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

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Message-ID: <642c6901-ad0d-b6b3-9ee9-3e0bd10aa610@tu-dresden.de>
Date:   Thu, 22 Mar 2018 14:18:59 +0100
From:   Thomas Ilsche <thomas.ilsche@...dresden.de>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
CC:     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 2018-03-21 23:15, Rafael J. Wysocki wrote:
> 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.
> 

The patch on top of idle-loop-v7.2 doesn't improve idle behavior on
SKL-SP. Overall it is pretty erratic, I have not seen any regular
patterns. Sometimes only few cpus are affected, here's a screenshot of
almost all cpus being affected after a short burst workload.

https://wwwpub.zih.tu-dresden.de/~tilsche/powernightmares/v7_2_reflect_skl_sp_idle.png