linux-kernel - Re: [RFT][PATCH v5 0/7] sched/cpuidle: Idle loop rework

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Date:   Wed, 21 Mar 2018 14:58:58 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Doug Smythies <dsmythies@...us.net>
Cc:     'Thomas Ilsche' <thomas.ilsche@...dresden.de>,
        'Linux PM' <linux-pm@...r.kernel.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>,
        'Peter Zijlstra' <peterz@...radead.org>,
        "'Rafael J. Wysocki'" <rafael@...nel.org>
Subject: Re: [RFT][PATCH v5 0/7] sched/cpuidle: Idle loop rework

On Wednesday, March 21, 2018 2:51:13 PM CET Doug Smythies wrote:
> On 2018.03.20 23:33 Rafael J. Wysocki wrote:
> > On Tuesday, March 20, 2018 10:03:50 PM CET Doug Smythies wrote:
> >> Summary: My results with kernel 4.16-rc6 and V8 of the patch set
> >> are completely different, and now show no clear difference
> >> (a longer test might reveal something). 
> >
> > Does this mean that you see the "powernightmares" pattern with the v8
> > again or are you referring to something else?
> 
> Sorry for not being clear.
> I do not see any "powernightmares" at all with V8.

Great!

> After this e-mail I did a 3 hour trace and saw none.
> 
> >> On 2018.03.20 10:16 Doug Smythies wrote:
> >>> On 2018.03.20 03:02 Thomas Ilsche wrote:
> >>>
> >>>...[snip]...
> >>>
> >>>> Consider the Skylake server system which has residencies in C1E of
> >>>> 20 us and C6 of 800 us. I use a small while(1) {usleep(300);}
> >>>> unsynchronized pinned to each core. While this is an artificial
> >>>> case, it is a very innocent one - easy to predict and regular. Between
> >>>> vanilla 4.16.0-rc5 and idle-loop/v6, the power consumption increases
> >>>> from 149.7 W to 158.1 W. On 4.16.0-rc5, the cores sleep almost
> >>>> entirely in C1E. With the patches applied, the cores spend ~75% of
> >>>> their sleep time in C6, ~25% in C1E. The average time/usage for C1E is
> >>>> also lower with v6 at ~350 us rather than the ~550 us in C6 (and in
> >>>> C1E with the baseline). Generally the new menu governor seems to chose
> >>>> C1E if the next timer is an enabled sched timer - which occasionally
> >>>> interrupts the sleep-interval into two C1E sleeps rather than one C6.
> >>>>
> >>>> Manually disabling C6, reduces power consumption back to 149.5 W.
> >>>
> >>> ...[snip]...
> >>>
> >>> Note that one of the tests that I normally do is a work/sleep
> >>> frequency sweep from 100 to 2100 Hz, typically at a lowish
> >>> workload. I didn't notice anything odd with this test:
> >>> 
> >>> http://fast.smythies.com/rjw_freq_sweep.png
> >
> > Would it be possible to produce this graph with the v8 of the
> > patchset?
> 
> Yes, sure.

Thanks!

> >>> However, your test is at 3333 Hz (well, minus overheads).
> >>> I did the same as you. And was surprised to confirm
> >>> your power findings. In my case package power goes from
> >>> ~8.6 watts to ~7.3 watts with idle state 4 (C6) disabled.
> >>>
> >>> I am getting different residency times than you though.
> >>> I also observe different overheads between idle state 4
> >>> being disabled or not. i.e. my actual loop frequency
> >>> drops from ~2801 Hz to ~2754 Hz.
> >>>
> >>> Example residencies over the previous minute:
> >>>
> >>> Idle state 4 (C6) disabled (seconds):
> >>>
> >>> Idle state 0: 0.001119
> >>> Idle state 1: 0.056638
> >>> Idle state 2: 13.100550
> >>> Idle state 3: 446.266744
> >>> Idle state 4: 0.000000
> >>>
> >>> Idle state 4 (C6) enabled (seconds):
> >>>
> >>> Idle state 0: 0.034502
> >>> Idle state 1: 1.949595
> >>> Idle state 2: 78.291793
> >>> Idle state 3: 96.467974
> >>> Idle state 4: 286.247524
> >>
> >> Now, with kernel 4.16-rc6 and V8 of the patch set and the poll fix
> >> I am unable to measure the processor package power difference
> >> between idle state 0 enabled or disabled (i.e. it is in the noise).
> >> also the loop time changes (overhead changes) are minimal. However,
> >> the overall loop time has dropped to ~2730 Hz, so there seems to be
> >> a little more overhead in general.
> >>
> >> I increased my loop frequency to ~3316 Hz. Similar.
> >> 
> >> I increased my loop frequency to ~15474 Hz. Similar.
> >> Compared to a stock 4.16-rc6 kernel: The loop rate dropped
> >> to 15,209 Hz and it (the stock kernel) used about 0.3 more
> >> watts (out of 10.97, or ~3% more).
> >
> > So do you prefer v6 or v8?  I guess the former?
> 
> Again sorry for not being clear.
> I was saying that V8 is great.

OK, thanks!

It's v7, actually. :-)

> I did more tests after the original e-mail was sent,
> and the noted slight overhead drop was not always there
> (i.e. it was inconsistent).

If possible, please also try the v7.2 replacement for patch [5/8]:

https://patchwork.kernel.org/patch/10299429/