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SubjectRE: [PATCH 3/3] nvme: complete request in work queue on CPU with flooded interrupts
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>>>Subject: Re: [PATCH 3/3] nvme: complete request in work queue on CPU
>>>with flooded interrupts
>>>
>>>On Tue, Aug 20, 2019 at 10:33:38AM -0700, Sagi Grimberg wrote:
>>>>
>>>> > From: Long Li <longli@microsoft.com>
>>>> >
>>>> > When a NVMe hardware queue is mapped to several CPU queues, it is
>>>> > possible that the CPU this hardware queue is bound to is flooded by
>>>> > returning I/O for other CPUs.
>>>> >
>>>> > For example, consider the following scenario:
>>>> > 1. CPU 0, 1, 2 and 3 share the same hardware queue 2. the hardware
>>>> > queue interrupts CPU 0 for I/O response 3. processes from CPU 1, 2
>>>> > and 3 keep sending I/Os
>>>> >
>>>> > CPU 0 may be flooded with interrupts from NVMe device that are I/O
>>>> > responses for CPU 1, 2 and 3. Under heavy I/O load, it is possible
>>>> > that CPU 0 spends all the time serving NVMe and other system
>>>> > interrupts, but doesn't have a chance to run in process context.
>>>> >
>>>> > To fix this, CPU 0 can schedule a work to complete the I/O request
>>>> > when it detects the scheduler is not making progress. This serves
>>>multiple purposes:
>>>> >
>>>> > 1. This CPU has to be scheduled to complete the request. The other
>>>> > CPUs can't issue more I/Os until some previous I/Os are completed.
>>>> > This helps this CPU get out of NVMe interrupts.
>>>> >
>>>> > 2. This acts a throttling mechanisum for NVMe devices, in that it
>>>> > can not starve a CPU while servicing I/Os from other CPUs.
>>>> >
>>>> > 3. This CPU can make progress on RCU and other work items on its
>>>queue.
>>>>
>>>> The problem is indeed real, but this is the wrong approach in my mind.
>>>>
>>>> We already have irqpoll which takes care proper budgeting polling
>>>> cycles and not hogging the cpu.
>>>
>>>The issue isn't unique to NVMe, and can be any fast devices which
>>>interrupts CPU too frequently, meantime the interrupt/softirq handler may
>>>take a bit much time, then CPU is easy to be lockup by the interrupt/sofirq
>>>handler, especially in case that multiple submission CPUs vs. single
>>>completion CPU.
>>>
>>>Some SCSI devices has the same problem too.
>>>
>>>Could we consider to add one generic mechanism to cover this kind of
>>>problem?
>>>
>>>One approach I thought of is to allocate one backup thread for handling such
>>>interrupt, which can be marked as IRQF_BACKUP_THREAD by drivers.
>>>
>>>Inside do_IRQ(), irqtime is accounted, before calling action->handler(),
>>>check if this CPU has taken too long time for handling IRQ(interrupt or
>>>softirq) and see if this CPU could be lock up. If yes, wakeup the backup

How do you know if this CPU is spending all the time in do_IRQ()?

Is it something like:
If (IRQ_time /elapsed_time > a threshold value)
wake up the backup thread

>>>thread to handle the interrupt for avoiding lockup this CPU.
>>>
>>>The threaded interrupt framework is there, and this way could be easier to
>>>implement. Meantime most time the handler is run in interrupt context and
>>>we may avoid the performance loss when CPU isn't busy enough.
>>>
>>>Any comment on this approach?
>>>
>>>Thanks,
>>>Ming

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