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

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Date:   Thu, 22 Mar 2018 18:21:28 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Doug Smythies <dsmythies@...us.net>
Cc:     "'Rafael J. Wysocki'" <rafael@...nel.org>,
        'Thomas Ilsche' <thomas.ilsche@...dresden.de>,
        '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>
Subject: Re: [RFT][PATCH v7 5/8] cpuidle: Return nohz hint from cpuidle_select()

On Thursday, March 22, 2018 4:41:54 PM CET Doug Smythies wrote:
> On 2018.03.21 23:25 Doug Smythies wrote:
> > 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.
> 
> I am not seeing any issues at all with V7.
> 
> >>
> >> 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).
> 
> I re-did the idle test on V7, and for longer.
> It is great.
> See line added to the idle graph for V7.2+:
> 
> http://fast.smythies.com/rjw_v72p_v7_idle.png
> 
> >
> > 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).
> 
> Now shown on the new graph. Link above.

Thanks for the data!

I will send another update of patch [5/8] shortly which is closer to the
original v7 of it than the v7.[1-2].