linux-kernel - Re: [PATCH v3 00/11] Add a percpu subsection for cache hot data

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Message-ID: <CAMzpN2gpHRtOtRuCJF_TKOFbEJ2xkksndCH+MfntfDuZHC0O1w@mail.gmail.com>
Date: Tue, 4 Mar 2025 04:26:38 -0500
From: Brian Gerst <brgerst@...il.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, 
	"H . Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	Ard Biesheuvel <ardb@...nel.org>, Uros Bizjak <ubizjak@...il.com>, 
	Linus Torvalds <torvalds@...uxfoundation.org>, Andy Lutomirski <luto@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 00/11] Add a percpu subsection for cache hot data

On Tue, Mar 4, 2025 at 3:47 AM Ingo Molnar <mingo@...nel.org> wrote:
>
>
> * Brian Gerst <brgerst@...il.com> wrote:
>
> > >
> > > -       PERCPU_SECTION(INTERNODE_CACHE_BYTES)
> > > +       PERCPU_SECTION(L1_CACHE_BYTES)
> > >         ASSERT(__per_cpu_hot_end - __per_cpu_hot_start <= 64, "percpu cache hot section too large")
> > >
> > >         RUNTIME_CONST_VARIABLES
> > >
> >
> > That is probably the right call.  The initial percpu section is just
> > used by the boot cpu early and as a template for the dynamically
> > allocated percpu memory, which should account for the proper
> > alignment for NUMA.
>
> Okay.
>
> Randconfig testing found another corner case with the attached config:
>
>     KSYMS   .tmp_vmlinux0.kallsyms.S
>     AS      .tmp_vmlinux0.kallsyms.o
>     LD      .tmp_vmlinux1
>   ld: percpu cache hot section too large
>   make[2]: *** [scripts/Makefile.vmlinux:77: vmlinux] Error 1
>
> (I haven't figured out the root cause yet.)

CONFIG_MPENTIUM4 sets X86_L1_CACHE_SHIFT to 7 (128 bytes).


Brian Gerst