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

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Date:   Wed, 17 Jan 2018 16:49:32 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     torvalds@...ux-foundation.org
Cc:     efault@....de, peterz@...radead.org, edumazet@...gle.com,
        dima@...sta.com, frederic@...nel.org, linux-kernel@...r.kernel.org,
        0x7f454c46@...il.com, akpm@...ux-foundation.org,
        fweisbec@...il.com, hannes@...essinduktion.org, mingo@...nel.org,
        alexander.levin@...izon.com, pabeni@...hat.com,
        paulmck@...ux.vnet.ibm.com, rrendec@...sta.com, riel@...hat.com,
        sgruszka@...hat.com, tglx@...utronix.de, wanpeng.li@...mail.com
Subject: Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd
 context

From: Linus Torvalds <torvalds@...ux-foundation.org>
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.

> 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.