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 21:46:02 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Arjan van de Ven <arjan@...ux.intel.com>
cc:     Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <ak@...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>, 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,
        daniel.lezcano@...aro.org
Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle
 periods

On Mon, 17 Jul 2017, Arjan van de Ven wrote:
> On 7/17/2017 12:23 PM, Peter Zijlstra wrote:
> > Now I think the problem is that the current predictor goes for an
> > average idle duration. This means that we, on average, get it wrong 50%
> > of the time. For performance that's bad.
> 
> that's not really what it does; it looks at next tick
> and then discounts that based on history;
> (with different discounts for different order of magnitude)

next tick is the worst thing to look at for interrupt heavy workloads as
the next tick (as computed by the nohz code) can be far away, while the I/O
interrupts come in at a high frequency.

That's where Daniel Lezcanos work of predicting interrupts comes in and
that's the right solution to the problem. The core infrastructure has been
merged, just the idle/cpufreq users are not there yet. All you need to do
is to select CONFIG_IRQ_TIMINGS and use the statistics generated there.

Thanks,

	tglx