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From:   "Doug Smythies" <dsmythies@telus.net>
To:     "'Thomas Ilsche'" <thomas.ilsche@tu-dresden.de>
Cc:     "'Rafael J. Wysocki'" <rjw@rjwysocki.net>,
        "'Thomas Gleixner'" <tglx@linutronix.de>,
        "'Paul McKenney'" <paulmck@linux.vnet.ibm.com>,
        "'Rik van Riel'" <riel@surriel.com>,
        "'Peter Zijlstra'" <peterz@infradead.org>,
        "'Aubrey Li'" <aubrey.li@linux.intel.com>,
        "'Mike Galbraith'" <mgalbraith@suse.de>,
        "'Frederic Weisbecker'" <fweisbec@gmail.com>,
        "'LKML'" <linux-kernel@vger.kernel.org>,
        "Doug Smythies" <dsmythies@telus.net>,
        "'Linux PM'" <linux-pm@vger.kernel.org>
References: <2142751.3U6XgWyF8u@aspire.rjw.lan> <2148754.TY7qXgFyZy@aspire.rjw.lan> xuDHe5JrTQdbpxuDMeg0da
In-Reply-To: xuDHe5JrTQdbpxuDMeg0da
Subject: RE: [RFT][PATCH v5 7/7] cpuidle: menu: Avoid selecting shallow states with stopped tick
Date:   Mon, 19 Mar 2018 11:21:58 -0700
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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@intel.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