linux-kernel - Re: [patch v3 08/36] x86/smpboot: Split up native_cpu_up() into separate phases and document them

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Message-ID: <87jzxh1zbg.ffs@tglx>
Date:   Tue, 09 May 2023 14:09:23 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        David Woodhouse <dwmw2@...radead.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Brian Gerst <brgerst@...il.com>,
        Arjan van de Veen <arjan@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Paul McKenney <paulmck@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Paul Menzel <pmenzel@...gen.mpg.de>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        Piotr Gorski <lucjan.lucjanov@...il.com>,
        Usama Arif <usama.arif@...edance.com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xenproject.org,
        Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Guo Ren <guoren@...nel.org>,
        linux-csky@...r.kernel.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, linux-parisc@...r.kernel.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        linux-riscv@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        Sabin Rapan <sabrapan@...zon.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [patch v3 08/36] x86/smpboot: Split up native_cpu_up() into
 separate phases and document them

On Tue, May 09 2023 at 12:31, Peter Zijlstra wrote:
> On Mon, May 08, 2023 at 09:43:39PM +0200, Thomas Gleixner wrote:
>> +	/*
>> +	 * Wait for the AP to mark itself online, so the core caller
>> +	 * can drop sparse_irq_lock.
>> +	 */
>> +	while (!cpu_online(cpu))
>> +		schedule();
>> +}
>
> These schedule() loops make me itch... this is basically Ye Olde yield()
> loop with all it's known 'benefits'.
>
> Now, I don't think it's horribly broken, we're explicitly waiting on
> another CPU and can't have priority inversions, but yuck!
>
> It could all be somewhat cleaned up with wait_var_event{_timeout}() and
> wake_up_var(), but I'm really not sure that's worth it. But at least it
> requires a comment to justify.

All of them are gone with the later patches and the control CPU will
just go straight to wait for the completion in the core code.