proc: add /proc/<pid>/timerslack_ns interface

This patch provides a proc/PID/timerslack_ns interface which exposes a
task's timerslack value in nanoseconds and allows it to be changed.

This allows power/performance management software to set timer slack for
other threads according to its policy for the thread (such as when the
thread is designated foreground vs.  background activity)

If the value written is non-zero, slack is set to that value.  Otherwise
sets it to the default for the thread.

This interface checks that the calling task has permissions to to use
PTRACE_MODE_ATTACH_FSCREDS on the target task, so that we can ensure
arbitrary apps do not change the timer slack for other apps.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Oren Laadan <orenl@cellrox.com>
Cc: Ruchi Kandoi <kandoiruchi@google.com>
Cc: Rom Lemarchand <romlem@android.com>
Cc: Android Kernel Team <kernel-team@android.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/proc.txt

@@ -43,6 +43,7 @@ Table of Contents
   3.7   /proc/<pid>/task/<tid>/children - Information about task children
   3.8   /proc/<pid>/fdinfo/<fd> - Information about opened file
   3.9   /proc/<pid>/map_files - Information about memory mapped files
+  3.10  /proc/<pid>/timerslack_ns - Task timerslack value
 
   4	Configuring procfs
   4.1	Mount options
@@ -1862,6 +1863,23 @@ time one can open(2) mappings from the listings of two processes and
 comparing their inode numbers to figure out which anonymous memory areas
 are actually shared.
 
+3.10	/proc/<pid>/timerslack_ns - Task timerslack value
+---------------------------------------------------------
+This file provides the value of the task's timerslack value in nanoseconds.
+This value specifies a amount of time that normal timers may be deferred
+in order to coalesce timers and avoid unnecessary wakeups.
+
+This allows a task's interactivity vs power consumption trade off to be
+adjusted.
+
+Writing 0 to the file will set the tasks timerslack to the default value.
+
+Valid values are from 0 - ULLONG_MAX
+
+An application setting the value must have PTRACE_MODE_ATTACH_FSCREDS level
+permissions on the task specified to change its timerslack_ns value.
+
+
 ------------------------------------------------------------------------------
 Configuring procfs
 ------------------------------------------------------------------------------