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

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Date:	Sat, 27 Feb 2016 18:17:29 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	David Miller <davem@...emloft.net>, edumazet@...gle.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	gregkh@...uxfoundation.org, dmaengine@...r.kernel.org,
	john.ogness@...utronix.de, bigeasy@...utronix.de,
	akpm@...ux-foundation.org
Subject: Re: Softirq priority inversion from "softirq: reduce latencies"

On sam., 2016-02-27 at 18:10 -0800, Peter Hurley wrote:
> On 02/27/2016 05:59 PM, Eric Dumazet wrote:
> > On sam., 2016-02-27 at 15:33 -0800, Peter Hurley wrote:
> >> On 02/27/2016 03:04 PM, David Miller wrote:
> >>> From: Peter Hurley <peter@...leysoftware.com>
> >>> Date: Sat, 27 Feb 2016 12:29:39 -0800
> >>>
> >>>> Not really. softirq raised from interrupt context will always execute
> >>>> on this cpu and not in ksoftirqd, unless load forces softirq loop abort.
> >>>
> >>> That guarantee never was specified.
> >>
> >> ??
> >>
> >> Neither is running network socket servers at normal priority as if they're
> >> higher priority than softirq.
> >>
> >>
> >>> Or are you saying that by design, on a system under load, your UART
> >>> will not function properly?
> >>>
> >>> Surely you don't mean that.
> >>
> >> No, that's not what I mean.
> >>
> >> What I mean is that bypassing the entire SOFTIRQ priority so that
> >> sshd can process one network packet makes a mockery of the point of softirq.
> >>
> >> This hack to workaround NET_RX looping over-and-over-and-over affects every
> >> subsystem, not just one uart.
> >>
> >> HI, TIMER, BLOCK; all of these are skipped: that's straight-up, a bug.
> > 
> > No idea what you talk about.
> > 
> > All pending softirq interrupts are processed. _Nothing_ is skipped.
> 
> An interrupt that schedules HI softirq while in NET_RX softirq should
> still run the HI softirq. But with your patch that won't happen.

Stop saying this. This never had been the case. I am glad my patch
finally show you are wrong.

> 
> 
> > Really, your system stability seems to depend on a completely
> > undocumented behavior of linux kernels before linux-3.8
> > 
> > If I understood, you expect that a tasklet activated from a softirq
> > handler is run from the same  __do_softirq() loop. This never has been
> > the case.
> 
> No.
> 
> The *interrupt handler* for DMA goes off while NET_RX softirq is running.
> That's what schedules the *DMA tasklet*.
> 
> That tasklet should run before any process.
> 
> But it doesn't because your patch bails out early from softirq.

Fine. Fix your driver.