linux-kernel - Re: [PATCH 9/9] x86/intel_rdt: Intel haswell Cache Allocation enumeration

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Date:	Wed, 29 Jul 2015 18:35:02 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vikas Shivappa <vikas.shivappa@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, vikas.shivappa@...el.com,
	x86@...nel.org, hpa@...or.com, tglx@...utronix.de,
	mingo@...nel.org, tj@...nel.org, matt.fleming@...el.com,
	will.auld@...el.com, glenn.p.williamson@...el.com,
	kanaka.d.juvva@...el.com
Subject: Re: [PATCH 9/9] x86/intel_rdt: Intel haswell Cache Allocation
 enumeration

On Wed, Jul 01, 2015 at 03:21:10PM -0700, Vikas Shivappa wrote:
> +	boot_cpu_data.x86_cache_max_closid = 4;
> +	boot_cpu_data.x86_cache_max_cbm_len = 20;

That's just vile. And I'm surprised it even works, I would've expected
boot_cpu_data to be const.

So the CQM code has paranoid things like:

	max_rmid = MAX_INT;
	for_each_possible_cpu(cpu)
		max_rmid = min(max_rmid, cpu_data(cpu)->x86_cache_max_rmid);

And then uses max_rmid. This has the advantage that if you mix parts in
a multi-socket environment and hotplug socket 0 to a later part which a
bigger {rm,clos}id your allocation isn't suddenly too small.

Please do similar things and only ever look at cpu_data once, at init
time.
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