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Date:   Tue, 9 May 2023 12:31:46 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        David Woodhouse <dwmw2@infradead.org>,
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Subject: Re: [patch v3 08/36] x86/smpboot: Split up native_cpu_up() into
 separate phases and document them
Message-ID: <20230509103146.GW83892@hirez.programming.kicks-ass.net>
References: <20230508181633.089804905@linutronix.de>
 <20230508185217.671595388@linutronix.de>
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And since I'm commenting on existing things anyway, let me continue...

On Mon, May 08, 2023 at 09:43:39PM +0200, Thomas Gleixner wrote:

> +static int wait_cpu_cpumask(unsigned int cpu, const struct cpumask *mask)
> +{
> +	unsigned long timeout;
>  
> +	/*
> +	 * Wait up to 10s for the CPU to report in.
> +	 */
> +	timeout = jiffies + 10*HZ;
> +	while (time_before(jiffies, timeout)) {
> +		if (cpumask_test_cpu(cpu, mask))
> +			return 0;
> +
> +		schedule();
>  	}
> +	return -1;
> +}

> +/*
> + * Bringup step three: Wait for the target AP to reach smp_callin().
> + * The AP is not waiting for us here so we don't need to parallelise
> + * this step. Not entirely clear why we care about this, since we just
> + * proceed directly to TSC synchronization which is the next sync
> + * point with the AP anyway.
> + */
> +static void wait_cpu_callin(unsigned int cpu)
> +{
> +	while (!cpumask_test_cpu(cpu, cpu_callin_mask))
> +		schedule();
> +}
> +
> +/*
> + * Bringup step four: Synchronize the TSC and wait for the target AP
> + * to reach set_cpu_online() in start_secondary().
> + */
> +static void wait_cpu_online(unsigned int cpu)
>  {
>  	unsigned long flags;
> +
> +	/*
> +	 * Check TSC synchronization with the AP (keep irqs disabled
> +	 * while doing so):
> +	 */
> +	local_irq_save(flags);
> +	check_tsc_sync_source(cpu);
> +	local_irq_restore(flags);
> +
> +	/*
> +	 * 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.