• Valentin Schneider's avatar
    sched/topology: Allow sched_asym_cpucapacity to be disabled · e284df70
    Valentin Schneider authored
    While the static key is correctly initialized as being disabled, it will
    remain forever enabled once turned on. This means that if we start with an
    asymmetric system and hotplug out enough CPUs to end up with an SMP system,
    the static key will remain set - which is obviously wrong. We should detect
    this and turn off things like misfit migration and capacity aware wakeups.
    
    As Quentin pointed out, having separate root domains makes this slightly
    trickier. We could have exclusive cpusets that create an SMP island - IOW,
    the domains within this root domain will not see any asymmetry. This means
    we can't just disable the key on domain destruction, we need to count how
    many asymmetric root domains we have.
    
    Consider the following example using Juno r0 which is 2+4 big.LITTLE, where
    two identical cpusets are created: they both span both big and LITTLE CPUs:
    
        asym0    asym1
      [       ][       ]
       L  L  B  L  L  B
    
      $ cgcreate -g cpuset:asym0
      $ cgset -r cpuset.cpus=0,1,3 asym0
      $ cgset -r cpuset.mems=0 asym0
      $ cgset -r cpuset.cpu_exclusive=1 asym0
    
      $ cgcreate -g cpuset:asym1
      $ cgset -r cpuset.cpus=2,4,5 asym1
      $ cgset -r cpuset.mems=0 asym1
      $ cgset -r cpuset.cpu_exclusive=1 asym1
    
      $ cgset -r cpuset.sched_load_balance=0 .
    
    (the CPU numbering may look odd because on the Juno LITTLEs are CPUs 0,3-5
    and bigs are CPUs 1-2)
    
    If we make one of those SMP (IOW remove asymmetry) by e.g. hotplugging its
    big core, we would end up with an SMP cpuset and an asymmetric cpuset - the
    static key must remain set, because we still have one asymmetric root domain.
    
    With the above example, this could be done with:
    
      $ echo 0 > /sys/devices/system/cpu/cpu2/online
    
    Which would result in:
    
        asym0   asym1
      [       ][    ]
       L  L  B  L  L
    
    When both SMP and asymmetric cpusets are present, all CPUs will observe
    sched_asym_cpucapacity being set (it is system-wide), but not all CPUs
    observe asymmetry in their sched domain hierarchy:
    
      per_cpu(sd_asym_cpucapacity, <any CPU in asym0>) == <some SD at DIE level>
      per_cpu(sd_asym_cpucapacity, <any CPU in asym1>) == NULL
    
    Change the simple key enablement to an increment, and decrement the key
    counter when destroying domains that cover asymmetric CPUs.
    Signed-off-by: default avatarValentin Schneider <valentin.schneider@arm.com>
    Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: default avatarDietmar Eggemann <dietmar.eggemann@arm.com>
    Cc: Dietmar.Eggemann@arm.com
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: hannes@cmpxchg.org
    Cc: lizefan@huawei.com
    Cc: morten.rasmussen@arm.com
    Cc: qperret@google.com
    Cc: tj@kernel.org
    Cc: vincent.guittot@linaro.org
    Fixes: df054e84 ("sched/topology: Add static_key for asymmetric CPU capacity optimizations")
    Link: https://lkml.kernel.org/r/20191023153745.19515-3-valentin.schneider@arm.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    e284df70
topology.c 56.9 KB