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Issue Description: We have seen cpu lock up issues from field if system has a large (more than 96) logical cpu count. SAS3.0 controller (Invader series) supports max 96 MSI-X vector and SAS3.5 product (Ventura) supports max 128 MSI-X vectors. This may be a generic issue (if PCI device support completion on multiple reply queues). Let me explain it w.r.t megaraid_sas supported h/w just to simplify the problem and possible changes to handle such issues. MegaRAID controller supports multiple reply queues in completion path. Driver creates MSI-X vectors for controller as "minimum of (FW supported Reply queues, 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. 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 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. megaraid_sas 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 irqbalancer as "exact". If rq_affinity is set to 2, submitter will be always interrupted via completion on same CPU. If irqbalancer is using "exact" policy, interrupt will be delivered to submitter CPU. Problem statement: If CPU count 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 is seen 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. MSI-X 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 MSI-X 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% 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: Use IRQ poll interface defined in "irq_poll.c". megaraid_sas driver will execute ISR routine in softirq context and it will always quit the loop based on budget provided in IRQ poll interface. Driver will switch to IRQ poll only when more than a threshold number of reply descriptors are handled in one ISR. Currently threshold is set as 1/4th of HBA queue depth. 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. Select CONFIG_IRQ_POLL from driver Kconfig for driver compilation. Signed-off-by: Kashyap Desai <kashyap.desai@broadcom.com> Signed-off-by: Shivasharan S <shivasharan.srikanteshwara@broadcom.com> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.