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

[longli@linuxonhyperv.com]

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