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

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Message-ID: <20170714153818.pjauqxebxyhs6ljp@hirez.programming.kicks-ass.net>
Date:   Fri, 14 Jul 2017 17:38:18 +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 Fri, Jul 14, 2017 at 11:56:33AM +0800, Li, Aubrey wrote:
> On 2017/7/14 2:28, Peter Zijlstra wrote:
> > 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.
> > 
> 
> Okay, maybe something like the following make sense? Give a hint to
> cpuidle_idle_call() to indicate a fast idle.
> 
> --------------------------------------------------------
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index ef63adc..3165e99 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -152,7 +152,7 @@ static void cpuidle_idle_call(void)
> 	 */
> 	rcu_idle_enter();
>  
> -	if (cpuidle_not_available(drv, dev)) {
> +	if (cpuidle_not_available(drv, dev) || this_is_a_fast_idle) {
> 		default_idle_call();
> 		goto exit_idle;
> 	}

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.