• Qais Yousef's avatar
    sched/rt: Make RT capacity-aware · 804d402f
    Qais Yousef authored
    Capacity Awareness refers to the fact that on heterogeneous systems
    (like Arm big.LITTLE), the capacity of the CPUs is not uniform, hence
    when placing tasks we need to be aware of this difference of CPU
    capacities.
    
    In such scenarios we want to ensure that the selected CPU has enough
    capacity to meet the requirement of the running task. Enough capacity
    means here that capacity_orig_of(cpu) >= task.requirement.
    
    The definition of task.requirement is dependent on the scheduling class.
    
    For CFS, utilization is used to select a CPU that has >= capacity value
    than the cfs_task.util.
    
    	capacity_orig_of(cpu) >= cfs_task.util
    
    DL isn't capacity aware at the moment but can make use of the bandwidth
    reservation to implement that in a similar manner CFS uses utilization.
    The following patchset implements that:
    
    https://lore.kernel.org/lkml/20190506044836.2914-1-luca.abeni@santannapisa.it/
    
    	capacity_orig_of(cpu)/SCHED_CAPACITY >= dl_deadline/dl_runtime
    
    For RT we don't have a per task utilization signal and we lack any
    information in general about what performance requirement the RT task
    needs. But with the introduction of uclamp, RT tasks can now control
    that by setting uclamp_min to guarantee a minimum performance point.
    
    ATM the uclamp value are only used for frequency selection; but on
    heterogeneous systems this is not enough and we need to ensure that the
    capacity of the CPU is >= uclamp_min. Which is what implemented here.
    
    	capacity_orig_of(cpu) >= rt_task.uclamp_min
    
    Note that by default uclamp.min is 1024, which means that RT tasks will
    always be biased towards the big CPUs, which make for a better more
    predictable behavior for the default case.
    
    Must stress that the bias acts as a hint rather than a definite
    placement strategy. For example, if all big cores are busy executing
    other RT tasks we can't guarantee that a new RT task will be placed
    there.
    
    On non-heterogeneous systems the original behavior of RT should be
    retained. Similarly if uclamp is not selected in the config.
    
    [ mingo: Minor edits to comments. ]
    Signed-off-by: default avatarQais Yousef <qais.yousef@arm.com>
    Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: default avatarDietmar Eggemann <dietmar.eggemann@arm.com>
    Reviewed-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: https://lkml.kernel.org/r/20191009104611.15363-1-qais.yousef@arm.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    804d402f
cpupri.c 7.03 KB