linux-kernel - Re: ARM router NAT performance affected by random/unrelated commits

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Date:   Tue, 21 May 2019 12:19:15 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Rafał Miłecki <zajec5@...il.com>
Cc:     Jo-Philipp Wich <jo@...n.io>,
        Network Development <netdev@...r.kernel.org>,
        John Crispin <john@...ozen.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-block@...r.kernel.org, Jonas Gorski <jonas.gorski@...il.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Felix Fietkau <nbd@....name>
Subject: Re: ARM router NAT performance affected by random/unrelated commits

On Tue, May 21, 2019 at 01:16:12PM +0200, Rafał Miłecki wrote:
> On Tue, 21 May 2019 at 12:45, Russell King - ARM Linux admin
> <linux@...linux.org.uk> wrote:
> > On Tue, May 21, 2019 at 12:28:48PM +0200, Rafał Miłecki wrote:
> > > I work on home routers based on Broadcom's Northstar SoCs. Those devices
> > > have ARM Cortex-A9 and most of them are dual-core.
> > >
> > > As for home routers, my main concern is network performance. That CPU
> > > isn't powerful enough to handle gigabit traffic so all kind of
> > > optimizations do matter. I noticed some unexpected changes in NAT
> > > performance when switching between kernels.
> > >
> > > My hardware is BCM47094 SoC (dual core ARM) with integrated network
> > > controller and external BCM53012 switch.
> >
> > Guessing, I'd say it's to do with the placement of code wrt cachelines.
> 
> That was my guess as well, that's why I tried "cachestat" tool.
> 
> 
> > You could try aligning some of the cache flushing code to a cache line
> > and see what effect that has.
> 
> Can you give me some extra hint on how to do that, please? I tried
> searching for it a bit but I didn't find any clear article on that
> matter.

IIRC, the cache line size on Cortex A9 is 32 bytes, so the assembler
directive would be ".align 5".  Place that in arch/arm/mm/cache-v7.S
before v7_dma_clean_range and v7_dma_inv_range.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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