linux-kernel - Re: [PATCH 0/6] cpuidle: menu: Fixes, optimizations and cleanups

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Date:   Tue, 9 Oct 2018 00:26:48 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Doug Smythies <dsmythies@...us.net>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH 0/6] cpuidle: menu: Fixes, optimizations and cleanups

On Tue, Oct 9, 2018 at 12:14 AM Doug Smythies <dsmythies@...us.net> wrote:
>
> On 2018.10.08 00:51 Rafael J. Wysocki wrote:
> > On Mon, Oct 8, 2018 at 8:02 AM Doug Smythies <dsmythies@...us.net> wrote:
> >>
> >> On 2018.10.03 23:56 Rafael J. Wysocki wrote:
> >>> On Tue, Oct 2, 2018 at 11:51 PM Rafael J. Wysocki <rjw@...ysocki.net> wrote:

[cut]

> >> Test 2: pipe test 2 CPUs, one core. CPU test:
> >>
> >> The average loop times graph is here:
> >> http://fast.smythies.com/linux-pm/k419/k419-rjw-pipe-1core.png
> >>
> >> The power and idle statistics graphs are here:
> >> http://fast.smythies.com/linux-pm/k419/k419-rjw-pipe-1core.htm
> >>
> >> Conclusions:
> >>
> >> Better performance at the cost of more power with
> >> the patch set, but late August had both better performance
> >> and less power.
> >>
> >> Overall idle entries and exits are about the same, but way
> >> way more idle state 0 entries and exits with the patch set.
> >
> >Same as above (and expected too).
>
> I Disagree. The significant transfer of idle entries from
> idle state 1 with kernel 4.19-rc6 to idle state 0 with the
> additional 8 patch set is virtually entirely due to this patch:
>
> "[PATCH 2/6] cpuidle: menu: Compute first_idx when latency_req is known"

OK

> As far as I can determine from all of this data, in particular the
> histogram data below, it seems to me that it now is selecting
> idle state 0 whereas before it was selecting idle state 1
> is the correct decision for those very short duration idle states
> (well, for my processor (older i7-2600K) at least).

At least, that's a matter of consistency IMO.

State 1 should not be selected if the final latency limit is below its
exit latency and that's what happens in that situation.

> Note: I did test my above assertion with kernels compiled with only
> the first 2 and then 3 of the 8 patch set.

I see.

Thanks,
Rafael