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From:   Thomas Gleixner <tglx@linutronix.de>
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To:     Peter Zijlstra <peterz@infradead.org>
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        David Woodhouse <dwmw2@infradead.org>,
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        Sabin Rapan <sabrapan@amazon.com>,
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        David Woodhouse <dwmw@amazon.co.uk>
Subject: Re: [patch v3 08/36] x86/smpboot: Split up native_cpu_up() into
 separate phases and document them
In-Reply-To: <20230509103146.GW83892@hirez.programming.kicks-ass.net>
References: <20230508181633.089804905@linutronix.de>
 <20230508185217.671595388@linutronix.de>
 <20230509103146.GW83892@hirez.programming.kicks-ass.net>
Date:   Tue, 09 May 2023 14:09:23 +0200
Message-ID: <87jzxh1zbg.ffs@tglx>
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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.