linux-kernel - Re: [PATCH 13/32] Documentation, x86: Documentation for Intel resource allocation user interface

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Message-ID: <20160714171645.GA19191@intel.com>
Date:	Thu, 14 Jul 2016 10:16:45 -0700
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Fenghua Yu <fenghua.yu@...el.com>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <h.peter.anvin@...el.com>,
	Tejun Heo <tj@...nel.org>, Borislav Petkov <bp@...e.de>,
	Stephane Eranian <eranian@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	David Carrillo-Cisneros <davidcc@...gle.com>,
	Ravi V Shankar <ravi.v.shankar@...el.com>,
	Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
	Sai Prakhya <sai.praneeth.prakhya@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH 13/32] Documentation, x86: Documentation for Intel
 resource allocation user interface

On Thu, Jul 14, 2016 at 08:53:17AM +0200, Thomas Gleixner wrote:
> > Happy to take suggestions for something in between those
> > extremes :-)
> 
> I'd suggest "resctrl" and the abbreviation dictionaries tell me that the most
> common ones for resource are: R, RESORC, RES

OK. "resctrl" it is.

> As a side effect that avoids the whole 'find all tasks' on mount machinery
> simply because the CPU defaults do not change at all.

That's a very good side effect.

It just means that the "tasks" file in the root of the hierachy will
need different read/write functions from those in sub-directories.

read: scan all tasks, print pid for ones with task->rdtgroup == NULL

write: remove task from the rdtgroup list that it was on; set task->rdtgroup = NULL;

> It covers almost everything except the case I outlined before:
> 
>    Isolated CPU	 	    Important Task runs on isolated CPU
>    5% exclusive cache	    10% exclusive cache
> 
> That's impossible with your scheme, but it's something which matters. You want
> to make sure that the system services on that isolated CPU stay cache hot
> without hurting the cache locality of your isolated task.

So the core part of __intel_rdt_sched_in() will look like:

	/*
	 * Precedence rules:
	 * Processes assigned to an rdtgroup use that group
	 * wherever they run. If they don't have an rdtgroup
	 * we see if the current cpu has one and use it.
	 * If no specific rdtgroup was provided, we use the
	 * root_rdtgroup
	 */
	rdtgrp = current->rdtgroup;
	if (!rdtgrp) {
		rdtgrp = per_cpu(cpu_rdtgroup, cpu);
		if (!rdtgrp)
			rdtgrp = root_rdtgroup;
	}

> > Otherwise we can revise the documentation to explain all this better.
> 
> That needs to be done in any case. The existing one does not really qualify as
> proper documentation. It's closer to a fairy tale :)

Yes. We will re-write.

-Tony