• Peter Williams's avatar
    sched: simplify move_tasks() · 43010659
    Peter Williams authored
    The move_tasks() function is currently multiplexed with two distinct
    capabilities:
    
    1. attempt to move a specified amount of weighted load from one run
    queue to another; and
    2. attempt to move a specified number of tasks from one run queue to
    another.
    
    The first of these capabilities is used in two places, load_balance()
    and load_balance_idle(), and in both of these cases the return value of
    move_tasks() is used purely to decide if tasks/load were moved and no
    notice of the actual number of tasks moved is taken.
    
    The second capability is used in exactly one place,
    active_load_balance(), to attempt to move exactly one task and, as
    before, the return value is only used as an indicator of success or failure.
    
    This multiplexing of sched_task() was introduced, by me, as part of the
    smpnice patches and was motivated by the fact that the alternative, one
    function to move specified load and one to move a single task, would
    have led to two functions of roughly the same complexity as the old
    move_tasks() (or the new balance_tasks()).  However, the new modular
    design of the new CFS scheduler allows a simpler solution to be adopted
    and this patch addresses that solution by:
    
    1. adding a new function, move_one_task(), to be used by
    active_load_balance(); and
    2. making move_tasks() a single purpose function that tries to move a
    specified weighted load and returns 1 for success and 0 for failure.
    
    One of the consequences of these changes is that neither move_one_task()
    or the new move_tasks() care how many tasks sched_class.load_balance()
    moves and this enables its interface to be simplified by returning the
    amount of load moved as its result and removing the load_moved pointer
    from the argument list.  This helps simplify the new move_tasks() and
    slightly reduces the amount of work done in each of
    sched_class.load_balance()'s implementations.
    
    Further simplification, e.g. changes to balance_tasks(), are possible
    but (slightly) complicated by the special needs of load_balance_fair()
    so I've left them to a later patch (if this one gets accepted).
    
    NB Since move_tasks() gets called with two run queue locks held even
    small reductions in overhead are worthwhile.
    
    [ mingo@elte.hu ]
    
    this change also reduces code size nicely:
    
       text    data     bss     dec     hex filename
       39216    3618      24   42858    a76a sched.o.before
       39173    3618      24   42815    a73f sched.o.after
    Signed-off-by: default avatarPeter Williams <pwil3058@bigpond.net.au>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
    43010659
sched.c 165 KB