• Martin Schwidefsky's avatar
    [PATCH] Directed yield: cpu_relax variants for spinlocks and rw-locks · ef6edc97
    Martin Schwidefsky authored
    On systems running with virtual cpus there is optimization potential in
    regard to spinlocks and rw-locks.  If the virtual cpu that has taken a lock
    is known to a cpu that wants to acquire the same lock it is beneficial to
    yield the timeslice of the virtual cpu in favour of the cpu that has the
    lock (directed yield).
    
    With CONFIG_PREEMPT="n" this can be implemented by the architecture without
    common code changes.  Powerpc already does this.
    
    With CONFIG_PREEMPT="y" the lock loops are coded with _raw_spin_trylock,
    _raw_read_trylock and _raw_write_trylock in kernel/spinlock.c.  If the lock
    could not be taken cpu_relax is called.  A directed yield is not possible
    because cpu_relax doesn't know anything about the lock.  To be able to
    yield the lock in favour of the current lock holder variants of cpu_relax
    for spinlocks and rw-locks are needed.  The new _raw_spin_relax,
    _raw_read_relax and _raw_write_relax primitives differ from cpu_relax
    insofar that they have an argument: a pointer to the lock structure.
    Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
    Cc: Ingo Molnar <mingo@elte.hu>
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: Haavard Skinnemoen <hskinnemoen@atmel.com>
    Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
    ef6edc97
spinlock.h 3.23 KB