• Helge Deller's avatar
    parisc: Add native high-resolution sched_clock() implementation · 54b66800
    Helge Deller authored
    Add a native implementation for the sched_clock() function which utilizes the
    processor-internal cycle counter (Control Register 16) as high-resolution time
    source.
    
    With this patch we now get much more fine-grained resolutions in various
    in-kernel time measurements (e.g. when viewing the function tracing logs), and
    probably a more accurate scheduling on SMP systems.
    
    There are a few specific implementation details in this patch:
    
    1. On a 32bit kernel we emulate the higher 32bits of the required 64-bit
    resolution of sched_clock() by increasing a per-cpu counter at every
    wrap-around of the 32bit cycle counter.
    
    2. In a SMP system, the cycle counters of the various CPUs are not syncronized
    (similiar to the TSC in a x86_64 system). To cope with this we define
    HAVE_UNSTABLE_SCHED_CLOCK and let the upper layers do the adjustment work.
    
    3. Since we need HAVE_UNSTABLE_SCHED_CLOCK, we need to provide a cmpxchg64()
    function even on a 32-bit kernel.
    
    4. A 64-bit SMP kernel which is started on a UP system will mark the
    sched_clock() implementation as "stable", which means that we don't expect any
    jumps in the returned counter. This is true because we then run only on one
    CPU.
    Signed-off-by: default avatarHelge Deller <deller@gmx.de>
    54b66800
time.c 8.92 KB