Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Wed, 17 Jan 2018 22:54:11 +0100 (CET)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     David Miller <davem@davemloft.net>
cc:     torvalds@linux-foundation.org, efault@gmx.de, peterz@infradead.org,
        edumazet@google.com, dima@arista.com, frederic@kernel.org,
        linux-kernel@vger.kernel.org, 0x7f454c46@gmail.com,
        akpm@linux-foundation.org, fweisbec@gmail.com,
        hannes@stressinduktion.org, mingo@kernel.org,
        alexander.levin@verizon.com, pabeni@redhat.com,
        paulmck@linux.vnet.ibm.com, rrendec@arista.com, riel@redhat.com,
        sgruszka@redhat.com, wanpeng.li@hotmail.com
Subject: Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd
 context
In-Reply-To: <20180117.164932.1269304606476934540.davem@davemloft.net>
Message-ID: <alpine.DEB.2.20.1801172251280.2326@nanos>
References: <1515782670.7007.3.camel@gmx.de> <20180117.153049.1803664333084879932.davem@davemloft.net> <CA+55aFwOv1SBOz6+0GxLxbK5riei83AQRJEHUTXUehSWfBq=Fg@mail.gmail.com> <20180117.164932.1269304606476934540.davem@davemloft.net>
User-Agent: Alpine 2.20 (DEB 67 2015-01-07)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, 17 Jan 2018, David Miller wrote:

> From: Linus Torvalds <torvalds@linux-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.

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