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

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Message-ID: <CAFULd4ZN1FXh3JMznJ=SiBxuYj74iaDSg-j6wZjbYxuTaS7wTA@mail.gmail.com>
Date: Tue, 4 Mar 2025 17:48:37 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Brian Gerst <brgerst@...il.com>
Cc: Ingo Molnar <mingo@...nel.org>, 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>, 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 5:42 PM Brian Gerst <brgerst@...il.com> wrote:
>
> On Tue, Mar 4, 2025 at 11:02 AM Uros Bizjak <ubizjak@...il.com> wrote:
> >
> > On Tue, Mar 4, 2025 at 4:00 PM Brian Gerst <brgerst@...il.com> wrote:
> > >
> > > On Tue, Mar 4, 2025 at 4:55 AM Ingo Molnar <mingo@...nel.org> wrote:
> > > >
> > > >
> > > > * Uros Bizjak <ubizjak@...il.com> wrote:
> > > >
> > > > > On Tue, Mar 4, 2025 at 10:48 AM Ingo Molnar <mingo@...nel.org> wrote:
> > > > > >
> > > > > >
> > > > > > * Brian Gerst <brgerst@...il.com> wrote:
> > > > > >
> > > > > > > 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).
> > > > > >
> > > > > > 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.
> > > > >
> > > > > Why not simply:
> > > > >
> > > > > ASSERT(__per_cpu_hot_end - __per_cpu_hot_start <= L1_CACHE_BYTES, "...")
> > > > >
> > > > > ?
> > > >
> > > > I don't think it's a great idea to randomly allow a larger section
> > > > depending on the .config ... The *actual* intended limit is 64, not 128
> > > > and not 4096, so I'd suggest we write it out as before.
> > >
> > > Change the assert to:
> > >        ASSERT(__per_cpu_hot_pad - __per_cpu_hot_start <= 64, "percpu
> > > cache hot section too large")
> > >
> > > We only care about the used portion, not the padded end.
> >
> > If this is the case, perhaps it is better to use __per_cpu_hot_end
> > to mark the end of the real data, as in the attached patch.
>
> That's a better approach.  There's not much value in the current _end
> symbol if it's not always _start+64.

Thanks, I'll submit a formal patch submission.

Uros.