• Paul E. McKenney's avatar
    rcu: Don't activate RCU core on NO_HZ_FULL CPUs · a096932f
    Paul E. McKenney authored
    Whenever a CPU receives a scheduling-clock interrupt, RCU checks to see
    if the RCU core needs anything from this CPU.  If so, RCU raises
    RCU_SOFTIRQ to carry out any needed processing.
    
    This approach has worked well historically, but it is undesirable on
    NO_HZ_FULL CPUs.  Such CPUs are expected to spend almost all of their time
    in userspace, so that scheduling-clock interrupts can be disabled while
    there is only one runnable task on the CPU in question.  Unfortunately,
    raising any softirq has the potential to wake up ksoftirqd, which would
    provide the second runnable task on that CPU, preventing disabling of
    scheduling-clock interrupts.
    
    What is needed instead is for RCU to leave NO_HZ_FULL CPUs alone,
    relying on the grace-period kthreads' quiescent-state forcing to
    do any needed RCU work on behalf of those CPUs.
    
    This commit therefore refrains from raising RCU_SOFTIRQ on any
    NO_HZ_FULL CPUs during any grace periods that have been in effect
    for less than one second.  The one-second limit handles the case
    where an inappropriate workload is running on a NO_HZ_FULL CPU
    that features lots of scheduling-clock interrupts, but no idle
    or userspace time.
    Reported-by: default avatarMike Galbraith <bitbucket@online.de>
    Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
    Tested-by: default avatarMike Galbraith <bitbucket@online.de>
    Toasted-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
    a096932f
tree.h 24 KB