linux-kernel - Re: [RFC 0/3] sched/idle: run-time support for setting idle polling

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Message-ID: <20150930154835.GP3604@twins.programming.kicks-ass.net>
Date:	Wed, 30 Sep 2015 17:48:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Luiz Capitulino <lcapitulino@...hat.com>,
	linux-kernel@...r.kernel.org, riel@...hat.com,
	rafael.j.wysocki@...el.com, mingo@...nel.org,
	Linux PM list <linux-pm@...r.kernel.org>,
	Len Brown <len.brown@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC 0/3] sched/idle: run-time support for setting idle polling

On Wed, Sep 23, 2015 at 03:17:59AM +0200, Rafael J. Wysocki wrote:
> On Tuesday, September 22, 2015 04:34:19 PM Luiz Capitulino wrote:
> > Hi,
> 
> Hi,
> 
> Please always CC patches related to power management to linux-pm@...r.kernel.org.
>
> Also CCing Len Brown who's the maintainer of the intel_idle driver and Peter Z.

And the sched people for touching kernel/sched (thanks Rafael)!

> > Some archs allow the system administrator to set the
> > idle thread behavior to spin instead of entering
> > sleep states. The x86 arch, for example, has a idle=
> > command-line parameter for this purpose.
> > 
> > However, the command-line parameter has two problems:
> > 
> >  1. You have to reboot if you change your mind
> >  2. This setting affects all system cores
> > 
> > The second point is relevant for systems where cores
> > are partitioned into bookkeeping and low-latency cores.
> > Usually, it's OK for bookkeeping cores to enter deeper
> > sleep states. It's only the low-latency cores that should
> > poll when entering idle.
> 
> This looks like a use case for PM QoS to me rather.  You'd need to make it
> work per-CPU rather than globally, but that really is about asking for
> minimum latency.

Agreed, this should very much hook into the PM QoS stuff.

> 
> > This series adds the following file:
> > 
> >  /sys/devices/system/cpu/cpu_idle
> > 
> > This file outputs and stores a cpumask of the cores
> > which will have idle polling behavior.
> 
> I don't like this interface at all.
> 
> You have a cpuidle directory per core already, so what's the reason to add an

Yeah this should very much not be exposed like this.

Ideally every cpuidle driver would get 'polling' as a
default state and the QoS constraints might select it if nothing else
matches.

And no mucking about with the force polling flag at _all_.
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