Re: [PATCH 0/3] fix interrupt swamp in NVMe

[John Garry]

On 20/08/2019 09:25, Ming Lei wrote:
> On Tue, Aug 20, 2019 at 2:14 PM <longli@xxxxxxxxxxxxxxxxx> wrote:
> > From: Long Li <longli@xxxxxxxxxxxxx>
> >
> > This patch set tries to fix interrupt swamp in NVMe devices.
> >
> > On large systems with many CPUs, a number of CPUs may share one NVMe hardware
> > queue. It may have this situation where several CPUs are issuing I/Os, and
> > all the I/Os are returned on the CPU where the hardware queue is bound to.
> > This may result in that CPU swamped by interrupts and stay in interrupt mode
> > for extended time while other CPUs continue to issue I/O. This can trigger
> > Watchdog and RCU timeout, and make the system unresponsive.
> >
> > This patch set addresses this by enforcing scheduling and throttling I/O when
> > CPU is starved in this situation.
> >
> > Long Li (3):
> >   sched: define a function to report the number of context switches on a
> >     CPU
> >   sched: export idle_cpu()
> >   nvme: complete request in work queue on CPU with flooded interrupts
> >
> >  drivers/nvme/host/core.c | 57 +++++++++++++++++++++++++++++++++++++++-
> >  drivers/nvme/host/nvme.h |  1 +
> >  include/linux/sched.h    |  2 ++
> >  kernel/sched/core.c      |  7 +++++
> >  4 files changed, 66 insertions(+), 1 deletion(-)
>
> Another simpler solution may be to complete request in threaded interrupt
> handler for this case. Meantime allow scheduler to run the interrupt thread
> handler on CPUs specified by the irq affinity mask, which was discussed by
> the following link:
>
> https://lore.kernel.org/lkml/e0e9478e-62a5-ca24-3b12-58f7d056383e@xxxxxxxxxx/
>
> Could you try the above solution and see if the lockup can be avoided?
> John Garry
> should have workable patch.

Yeah, so we experimented with changing the interrupt handling in the 
SCSI driver I maintain to use a threaded handler IRQ handler plus patch 
below, and saw a significant throughput boost:

--->8

Subject: [PATCH] genirq: Add support to allow thread to use hard irq 
affinity

Currently the cpu allowed mask for the threaded part of a threaded irq
handler will be set to the effective affinity of the hard irq.

Typically the effective affinity of the hard irq will be for a single
cpu. As such, the threaded handler would always run on the same cpu as
the hard irq.

We have seen scenarios in high data-rate throughput testing that the
cpu handling the interrupt can be totally saturated handling both the
hard interrupt and threaded handler parts, limiting throughput.

Add IRQF_IRQ_AFFINITY flag to allow the driver requesting the threaded
interrupt to decide on the policy of which cpu the threaded handler
may run.

Signed-off-by: John Garry <john.garry@xxxxxxxxxx>

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index 5b8328a99b2a..48e8b955989a 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -61,6 +61,9 @@
  *                interrupt handler after suspending interrupts. For 
system
  *                wakeup devices users need to implement wakeup 
detection in
  *                their interrupt handlers.
+ * IRQF_IRQ_AFFINITY - Use the hard interrupt affinity for setting the cpu
+ *                allowed mask for the threaded handler of a threaded 
interrupt
+ *                handler, rather than the effective hard irq affinity.
  */
 #define IRQF_SHARED		0x00000080
 #define IRQF_PROBE_SHARED	0x00000100
@@ -74,6 +77,7 @@
 #define IRQF_NO_THREAD		0x00010000
 #define IRQF_EARLY_RESUME	0x00020000
 #define IRQF_COND_SUSPEND	0x00040000
+#define IRQF_IRQ_AFFINITY	0x00080000

 #define IRQF_TIMER		(__IRQF_TIMER | IRQF_NO_SUSPEND | IRQF_NO_THREAD)

diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index e8f7f179bf77..cb483a055512 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -966,9 +966,13 @@ irq_thread_check_affinity(struct irq_desc *desc, 
struct irqaction *action)
 	 * mask pointer. For CPU_MASK_OFFSTACK=n this is optimized out.
 	 */
 	if (cpumask_available(desc->irq_common_data.affinity)) {
+		struct irq_data *irq_data = &desc->irq_data;
 		const struct cpumask *m;

-		m = irq_data_get_effective_affinity_mask(&desc->irq_data);
+		if (action->flags & IRQF_IRQ_AFFINITY)
+			m = desc->irq_common_data.affinity;
+		else
+			m = irq_data_get_effective_affinity_mask(irq_data);
 		cpumask_copy(mask, m);
 	} else {
 		valid = false;
--
2.17.1

As Ming mentioned in that same thread, we could even make this policy 
for managed interrupts.

Cheers,
John

> Thanks,
> Ming Lei
>
> .