Doug / smarc-fsl-linux-kernel

15 Jan, 2009

1 commit

  • 0d66bf6d3   mutex: implement adaptive spinning ... Browse Code ยป

    Change mutex contention behaviour such that it will sometimes busy wait on
    acquisition - moving its behaviour closer to that of spinlocks.

    This concept got ported to mainline from the -rt tree, where it was originally
    implemented for rtmutexes by Steven Rostedt, based on work by Gregory Haskins.

    Testing with Ingo's test-mutex application (http://lkml.org/lkml/2006/1/8/50)
    gave a 345% boost for VFS scalability on my testbox:

    # ./test-mutex-shm V 16 10 | grep "^avg ops"
    avg ops/sec: 296604

    # ./test-mutex-shm V 16 10 | grep "^avg ops"
    avg ops/sec: 85870

    The key criteria for the busy wait is that the lock owner has to be running on
    a (different) cpu. The idea is that as long as the owner is running, there is a
    fair chance it'll release the lock soon, and thus we'll be better off spinning
    instead of blocking/scheduling.

    Since regular mutexes (as opposed to rtmutexes) do not atomically track the
    owner, we add the owner in a non-atomic fashion and deal with the races in
    the slowpath.

    Furthermore, to ease the testing of the performance impact of this new code,
    there is means to disable this behaviour runtime (without having to reboot
    the system), when scheduler debugging is enabled (CONFIG_SCHED_DEBUG=y),
    by issuing the following command:

    # echo NO_OWNER_SPIN > /debug/sched_features

    This command re-enables spinning again (this is also the default):

    # echo OWNER_SPIN > /debug/sched_features

    Signed-off-by: Peter Zijlstra
    Signed-off-by: Ingo Molnar

    Peter Zijlstra