linux-kernel - Re: [patch 5/8] Immediate Values - x86 Optimization (simplified)

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Message-Id: <200711201006.34954.rusty@rustcorp.com.au>
Date:	Tue, 20 Nov 2007 10:06:34 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@....de>, "H. Peter Anvin" <hpa@...or.com>,
	Chuck Ebbert <cebbert@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 5/8] Immediate Values - x86 Optimization (simplified)

On Tuesday 20 November 2007 01:28:03 Mathieu Desnoyers wrote:
> * Rusty Russell (rusty@...tcorp.com.au) wrote:
> > I think it would be easier to just fast-path the num_online_cpus == 1
> > case, even if you want to keep this "update_early" interface.
>
> Nope, that could lead to problems. I call core_immediate_update()
> _very_ early, before boot_cpu_init() is called.

Ah, I see the problem.  It would in fact be clearer for us to move 
boot_cpu_init() up to just after smp_setup_processor_id() in start_kernel 
anyway, not just for this code, but in general.

> Therefore, 
> cpu_online_map is not set yet. I am not sure the benefit of using
> num_online_cpus outweights the added fragility wrt other boot process
> initializations.

I think it's still a win, though worth a comment that we always go via the 
non-IPI path for the early boot case.

Cheers,
Rusty.
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