• John Stultz's avatar
    timer: convert timer_slack_ns from unsigned long to u64 · da8b44d5
    John Stultz authored
    This patchset introduces a /proc/<pid>/timerslack_ns interface which
    would allow controlling processes to be able to set the timerslack value
    on other processes in order to save power by avoiding wakeups (Something
    Android currently does via out-of-tree patches).
    
    The first patch tries to fix the internal timer_slack_ns usage which was
    defined as a long, which limits the slack range to ~4 seconds on 32bit
    systems.  It converts it to a u64, which provides the same basically
    unlimited slack (500 years) on both 32bit and 64bit machines.
    
    The second patch introduces the /proc/<pid>/timerslack_ns interface
    which allows the full 64bit slack range for a task to be read or set on
    both 32bit and 64bit machines.
    
    With these two patches, on a 32bit machine, after setting the slack on
    bash to 10 seconds:
    
    $ time sleep 1
    
    real    0m10.747s
    user    0m0.001s
    sys     0m0.005s
    
    The first patch is a little ugly, since I had to chase the slack delta
    arguments through a number of functions converting them to u64s.  Let me
    know if it makes sense to break that up more or not.
    
    Other than that things are fairly straightforward.
    
    This patch (of 2):
    
    The timer_slack_ns value in the task struct is currently a unsigned
    long.  This means that on 32bit applications, the maximum slack is just
    over 4 seconds.  However, on 64bit machines, its much much larger (~500
    years).
    
    This disparity could make application development a little (as well as
    the default_slack) to a u64.  This means both 32bit and 64bit systems
    have the same effective internal slack range.
    
    Now the existing ABI via PR_GET_TIMERSLACK and PR_SET_TIMERSLACK specify
    the interface as a unsigned long, so we preserve that limitation on
    32bit systems, where SET_TIMERSLACK can only set the slack to a unsigned
    long value, and GET_TIMERSLACK will return ULONG_MAX if the slack is
    actually larger then what can be stored by an unsigned long.
    
    This patch also modifies hrtimer functions which specified the slack
    delta as a unsigned long.
    Signed-off-by: default avatarJohn Stultz <john.stultz@linaro.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: Kees Cook <keescook@chromium.org>
    Cc: Android Kernel Team <kernel-team@android.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    da8b44d5
hrtimer.h 14.2 KB