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SubjectRE: [PATCH 3/3] nvme: complete request in work queue on CPU with flooded interrupts
Date
>>>Subject: RE: [PATCH 3/3] nvme: complete request in work queue on CPU
>>>with flooded interrupts
>>>
>>>>>>Subject: Re: [PATCH 3/3] nvme: complete request in work queue on
>>>CPU
>>>>>>with flooded interrupts
>>>>>>
>>>>>>
>>>>>>> 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.
>>>>>>
>>>>>>I've sent rfc for this particular problem before [1]. At the time
>>>>>>IIRC, Christoph suggested that we will poll the first batch directly
>>>>>>from the irq context and reap the rest in irqpoll handler.
>>>
>>>Thanks for the pointer. I will test and report back.

Sagi,

Here are the test results.

Benchmark command:
fio --bs=4k --ioengine=libaio --iodepth=64 --filename=/dev/nvme0n1:/dev/nvme1n1:/dev/nvme2n1:/dev/nvme3n1:/dev/nvme4n1:/dev/nvme5n1:/dev/nvme6n1:/dev/nvme7n1:/dev/nvme8n1:/dev/nvme9n1 --direct=1 --runtime=90 --numjobs=80 --rw=randread --name=test --group_reporting --gtod_reduce=1

With your patch: 1720k IOPS
With threaded interrupts: 1320k IOPS
With just interrupts: 3720k IOPS

Interrupts are the fastest but we need to find a way to throttle it.

Thanks

Long


>>>
>>>>>>
>>>>>>[1]:
>>>>>>https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fl
>>>ists.
>>>>>>infradead.org%2Fpipermail%2Flinux-nvme%2F2016-
>>>>>>October%2F006497.html&amp;data=02%7C01%7Clongli%40microsoft.co
>>>m%
>>>>>>7C0ebf36eff15c4182116608d725948b93%7C72f988bf86f141af91ab2d7cd0
>>>11d
>>>>>>b47%7C1%7C0%7C637019192254250361&amp;sdata=fJ%2Fkc8HLSmfzaY
>>>3BY
>>>>>>E66zlZKD6FjcXgMJZzVGCVqI%2FU%3D&amp;reserved=0
>>>>>>
>>>>>>How about something like this instead:
>>>>>>--
>>>>>>diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c index
>>>>>>71127a366d3c..84bf16d75109 100644
>>>>>>--- a/drivers/nvme/host/pci.c
>>>>>>+++ b/drivers/nvme/host/pci.c
>>>>>>@@ -24,6 +24,7 @@
>>>>>> #include <linux/io-64-nonatomic-lo-hi.h>
>>>>>> #include <linux/sed-opal.h>
>>>>>> #include <linux/pci-p2pdma.h>
>>>>>>+#include <linux/irq_poll.h>
>>>>>>
>>>>>> #include "trace.h"
>>>>>> #include "nvme.h"
>>>>>>@@ -32,6 +33,7 @@
>>>>>> #define CQ_SIZE(q) ((q)->q_depth * sizeof(struct nvme_completion))
>>>>>>
>>>>>> #define SGES_PER_PAGE (PAGE_SIZE / sizeof(struct nvme_sgl_desc))
>>>>>>+#define NVME_POLL_BUDGET_IRQ 256
>>>>>>
>>>>>> /*
>>>>>> * These can be higher, but we need to ensure that any command
>>>>>>doesn't @@ -189,6 +191,7 @@ struct nvme_queue {
>>>>>> u32 *dbbuf_cq_db;
>>>>>> u32 *dbbuf_sq_ei;
>>>>>> u32 *dbbuf_cq_ei;
>>>>>>+ struct irq_poll iop;
>>>>>> struct completion delete_done; };
>>>>>>
>>>>>>@@ -1015,6 +1018,23 @@ static inline int nvme_process_cq(struct
>>>>>>nvme_queue *nvmeq, u16 *start,
>>>>>> return found;
>>>>>> }
>>>>>>
>>>>>>+static int nvme_irqpoll_handler(struct irq_poll *iop, int budget) {
>>>>>>+ struct nvme_queue *nvmeq = container_of(iop, struct
>>>>>>+nvme_queue,
>>>>>>iop);
>>>>>>+ struct pci_dev *pdev = to_pci_dev(nvmeq->dev->dev);
>>>>>>+ u16 start, end;
>>>>>>+ int completed;
>>>>>>+
>>>>>>+ completed = nvme_process_cq(nvmeq, &start, &end, budget);
>>>>>>+ nvme_complete_cqes(nvmeq, start, end);
>>>>>>+ if (completed < budget) {
>>>>>>+ irq_poll_complete(&nvmeq->iop);
>>>>>>+ enable_irq(pci_irq_vector(pdev, nvmeq->cq_vector));
>>>>>>+ }
>>>>>>+
>>>>>>+ return completed;
>>>>>>+}
>>>>>>+
>>>>>> static irqreturn_t nvme_irq(int irq, void *data)
>>>>>> {
>>>>>> struct nvme_queue *nvmeq = data; @@ -1028,12 +1048,16 @@
>>>>>>static irqreturn_t nvme_irq(int irq, void *data)
>>>>>> rmb();
>>>>>> if (nvmeq->cq_head != nvmeq->last_cq_head)
>>>>>> ret = IRQ_HANDLED;
>>>>>>- nvme_process_cq(nvmeq, &start, &end, -1);
>>>>>>+ nvme_process_cq(nvmeq, &start, &end,
>>>NVME_POLL_BUDGET_IRQ);
>>>>>> nvmeq->last_cq_head = nvmeq->cq_head;
>>>>>> wmb();
>>>>>>
>>>>>> if (start != end) {
>>>>>> nvme_complete_cqes(nvmeq, start, end);
>>>>>>+ if (nvme_cqe_pending(nvmeq)) {
>>>>>>+ disable_irq_nosync(irq);
>>>>>>+ irq_poll_sched(&nvmeq->iop);
>>>>>>+ }
>>>>>> return IRQ_HANDLED;
>>>>>> }
>>>>>>
>>>>>>@@ -1347,6 +1371,7 @@ static enum blk_eh_timer_return
>>>>>>nvme_timeout(struct request *req, bool reserved)
>>>>>>
>>>>>> static void nvme_free_queue(struct nvme_queue *nvmeq) {
>>>>>>+ irq_poll_disable(&nvmeq->iop);
>>>>>> dma_free_coherent(nvmeq->dev->dev, CQ_SIZE(nvmeq),
>>>>>> (void *)nvmeq->cqes, nvmeq->cq_dma_addr);
>>>>>> if (!nvmeq->sq_cmds)
>>>>>>@@ -1481,6 +1506,7 @@ static int nvme_alloc_queue(struct nvme_dev
>>>>>>*dev, int qid, int depth)
>>>>>> nvmeq->dev = dev;
>>>>>> spin_lock_init(&nvmeq->sq_lock);
>>>>>> spin_lock_init(&nvmeq->cq_poll_lock);
>>>>>>+ irq_poll_init(&nvmeq->iop, NVME_POLL_BUDGET_IRQ,
>>>>>>nvme_irqpoll_handler);
>>>>>> nvmeq->cq_head = 0;
>>>>>> nvmeq->cq_phase = 1;
>>>>>> nvmeq->q_db = &dev->dbs[qid * 2 * dev->db_stride];
>>>>>>--
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