RE: [PATCH 3/3] nvme: complete request in work queue on CPU with flooded interrupts

[Long Li <longli@microsoft.com>]

>>>Subject: Re: [PATCH 3/3] nvme: complete request in work queue on CPU
>>>with flooded interrupts
>>>
>>>On Mon, Aug 19, 2019 at 11:14:29PM -0700, longli@linuxonhyperv.com
>>>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.
>>>
>>>Ideally -- and there is some code to affect this, the load-balancer will move
>>>tasks away from this CPU.
>>>
>>>> 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:
>>>
>>>Suppose the task waiting for the IO completion is a RT task, and you've just
>>>queued it to a regular work. This is an instant priority inversion.

This is a choice. We can either not "lock up" the CPU, or finish the I/O on time from IRQ handler. I think throttling only happens in extreme conditions, which is rare. The purpose is to make the whole system responsive and happy.

>>>
>>>> 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.
>>>>
>>>> Signed-off-by: Long Li <longli@microsoft.com>
>>>> ---
>>>>  drivers/nvme/host/core.c | 57
>>>> +++++++++++++++++++++++++++++++++++++++-
>>>>  drivers/nvme/host/nvme.h |  1 +
>>>>  2 files changed, 57 insertions(+), 1 deletion(-)
>>>
>>>WTH does this live in the NVME driver? Surely something like this should be
>>>in the block layer. I'm thinking there's fiber channel connected storage that
>>>should be able to trigger much the same issues.

Yes this can be done in block layer. I'm not sure the best way to accomplish this so implemented a NVMe patch to help test. The test results are promising in that we are getting 99.5% of performance while avoided CPU lockup. The challenge is to find a way to throttle a fast storage device.

>>>
>>>> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c index
>>>> 6a9dd68c0f4f..576bb6fce293 100644
>>>> --- a/drivers/nvme/host/core.c
>>>> +++ b/drivers/nvme/host/core.c
>>>
>>>> @@ -260,9 +270,54 @@ static void nvme_retry_req(struct request *req)
>>>>  	blk_mq_delay_kick_requeue_list(req->q, delay);  }
>>>>
>>>> +static void nvme_complete_rq_work(struct work_struct *work) {
>>>> +	struct nvme_request *nvme_rq =
>>>> +		container_of(work, struct nvme_request, work);
>>>> +	struct request *req = blk_mq_rq_from_pdu(nvme_rq);
>>>> +
>>>> +	nvme_complete_rq(req);
>>>> +}
>>>> +
>>>> +
>>>>  void nvme_complete_rq(struct request *req)  {
>>>> -	blk_status_t status = nvme_error_status(req);
>>>> +	blk_status_t status;
>>>> +	int cpu;
>>>> +	u64 switches;
>>>> +	struct nvme_request *nvme_rq;
>>>> +
>>>> +	if (!in_interrupt())
>>>> +		goto skip_check;
>>>> +
>>>> +	nvme_rq = nvme_req(req);
>>>> +	cpu = smp_processor_id();
>>>> +	if (idle_cpu(cpu))
>>>> +		goto skip_check;
>>>> +
>>>> +	/* Check if this CPU is flooded with interrupts */
>>>> +	switches = get_cpu_rq_switches(cpu);
>>>> +	if (this_cpu_read(last_switch) == switches) {
>>>> +		/*
>>>> +		 * If this CPU hasn't made a context switch in
>>>> +		 * MAX_SCHED_TIMEOUT ns (and it's not idle), schedule a
>>>work to
>>>> +		 * complete this I/O. This forces this CPU run non-interrupt
>>>> +		 * code and throttle the other CPU issuing the I/O
>>>> +		 */
>>>
>>>What if there was only a single task on that CPU? Then we'd never
>>>need/want to context switch in the first place.
>>>
>>>AFAICT all this is just a whole bunch of gruesome hacks piled on top one
>>>another.
>>>
>>>> +		if (sched_clock() - this_cpu_read(last_clock)
>>>> +				> MAX_SCHED_TIMEOUT) {
>>>> +			INIT_WORK(&nvme_rq->work,
>>>nvme_complete_rq_work);
>>>> +			schedule_work_on(cpu, &nvme_rq->work);
>>>> +			return;
>>>> +		}
>>>> +
>>>> +	} else {
>>>> +		this_cpu_write(last_switch, switches);
>>>> +		this_cpu_write(last_clock, sched_clock());
>>>> +	}
>>>> +
>>>> +skip_check:
>>>
>>>Aside from everything else; this is just sodding poor coding style. What is
>>>wrong with something like:
>>>
>>>	if (nvme_complete_throttle(...))
>>>		return;
>>>
>>>> +	status = nvme_error_status(req);
>>>>
>>>>  	trace_nvme_complete_rq(req);
>>>>