Re: [PATCH 1/4] softirq: implement IRQ flood detection mechanism

Sagi Grimberg
2019-09-20T17:09:56Z

>> It seems like we're attempting to stay in irq context for as long as we
>> can instead of scheduling to softirq/thread context if we have more than
>> a minimal amount of work to do. Without at least understanding why
>> softirq/thread degrades us so much this code seems like the wrong
>> approach to me. Interrupt context will always be faster, but it is
>> not a sufficient reason to spend as much time as possible there, is it?
>
> If extra latency is added in IO completion path, this latency will be
> introduced in the submission path, because the hw queue depth is fixed,
> which is often small. Especially in case of multiple submission vs.
> single(shared) completion, the whole hw queue tags can be exhausted
> easily.

This is why the patch is reaping the first batch from hard-irq, but only
if it has more will defer to softirq. So I'm not sure the short QD use
case applies here...

> I guess no such effect for networking IO.

Maybe, maybe not...