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Date: Tue, 4 Mar 2025 10:47:54 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Brian Gerst <brgerst@gmail.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	"H . Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>, Ard Biesheuvel <ardb@kernel.org>,
	Uros Bizjak <ubizjak@gmail.com>,
	Linus Torvalds <torvalds@linuxfoundation.org>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v3 00/11] Add a percpu subsection for cache hot data
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* Brian Gerst <brgerst@gmail.com> wrote:

> On Tue, Mar 4, 2025 at 3:47 AM Ingo Molnar <mingo@kernel.org> wrote:
> >
> >
> > * Brian Gerst <brgerst@gmail.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).

Hm, to resolve this I'd go for the easy out of explicitly using '64' as 
the size limit - like we did it in the C space.

Thanks,

	Ingo