• Dario Faggioli's avatar
    sched/deadline: Add bandwidth management for SCHED_DEADLINE tasks · 332ac17e
    Dario Faggioli authored
    
    
    In order of deadline scheduling to be effective and useful, it is
    important that some method of having the allocation of the available
    CPU bandwidth to tasks and task groups under control.
    This is usually called "admission control" and if it is not performed
    at all, no guarantee can be given on the actual scheduling of the
    -deadline tasks.
    
    Since when RT-throttling has been introduced each task group have a
    bandwidth associated to itself, calculated as a certain amount of
    runtime over a period. Moreover, to make it possible to manipulate
    such bandwidth, readable/writable controls have been added to both
    procfs (for system wide settings) and cgroupfs (for per-group
    settings).
    
    Therefore, the same interface is being used for controlling the
    bandwidth distrubution to -deadline tasks and task groups, i.e.,
    new controls but with similar names, equivalent meaning and with
    the same usage paradigm are added.
    
    However, more discussion is needed in order to figure out how
    we want to manage SCHED_DEADLINE bandwidth at the task group level.
    Therefore, this patch adds a less sophisticated, but actually
    very sensible, mechanism to ensure that a certain utilization
    cap is not overcome per each root_domain (the single rq for !SMP
    configurations).
    
    Another main difference between deadline bandwidth management and
    RT-throttling is that -deadline tasks have bandwidth on their own
    (while -rt ones doesn't!), and thus we don't need an higher level
    throttling mechanism to enforce the desired bandwidth.
    
    This patch, therefore:
    
     - adds system wide deadline bandwidth management by means of:
        * /proc/sys/kernel/sched_dl_runtime_us,
        * /proc/sys/kernel/sched_dl_period_us,
       that determine (i.e., runtime / period) the total bandwidth
       available on each CPU of each root_domain for -deadline tasks;
    
     - couples the RT and deadline bandwidth management, i.e., enforces
       that the sum of how much bandwidth is being devoted to -rt
       -deadline tasks to stay below 100%.
    
    This means that, for a root_domain comprising M CPUs, -deadline tasks
    can be created until the sum of their bandwidths stay below:
    
        M * (sched_dl_runtime_us / sched_dl_period_us)
    
    It is also possible to disable this bandwidth management logic, and
    be thus free of oversubscribing the system up to any arbitrary level.
    Signed-off-by: default avatarDario Faggioli <raistlin@linux.it>
    Signed-off-by: default avatarJuri Lelli <juri.lelli@gmail.com>
    Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
    Link: http://lkml.kernel.org/r/1383831828-15501-12-git-send-email-juri.lelli@gmail.com
    
    Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
    332ac17e
sched.h 82.5 KB