netdev - Re: Softirq priority inversion from "softirq: reduce latencies"

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Date:	Mon, 29 Feb 2016 20:14:36 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Hurley <peter@...leysoftware.com>
cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Francois Romieu <romieu@...zoreil.com>,
	Eric Dumazet <edumazet@...gle.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>,
	dmaengine@...r.kernel.org, John Ogness <john.ogness@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Softirq priority inversion from "softirq: reduce latencies"

On Mon, 29 Feb 2016, Peter Hurley wrote:
> On 02/29/2016 10:24 AM, Eric Dumazet wrote:
> >> Just to be clear
> >>
> >> 		if (time_before(jiffies, end) && !need_resched() &&
> >> 		    --max_restart)
> >> 			goto restart;
> >>
> >> aborts softirq *even if 0ns have elapsed*, if NET_RX has woken a process.
> > 
> > Sure, now remove the 1st and 2nd condition.
> 
> Well just removing the 2nd condition has everything working fine,
> because that fixes the priority inversion.

No. It does not fix anything. It hides the shortcomings of the driver.
 
> However, when system resources are _not_ contended, it makes no
> sense to be forced to revert to ksoftirqd resolution, which is strictly
> intended as fallback.

No. You claim it is simply because your driver does not handle that situation
properly.
 
> Or flipping your argument on its head, why not just _always_ execute
> softirq in ksoftirqd?

Which is what that change effectivley does. And that makes a lot of sense,
because you get the softirq load under scheduler control and do not let the
softirq run as a context stealing entity which is completely uncontrollable by
the scheduler.

Running the softirq on return from interrupt can cause real priority
inversions.

Thanks,

	tglx