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

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Message-ID: <3d2d06e2-a2ee-bcf7-7727-07f72de5d32d@linux.intel.com>
Date:   Fri, 14 Jul 2017 08:52:28 -0700
From:   Arjan van de Ven <arjan@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>
Cc:     Andi Kleen <ak@...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 7/14/2017 8:38 AM, Peter Zijlstra wrote:
> No, that's wrong. We want to fix the normal C state selection process to
> pick the right C state.
>
> The fast-idle criteria could cut off a whole bunch of available C
> states. We need to understand why our current C state pick is wrong and
> amend the algorithm to do better. Not just bolt something on the side.

I can see a fast path through selection if you know the upper bound of any
selection is just 1 state.

But also, how much of this is about "C1 be fast" versus "selecting C1 is slow"

a lot of the patches in the thread seem to be about making a lighter/faster C1,
which is reasonable (you can even argue we might end up with 2 C1s, one fast
one full feature)