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

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Date:	Tue, 22 Sep 2015 16:34:19 -0400
From:	Luiz Capitulino <lcapitulino@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	riel@...hat.com, rafael.j.wysocki@...el.com, mingo@...nel.org
Subject: [RFC 0/3] sched/idle: run-time support for setting idle polling

Hi,

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 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.

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?

 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(-)

-- 
2.1.0
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