have set 'smp_affinity_enable' as 1 at default already, then pci_alloc_irq_vectors(PCI_IRQ_AFFINITY) can do IRQ vectors spread on CPUs perfectly for you. But the following Hannes's patch is required for the conversion. https://marc.info/?l=linux-block&m=149130770004507&w=2 > > Example - one CPU (e.g. CPU A) is busy submitting the IOs and another CPU > (e.g. CPU B) is busy with processing the corresponding IO's reply > descriptors from reply descriptor queue upon receiving the interrupts from > HBA. If the CPU A is continuously pumping the IOs then always CPU B (which > is executing the ISR) will see the valid reply descriptors in the reply > descriptor queue and it will be continuously processing those reply > descriptor in a loop without quitting the ISR handler. Mpt3sas driver > will exit ISR handler if it finds unused reply descriptor in the reply > descriptor queue. Since CPU A will be continuously sending the IOs, CPU B > may always see a valid reply descriptor (posted by HBA Firmware after > processing the IO) in the reply descriptor queue. In worst case, driver > will not quit from this loop in the ISR handler. Eventually, CPU lockup > will be detected by watchdog. > > Above mentioned behavior is not common if "rq_affinity" set to 2 or > affinity_hint is honored by irqbalance as "exact". > If rq_affinity is set to 2, submitter will be always interrupted via > completion on same CPU. > If irqbalance is using "exact" policy, interrupt will be delivered to > submitter CPU. Now you have used pci_alloc_irq_vectors(PCI_IRQ_AFFINITY) to get msix vector number, the irq affinity can't be changed by userspace any more. > > Problem statement - > If CPU counts to MSI-X vectors (reply descriptor Queues) count ratio is > not 1:1, we still have exposure of issue explained above and for that we > don't have any solution. > > Exposure of soft/hard lockup if CPU count is more than MSI-x supported by > device. > > If CPUs count to MSI-x vectors count ratio is not 1:1, (Other way, if CPU > counts to MSI-x vector count ratio is something like X:1, where X > 1) > then 'exact' irqbalance policy OR rq_affinity = 2 won't help to avoid CPU > hard/soft lockups. There won't be any one to one mapping between CPU to > MSI-x vector instead one MSI-x interrupt (or reply descriptor queue) is > shared with group/set of CPUs and there is a possibility of having a loop > in the IO path within that CPU group and may observe lockups. > > For example: Consider a system having two NUMA nodes and each node having > four logical CPUs and also consider that number of MSI-x vectors enabled > on the HBA is two, then CPUs count to MSI-x vector count ratio as 4:1. > e.g. > MSIx vector 0 is affinity to CPU 0, CPU 1, CPU 2 & CPU 3 of NUMA node 0 > and MSI-x vector 1 is affinity to CPU 4, CPU 5, CPU 6 & CPU 7 of NUMA node > 1. > > numactl --hardware > available: 2 nodes (0-1) > node 0 cpus: 0 1 2 3 --> > MSI-x 0 > node 0 size: 65536 MB > node 0 free: 63176 MB > node 1 cpus: 4 5 6 7 > -->MSI-x 1 > node 1 size: 65536 MB > node 1 free: 63176 MB > > Assume that user started an application which uses all the CPUs of NUMA > node 0 for issuing the IOs. > Only one CPU from affinity list (it can be any cpu since this behavior > depends upon irqbalance) CPU0 will receive the interrupts from MSIx vector > 0 for all the IOs. Eventually, CPU 0 IO submission percentage will be > decreasing and ISR processing percentage will be increasing as it is more > busy with processing the interrupts. Gradually IO submission percentage on > CPU 0 will be zero and it's ISR processing percentage will be 100 > percentage as IO loop has already formed within the NUMA node 0, i.e. CPU > 1, CPU 2 & CPU 3 will be continuously busy with submitting the heavy IOs > and only CPU 0 is busy in the ISR path as it always find the valid reply > descriptor in the reply descriptor queue. Eventually, we will observe the > hard lockup here. > > Chances of occurring of hard/soft lockups are directly proportional to > value of X. If value of X is high, then chances of observing CPU lockups > is high. > > Solution - > Fix - 1 Use IRQ poll interface defined in " irq_poll.c". mpt3sas driver > will execute ISR routine in Softirq context and it will always quit the > loop based on budget provided in IRQ poll interface. > > In these scenarios( i.e. where CPUs count to MSI-X vectors count ratio is > X:1 (where X > 1)), IRQ poll interface will avoid CPU hard lockups due > to voluntary exit from the reply queue processing based on budget. Note - > Only one MSI-x vector is busy doing processing. Irqstat ouput - > > IRQs / 1 second(s) > IRQ# TOTAL NODE0 NODE1 NODE2 NODE3 NAME > 44 122871 122871 0 0 0 IR-PCI-MSI-edge > mpt3sas0-msix0 > 45 0 0 0 0 0 IR-PCI-MSI-edge > mpt3sas0-msix1 > > Fix-2 - Above fix will avoid lockups, but there can be some performance > issue if very few reply queue is busy. Driver should round robin the reply > queue, so that each reply queue is load balanced. Irqstat ouput after > driver does reply queue load balance- > > IRQs / 1 second(s) > IRQ# TOTAL NODE0 NODE1 NODE2 NODE3 NAME > 44 62871 62871 0 0 0 IR-PCI-MSI-edge mpt3sas0-msix0 > 45 62718 62718 0 0 0 IR-PCI-MSI-edge mpt3sas0-msix1 > > In Summary, > CPU completing IO which is not contributing to IO submission, may cause > cpu lockup. > If CPUs count to MSI-X vector count ratio is X:1 (where X > 1) then using > irq poll interface, we can avoid the CPU lockups and by equally > distributing the interrupts among the enabled MSI-x interrupts we can > avoid performance issues. > > We are planning to use both the fixes only if cpu count is more than FW > supported MSI-x vector. > Please review and provide your feedback. I have appended both the patches. > Please take a look at pci_alloc_irq_vectors(PCI_IRQ_AFFINITY) and SCSI_MQ/blk_mq, you issue can be solved without much difficulty. One annoying thing is that SCSI driver has to support both MQ and non-MQ path. Long time ago, I submitted patch to support force-MQ in driver, but it is rejected. Thanks, Ming
