linux-kernel - RE: [RFT][PATCH v5 7/7] cpuidle: menu: Avoid selecting shallow states with stopped tick

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Message-ID: <003301d3bfaf$2d5e4090$881ac1b0$@net>
Date:   Mon, 19 Mar 2018 11:21:58 -0700
From:   "Doug Smythies" <dsmythies@...us.net>
To:     "'Thomas Ilsche'" <thomas.ilsche@...dresden.de>
Cc:     "'Rafael J. Wysocki'" <rjw@...ysocki.net>,
        "'Thomas Gleixner'" <tglx@...utronix.de>,
        "'Paul McKenney'" <paulmck@...ux.vnet.ibm.com>,
        "'Rik van Riel'" <riel@...riel.com>,
        "'Peter Zijlstra'" <peterz@...radead.org>,
        "'Aubrey Li'" <aubrey.li@...ux.intel.com>,
        "'Mike Galbraith'" <mgalbraith@...e.de>,
        "'Frederic Weisbecker'" <fweisbec@...il.com>,
        "'LKML'" <linux-kernel@...r.kernel.org>,
        "Doug Smythies" <dsmythies@...us.net>,
        "'Linux PM'" <linux-pm@...r.kernel.org>
Subject: RE: [RFT][PATCH v5 7/7] cpuidle: menu: Avoid selecting shallow states with stopped tick

On 2018.03.19 05:47 Thomas Ilsche wrote:
> On 2018-03-15 23:19, Rafael J. Wysocki wrote:
>> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>> 
>> If the scheduler tick has been stopped already and the governor
>> selects a shallow idle state, the CPU can spend a long time in that
>> state if the selection is based on an inaccurate prediction of idle
>> time.  That effect turns out to be noticeable, so it needs to be
>> mitigated.
>
> What are some common causes for that situation?
> How could I trigger this for testing?

It appeared quite readily with my simple 100% load
on one CPU test. Back then (V3) there only 6 patches in the set,
and before the re-spin there ended up being a patch 7 of 6, which
made a significant difference in both package power and the
histograms of times in each idle state.

Reference:
https://marc.info/?l=linux-pm&m=152075419526696&w=2

... Doug