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

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Date:	Wed, 23 Sep 2015 03:17:59 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Luiz Capitulino <lcapitulino@...hat.com>
Cc:	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>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC 0/3] sched/idle: run-time support for setting idle polling

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.

> 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.

> 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
extra mask file really? 

> This implementation seems to work fine on x86, however
> it's RFC because of the following points (for which
> feedback is greatly appreciated):
> 
>  o I believe this implementation should work for all archs,
>    but I can't confirm it as my machines and experience is
>    limited to x86
> 
>  o Some x86 cpufreq drivers explicitly check if idle=poll
>    was passed. Does anyone know if this is an optmization
>    or is there actually a conflict between idle=poll and
>    driver operation?

idle=poll is used as a workaround for platform defects on some systems IIRC.

>  o This series maintains cpu_idle_poll_ctrl() semantics
>    which led to a more complex implementation. That is, today
>    cpu_idle_poll_ctrl() increments or decrements a counter.
>    A lot of arch code seems to count on this semantic, where
>    cpu_idle_poll_ctrl(enable or false) calls have to match to
>    enable or disable idle polling
> 
> Luiz Capitulino (3):
>   sched/idle: cpu_idle_poll(): drop unused return code
>   sched/idle: make cpu_idle_force_poll per-cpu
>   sched/idle: run-time support for setting idle polling
> 
>  drivers/base/cpu.c  | 44 ++++++++++++++++++++++++
>  include/linux/cpu.h |  2 ++
>  kernel/sched/idle.c | 96 +++++++++++++++++++++++++++++++++++++++++++++--------
>  3 files changed, 129 insertions(+), 13 deletions(-)

Thanks,
Rafael

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