linux-kernel - Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

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Date:   Mon, 17 Jul 2017 11:21:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Arjan van de Ven <arjan@...ux.intel.com>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Christoph Lameter <cl@...ux.com>,
        Aubrey Li <aubrey.li@...el.com>, tglx@...utronix.de,
        len.brown@...el.com, rjw@...ysocki.net, tim.c.chen@...ux.intel.com,
        paulmck@...ux.vnet.ibm.com, yang.zhang.wz@...il.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

On Fri, Jul 14, 2017 at 09:03:14AM -0700, Andi Kleen wrote:
> fast idle doesn't have an upper bound.
> 
> If the prediction exceeds the fast idle threshold any C state can be used.
> 
> It's just another state (fast C1), but right now it has an own threshold
> which may be different from standard C1.

Given it uses the same estimate we end up with:

select_c_state(idle_est)
{
	if (idle_est < fast_threshold)
		return C1;

	if (idle_est < C1_threshold)
		return C1;
	if (idle_est < C2_threshold)
		return C2;
	/* ... */

	return C6
}

Now, unless you're mister Turnbull, C2 will never get selected when
fast_threshold > C2_threshold.

Which is wrong. If you want to effectively scale the selection of C1,
why not also change the C2 and further criteria.