From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Huacai Chen <chenhuacai@kernel.org>,
Huacai Chen <chenhuacai@loongson.cn>,
Andrew Morton <akpm@linux-foundation.org>,
Ingo Molnar <mingo@redhat.com>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
stable@vger.kernel.org, Tiezhu Yang <yangtiezhu@loongson.cn>
Subject: Re: [PATCH] lib/smp_processor_id: Don't warn if there is only one CPU
Date: Thu, 12 Nov 2026 16:07:49 +0100 [thread overview]
Message-ID: <20261112150749.GA1813469@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <20261112094059.18f66c2a@fedora>
On Thu, Nov 12, 2026 at 09:40:59AM -0500, Steven Rostedt wrote:
> On Thu, 12 Nov 2026 09:13:48 +0100
> Peter Zijlstra <peterz@infradead.org> wrote:
>
> > num_possible_cpus() is what I was thinking of. Notably, there is no
> > requirement that the CPU bitmap is dense. You can have nr_cpu_ids at 256
> > and use 255 for your boot CPU. It would be completely daft to do so, but
> > it is still valid.
>
> Hmm, I was under the impression that there would always be a CPU 0.
>
> Is it possible to boot with a non zero CPU setup?
Yes. Voyager did that -- although that code is now long gone.
In general we must not assume boot cpu is 0 -- although I don't know of
any architecture that currently doesn't have that. Might be fun to have
it as a debug option to catch out assumptions.
next prev parent reply other threads:[~2026-11-12 15:07 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-11-11 9:24 [PATCH] lib/smp_processor_id: Don't warn if there is only one CPU Huacai Chen
2026-11-11 17:06 ` Peter Zijlstra
2026-11-12 3:14 ` Huacai Chen
2026-11-12 8:13 ` Peter Zijlstra
2026-11-12 14:40 ` Steven Rostedt
2026-11-12 15:07 ` Peter Zijlstra [this message]
2026-11-13 2:06 ` Huacai Chen
2026-11-12 4:00 ` Tiezhu Yang
2026-11-12 9:24 ` sashiko-bot
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