Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd context

[Thomas Gleixner]

On Wed, 17 Jan 2018, David Miller wrote:

> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Date: Wed, 17 Jan 2018 13:06:58 -0800
> 
> > It was in some way always a "poor mans interrupt thread" (with no
> > blocking like a real thread context, but at least not impacting actual
> > interrupt latency).
> 
> Or in this loopback device case (and tunnel decapsulation) a poor
> man's longjmp, releasing the current stack frame to keep the depth
> in check.
> 
> Anyways...
> 
> > That said, this made me wonder a bit. I wonder how bounded the latency
> > is for raising a softirq from process context. We only _check_ the
> > softirq on the last hardirq exit, I think.
> 
> System call return checks it, otherwise this situation would be
> completely bolixed.

Errm. No.

> 
> > I wonder if we should run softirqs on return to user mode (and make
> > softirq set a thread flag if not in interrupt context).
> 
> I'm pretty sure we already do.

Nope.

raise_softirq() -> raise_softirq_irqoff()

	set_softirq_bit();

	if (!in_interrupt())
		wake_softirqd();

So if the caller is not in hard or soft interrupt context, which includes
bottom half disabled regions softirqd is woken.

If the caller is in a bottom half disabled region then local_bh_enable()
will run the pending softirqs.

Thanks,

	tglx