linux-kernel - Re: [RFT][PATCH v7 5/8] cpuidle: Return nohz hint from cpuidle

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Date:   Wed, 21 Mar 2018 23:15:01 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Thomas Ilsche <thomas.ilsche@...dresden.de>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Doug Smythies <dsmythies@...us.net>,
        Rik van Riel <riel@...riel.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Mike Galbraith <mgalbraith@...e.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFT][PATCH v7 5/8] cpuidle: Return nohz hint from cpuidle_select()

On Wed, Mar 21, 2018 at 6:59 PM, Thomas Ilsche
<thomas.ilsche@...dresden.de> wrote:
> On 2018-03-21 15:36, Rafael J. Wysocki wrote:
>>
>>
>> So please disregard this one entirely and take the v7.2 replacement
>> instead of it:https://patchwork.kernel.org/patch/10299429/
>>
>> The current versions (including the above) is in the git branch at
>>
>>   git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git \
>>   idle-loop-v7.2
>
>
> With v7.2 (tested on SKL-SP from git) I see similar behavior in idle
> as with v5: several cores which just keep the sched tick enabled.
> Worse yet, some go only in C1 (not even C1E!?) despite sleeping the
> full sched tick.
> The resulting power consumption is ~105 W instead of ~ 70 W.
>
> https://wwwpub.zih.tu-dresden.de/~tilsche/powernightmares/v7_2_skl_sp_idle.png
>
> I have briefly ran v7 and I believe it was also affected.

Then it looks like menu_select() stubbornly thinks that the idle
duration will be within the tick boundary on those cores.

That may be because the bumping up of the correction factor in
menu_reflect() is too conservative or it may be necessary to do
something radical to measured_us in menu_update() in case of a tick
wakeup combined with a large next_timer_us value.

For starters, please see if the attached patch (on top of the
idle-loop-v7.2 git branch) changes this behavior in any way.

View attachment "cpuidle-menu-menu_reflect-debug.patch" of type "text/x-patch" (672 bytes)