linux-kernel - Re: [PATCH v4 1/2] idle: add support for tasks that inject idle

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Date:   Mon, 28 Nov 2016 23:22:50 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Eduardo Valentin <edubezval@...il.com>,
        Zhang Rui <rui.zhang@...el.com>,
        Petr Mladek <pmladek@...e.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH v4 1/2] idle: add support for tasks that inject idle

On Mon, Nov 28, 2016 at 10:46 PM, Jacob Pan
<jacob.jun.pan@...ux.intel.com> wrote:
> On Mon, 28 Nov 2016 22:39:07 +0100
> "Rafael J. Wysocki" <rafael@...nel.org> wrote:
>
>> On Mon, Nov 28, 2016 at 10:33 PM, Jacob Pan
>> <jacob.jun.pan@...ux.intel.com> wrote:
>> > From: Peter Zijlstra <peterz@...radead.org>
>> >
>> > Idle injection drivers such as Intel powerclamp and ACPI PAD
>> > drivers use realtime tasks to take control of CPU then inject idle.
>> > There are two issues with this approach:
>> >
>> >  1. Low efficiency: injected idle task is treated as busy so sched
>> > ticks do not stop during injected idle period, the result of these
>> >     unwanted wakeups can be ~20% loss in power savings.
>> >
>> >  2. Idle accounting: injected idle time is presented to user as
>> > busy.
>> >
>> > This patch addresses the issues by introducing a new PF_IDLE flag
>> > which allows any given task to be treated as idle task while the
>> > flag is set. Therefore, idle injection tasks can run through the
>> > normal flow of NOHZ idle enter/exit to get the correct accounting
>> > as well as tick stop when possible.
>> >
>> > The implication is that idle task is then no longer limited to PID
>> > == 0.
>> >
>> > Acked-by: Ingo Molnar <mingo@...nel.org>
>> > Signed-off-by: Peter Zijlstra <peterz@...radead.org>
>> > Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
>>
>> Have you made any changes to the original Peter's patch, or is this
>> just a resend of that?
> No changes made to Peter's patch. I just rebased to v4.9-rc7 and tested
> it.

OK, thanks!