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

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Date:   Fri, 14 Jul 2017 09:03:14 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 05:58:53PM +0200, Peter Zijlstra wrote:
> On Fri, Jul 14, 2017 at 08:52:28AM -0700, Arjan van de Ven wrote:
> > 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.

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.

> > 
> > But also, how much of this is about "C1 be fast" versus "selecting C1 is slow"
> 
> I got the impression its about we need to select C1 for longer. But the
> fact that the patches don't in fact answer any of these questions,
> they're wrong in principle ;-)

For that workload. Tuning idle thresholds is a complex trade off.

-Andi