linux-kernel - Re: [PATCH v3 08/11] slub: Replace cmpxchg

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Date:   Thu, 25 May 2023 15:10:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>, dennis@...nel.org,
        Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        Heiko Carstens <hca@...ux.ibm.com>, gor@...ux.ibm.com,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        borntraeger@...ux.ibm.com, Sven Schnelle <svens@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Joerg Roedel <joro@...tes.org>,
        suravee.suthikulpanit@....com, Robin Murphy <robin.murphy@....com>,
        David Woodhouse <dwmw2@...radead.org>,
        Baolu Lu <baolu.lu@...ux.intel.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S . Miller" <davem@...emloft.net>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-s390@...r.kernel.org, iommu@...ts.linux.dev,
        Linux-Arch <linux-arch@...r.kernel.org>,
        linux-crypto@...r.kernel.org,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH v3 08/11] slub: Replace cmpxchg_double()

On Thu, May 25, 2023 at 12:52:06PM +0200, Arnd Bergmann wrote:
> On Thu, May 25, 2023, at 12:29, Peter Zijlstra wrote:
> > On Wed, May 24, 2023 at 11:32:47AM +0200, Peter Zijlstra wrote:
> >> On Mon, May 15, 2023 at 09:57:07AM +0200, Peter Zijlstra wrote:
> >
> > This then also means I need to look at this_cpu_cmpxchg128 and
> > this_cpu_cmoxchg64 behaviour when we dont have the CPUID feature.
> >
> > Because current verions seem to assume the instruction is present.
> 
> As far as I could tell when reviewing your series, this_cpu_cmpxchg64()
> is always available on all architectures. Depending on compile-time
> feature detection this would be either a native instruction that
> is guaranteed to work, or the irq-disabled version. On x86, this
> is handled at runtime with alternative_io().
> 
> this_cpu_cmpxchg128() clearly needed the system_has_cmpxchg128()
> check, same as system_has_cmpxchg_double() today.

So, having just dug through all that, on x86:

this_cpu_cmpxchg64() is:

 X86_CMPXCHG64=n -> fallback, irrespective of CX8
 X86_CMPXCHG64=y -> cmpxchg8b
 X86_64          -> cmpxchg


I've changed it to be similar between 32bit and 64bit such that both:

  cmpxchg#b when CX#, otherwise this_cpu_cmpxchg#b_emu