linux-kernel - Re: Performance regression in v3.14

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Message-ID: <20140528165337.GB2296@intel.com>
Date:	Thu, 29 May 2014 00:53:38 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Doug Smythies <dsmythies@...us.net>
Cc:	'Johan Hovold' <jhovold@...il.com>,
	'Dirk Brandewie' <dirk.brandewie@...il.com>,
	'Viresh Kumar' <viresh.kumar@...aro.org>,
	dirk.j.brandewie@...el.com,
	"'Rafael J. Wysocki'" <rafael.j.wysocki@...el.com>,
	cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
	'Linux Kernel Mailing List' <linux-kernel@...r.kernel.org>,
	'Greg Kroah-Hartman' <gregkh@...uxfoundation.org>,
	'Stratos Karafotis' <stratosk@...aphore.gr>
Subject: Re: Performance regression in v3.14

> That is not true. Yes, and due to the setpoint being less than
> 100, which is needed or the driver won't work at all, there is
> a tendency to drive the target pstate upwards.
> However that is tempered by both the PID proportional gain,
> and ultimately integer math. More importantly, the CPU
> itself tells the driver when it is operating below the target
> pstate and driver responds.
> 
> Additionally, the tendency to drive up the target pstate
> too much is exasperated by some extra rounding up at a
> couple of spots. Dirk has a pending fix.
> 
> > And a few iterations
> > later, we will reach max (possible) frequency,
> > then we are effectively performance governor
> > (highest frequency all the time).
> 
> Please do not confuse highest target pstate with
> highest frequency. They are not the same. The processor
> itself can back off.
> 
Hi Doug,

All you said is about the hardware will not give whatever software wants
(e.g., requested freq too high). Agreed.

But does it matter to this discussion?

Yuyang 
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