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

Ming Lei
2019-12-10T01:44:01Z

On Mon, Dec 09, 2019 at 02:30:59PM +0000, John Garry wrote:
> 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?

That is why I ask you for the details of your test.

If you mean hisilicon SAS, the interrupt load should have been distributed
well given the device has multiple reply queues for distributing interrupt
load.

>
> >
> > 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.

The result just shows the saturation, we need to root cause it instead
of workaround it via random changes.

>
> 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

I am pretty clear the reason for Azure, which is caused by aggressive interrupt
coalescing, and this behavior shouldn't be very common, and it can be
addressed by the following patch:

http://lists.infradead.org/pipermail/linux-nvme/2019-November/028008.html

Then please share your lockup story, such as, which HBA/drivers, test steps,
if you complete IOs from multiple disks(LUNs) on single CPU, if you have
multiple queues, how many active LUNs involved in the test, ...

Thanks,
Ming