linux-kernel - Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAJZ5v0g=ZNssYWwtHs5jNN5ercasie41bXrs0CyKJSNV+RMAJA@mail.gmail.com>
Date:	Sat, 13 Feb 2016 00:14:12 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Ashwin Chaugule <ashwin.chaugule@...aro.org>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steve Muckle <steve.muckle@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Juri Lelli <juri.lelli@....com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks

On Fri, Feb 12, 2016 at 5:53 PM, Ashwin Chaugule
<ashwin.chaugule@...aro.org> wrote:
> On 12 February 2016 at 11:15, Rafael J. Wysocki <rafael@...nel.org> wrote:
>> On Fri, Feb 12, 2016 at 5:01 PM, Rafael J. Wysocki <rafael@...nel.org> wrote:
>>> On Fri, Feb 12, 2016 at 3:10 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>>>> On Thu, Feb 11, 2016 at 10:52:20AM -0800, Steve Muckle wrote:
>>>>> On 02/11/2016 09:30 AM, Peter Zijlstra wrote:
>>>>> >> My concern above is that pokes are guaranteed to keep occurring when
>>>>> >> > there is only RT or DL activity so nothing breaks.
>>>>> >
>>>>> > The hook in their respective tick handler should ensure stuff is called
>>>>> > sporadically and isn't stalled.
>>>>>
>>>>> But that's only true if the RT/DL tasks happen to be running when the
>>>>> tick arrives right?
>>>>>
>>>>> Couldn't we have RT/DL activity which doesn't overlap with the tick? And
>>>>> if no CFS tasks happen to be executing on that CPU, we'll never trigger
>>>>> the cpufreq update. This could go on for an arbitrarily long time
>>>>> depending on the periodicity of the work.
>>>>
>>>> Possible yes, but why do we care? Such a CPU would be so much idle that
>>>> cpufreq doesn't matter one way or another, right?
>>>
>>> Well, in theory you can get 50% or so of the time active in bursts
>>> that happen to fit between ticks.  If we happen to do those in the
>>> lowest P-state, we may burn more energy than necessary on platforms
>>> where more idle is preferred.
>>
>> At least intel_pstate should be able to figure out which P-state to
>> use then on the APERF/MPERF basis.
>
> Speaking for the generic case, it would be great to make use of such
> feedback counters for selecting the next freq request. Use (num of
> cycles used/total cycles) to figure out %ON time for the CPU. I
> understand its not the goal for this patch series, but in the future
> if we can do this in your callbacks where possible, then I think we
> will do better than Ondemand.

Yes, we can do that at least in principle.  intel_pstate is a proof of that.

Thanks,
Rafael