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

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Message-ID: <20170713182820.sn3fjitnd3mca27p@hirez.programming.kicks-ass.net>
Date:   Thu, 13 Jul 2017 20:28:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "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,
        arjan@...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 Thu, Jul 13, 2017 at 11:13:28PM +0800, Li, Aubrey wrote:
> On 2017/7/13 22:53, Peter Zijlstra wrote:

> > Fixing C-state selection by creating an alternative idle path sounds so
> > very wrong.
> 
> This only happens on the arch which has multiple hardware idle cstates, like
> Intel's processor. As long as we want to support multiple cstates, we have to
> make a selection(with cost of timestamp update and computation). That's fine
> in the normal idle path, but if we want a fast idle switch, we can make a 
> tradeoff to use a low-latency one directly, that's why I proposed a fast idle
> path, so that we don't need to mix fast idle condition judgement in both idle
> entry and idle exit path.

That doesn't make sense. If you can decide to pick a shallow C state in
any way, you can fix the general selection too.