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

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(-)

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
@@ -28,6 +28,7 @@
#include <linux/t10-pi.h>
#include <linux/pm_qos.h>
#include <asm/unaligned.h>
+#include <linux/sched.h>

#define CREATE_TRACE_POINTS
#include "trace.h"
@@ -97,6 +98,15 @@ static dev_t nvme_chr_devt;
static struct class *nvme_class;
static struct class *nvme_subsys_class;

+/*
+ * The following are for detecting if this CPU is flooded with interrupts.
+ * The timestamp for the last context switch is recorded. If that is at least
+ * MAX_SCHED_TIMEOUT ago, try to recover from interrupt flood
+ */
+static DEFINE_PER_CPU(u64, last_switch);
+static DEFINE_PER_CPU(u64, last_clock);
+#define MAX_SCHED_TIMEOUT 2000000000 // 2 seconds in ns
+
static int nvme_revalidate_disk(struct gendisk *disk);
static void nvme_put_subsystem(struct nvme_subsystem *subsys);
static void nvme_remove_invalid_namespaces(struct nvme_ctrl *ctrl,
@@ -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
+ */
+ 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:
+ status = nvme_error_status(req);

trace_nvme_complete_rq(req);

diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index 0a4a7f5e0de7..a8876e69e476 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -113,6 +113,7 @@ struct nvme_request {
u8 flags;
u16 status;
struct nvme_ctrl *ctrl;
+ struct work_struct work;
};

/*
--
2.17.1
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