Re: [PATCH RFC 1/1] genirq: Make threaded handler use irq affinity for managed interrupt

[John Garry]

On 07/12/2019 08:03, Ming Lei wrote:
> On Fri, Dec 06, 2019 at 10:35:04PM +0800, John Garry wrote:
> > 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.

Hi Ming,

> Frankly speaking, I never observed that single CPU is saturated by one storage
> completion queue's interrupt load. Because CPU is still much quicker than
> current storage device.
>
> If there are more drives, one CPU won't handle more than one queue(drive)'s
> interrupt if (nr_drive * nr_hw_queues) < nr_cpu_cores.

Are things this simple? I mean, can you guarantee that fio processes are 
evenly distributed as such?

> So could you describe your case in a bit detail? Then we can confirm
> if this change is really needed.

The issue is that the CPU is saturated in servicing the hard and 
threaded part of the interrupt together - here's the sort of thing which 
we saw previously:
Before:
CPU	%usr	%sys	%irq	%soft	%idle
all	2.9	13.1	1.2	4.6	78.2				
0	0.0	29.3	10.1	58.6	2.0
1	18.2	39.4	0.0	1.0	41.4
2	0.0	2.0	0.0	0.0	98.0

CPU0 has no effectively no idle.

Then, by allowing the threaded part to roam:
After:
CPU	%usr	%sys	%irq	%soft	%idle
all	3.5	18.4	2.7	6.8	68.6
0	0.0	20.6	29.9	29.9	19.6
1	0.0	39.8	0.0	50.0	10.2

Note: I think that I may be able to reduce the irq hard part load in the 
endpoint driver, but not that much such that we see still this issue.

> > For when the interrupt is managed, allow the threaded part to run on all
> > cpus in the irq affinity mask.
>
> I remembered that performance drop is observed by this approach in some
> test.

From checking the thread about the NVMe interrupt swamp, just switching 
to threaded handler alone degrades performance. I didn't see any 
specific results for this change from Long Li - 
https://lkml.org/lkml/2019/8/21/128

Thanks,
John