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

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Date:   Wed, 21 Mar 2018 23:24:44 -0700
From:   "Doug Smythies" <dsmythies@...us.net>
To:     "'Rafael J. Wysocki'" <rafael@...nel.org>,
        "'Thomas Ilsche'" <thomas.ilsche@...dresden.de>
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>,
        "'Rik van Riel'" <riel@...riel.com>,
        "'Aubrey Li'" <aubrey.li@...ux.intel.com>,
        "'Mike Galbraith'" <mgalbraith@...e.de>,
        "'LKML'" <linux-kernel@...r.kernel.org>,
        "Doug Smythies" <dsmythies@...us.net>
Subject: RE: [RFT][PATCH v7 5/8] cpuidle: Return nohz hint from cpuidle_select()

On 2018.03.21 15:15 Rafael J. Wysocki wrote:
> On Wed, Mar 21, 2018 at 6:59 PM, Thomas Ilsche 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.

O.K. I am seeing some weirdness.
On my system with both V7.2 and V7.2 plus this patch, I observe
A spike in Idle State 1 residency every 34+ minutes. And slightly
higher average idle power than before. (I might not have done V7
idle tests long enough).

It can be seen in the frequency sweep I did earlier today, with V7.2:

http://fast.smythies.com/rjw_freq_sweep_72_combined.png

Despite the note on the graph that says it might be real, I don't think
it is (I forgot to delete the note).

With V7.2+ sometimes the event occurs at 17 minute intervals.
Here is a idle graph (for reference: we have seen idle package power
pretty steady at ~3.7 watts before).

http://fast.smythies.com/rjw_v72p_idle.png

... Doug