• Vincent Guittot's avatar
    sched: Fix init NOHZ_IDLE flag · 25f55d9d
    Vincent Guittot authored
    On my SMP platform which is made of 5 cores in 2 clusters, I
    have the nr_busy_cpu field of sched_group_power struct that is
    not null when the platform is fully idle - which makes the
    scheduler unhappy.
    
    The root cause is:
    
    During the boot sequence, some CPUs reach the idle loop and set
    their NOHZ_IDLE flag while waiting for others CPUs to boot. But
    the nr_busy_cpus field is initialized later with the assumption
    that all CPUs are in the busy state whereas some CPUs have
    already set their NOHZ_IDLE flag.
    
    More generally, the NOHZ_IDLE flag must be initialized when new
    sched_domains are created in order to ensure that NOHZ_IDLE and
    nr_busy_cpus are aligned.
    
    This condition can be ensured by adding a synchronize_rcu()
    between the destruction of old sched_domains and the creation of
    new ones so the NOHZ_IDLE flag will not be updated with old
    sched_domain once it has been initialized. But this solution
    introduces a additionnal latency in the rebuild sequence that is
    called during cpu hotplug.
    
    As suggested by Frederic Weisbecker, another solution is to have
    the same rcu lifecycle for both NOHZ_IDLE and sched_domain
    struct. A new nohz_idle field is added to sched_domain so both
    status and sched_domain will share the same RCU lifecycle and
    will be always synchronized. In addition, there is no more need
    to protect nohz_idle against concurrent access as it is only
    modified by 2 exclusive functions called by local cpu.
    
    This solution has been prefered to the creation of a new struct
    with an extra pointer indirection for sched_domain.
    
    The synchronization is done at the cost of :
    
     - An additional indirection and a rcu_dereference for accessing nohz_idle.
     - We use only the nohz_idle field of the top sched_domain.
    Signed-off-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
    Acked-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: linaro-kernel@lists.linaro.org
    Cc: peterz@infradead.org
    Cc: fweisbec@gmail.com
    Cc: pjt@google.com
    Cc: rostedt@goodmis.org
    Cc: efault@gmx.de
    Link: http://lkml.kernel.org/r/1366729142-14662-1-git-send-email-vincent.guittot@linaro.org
    [ Fixed !NO_HZ build bug. ]
    Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
    25f55d9d
fair.c 159 KB