linux-kernel - RE: [RFT][PATCH v7 6/8] sched: idle: Select idle state before stopping the tick

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Date:   Wed, 28 Mar 2018 13:41:35 -0700
From:   "Doug Smythies" <dsmythies@...us.net>
To:     "'Thomas Ilsche'" <thomas.ilsche@...dresden.de>,
        "'Rafael J. Wysocki'" <rafael@...nel.org>
Cc:     "'Peter Zijlstra'" <peterz@...radead.org>,
        "'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>,
        "Doug Smythies" <dsmythies@...us.net>
Subject: RE: [RFT][PATCH v7 6/8] sched: idle: Select idle state before stopping the tick

On 2018.03.28 08:15 Thomas Ilsche wrote:
> On 2018-03-28 12:56, Rafael J. Wysocki wrote:
>> On Wed, Mar 28, 2018 at 12:37 PM, Rafael J. Wysocki <rafael@...nel.org> wrote:
>>> On Wed, Mar 28, 2018 at 10:38 AM, Thomas Ilsche
>>> <thomas.ilsche@...dresden.de> wrote:
>>>> On 2018-03-28 10:13, Rafael J. Wysocki wrote:
>>>>>
>> 
>> [cut]
>> 
>>>
>>> So I do
>>>
>>> $ for cpu in 0 1 2 3; do taskset -c $cpu sh -c 'while true; do usleep
>>> 500; done' & done
>>>
>>> which is a shell kind of imitation of the above and I cannot see this
>>> issue at all.
>>>
>>> I count the number of times data->next_timer_us in menu_select() is
>>> greater than TICK_USEC and while this "workload" is running, that
>>> number is exactly 0.
>>>
>>> I'll try with a C program still.
>> 
>> And with a C program I see data->next_timer_us greater than TICK_USEC
>> while it is running, so let me dig deeper.
>> 
>
> I can confirm that a shell-loop behaves differently like you describe.
> Even if it's just a shell-loop calling "main{usleep(500);}" binary.

I normally use the C program method.
The timer there returns with the need_sched() flag set.

I do not seem to have usleep on my system, but when using sleep in a
shell loop, the timer returns without the need_resched() flag set.

Most of my test results involving varying the value of
POLL_IDLE_COUNT are total garbage, because I was using the
C program method, and thus exiting the poll_idle loop based
on the need_resched() flag and not the POLL_IDLE_COUNT
setting. 

I don't know if I can re-do the work, because I
do not have a good way to get my system to use Idle
State 0 with any real workflow, and I seem to get into
side effect issues when I disable other idle states to 
force more use of idle state 0.

... Doug