linux-kernel - Re: [Problem] Cache line starvation

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Date:   Wed, 3 Oct 2018 12:43:57 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Catalin Marinas <catalin.marinas@....com>, bigeasy@...utronix.de,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        daniel.wagner@...mens.com, Will Deacon <will.deacon@....com>,
        x86@...nel.org, Linus Torvalds <torvalds@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.com>, boqun.feng@...il.com,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [Problem] Cache line starvation

On Wed, 3 Oct 2018, Peter Zijlstra wrote:
> On Wed, Oct 03, 2018 at 10:07:05AM +0200, Thomas Gleixner wrote:
> > On Wed, 3 Oct 2018, Catalin Marinas wrote:
> > 
> > > On Fri, 21 Sep 2018 at 13:22, Peter Zijlstra <peterz@...radead.org> wrote:
> > > > On Fri, Sep 21, 2018 at 02:02:26PM +0200, Sebastian Andrzej Siewior wrote:
> > > > > We reproducibly observe cache line starvation on a Core2Duo E6850 (2
> > > > > cores), a i5-6400 SKL (4 cores) and on a NXP LS2044A ARM Cortex-A72 (4
> > > > > cores).
> 
> > Except that the ARM64 ticket locks are not preventing the starvation
> > issue. Neither do the qrlocks on ARM64 on later kernels.
> 
> That A72 is ARMv8-A afaict, that doesn't have the fancy LSE bits (which
> are new in ARMv8.1) on and thus ends up using LL/SC. And LL/SC has
> similar issues as cmpxchg loops.
> 
> AFAICT Cortex-A75 is the first that has LSE on (that's an ARMv8.2-A).

I know. That doesn't help though. We need a solution for the v8-A and the
magic delay loop we have right now to work around it is more that
disturbing.

Thanks,

	tglx