• Tejun Heo's avatar
    sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem · 1ed13287
    Tejun Heo authored
    Note: This commit was originally committed as d59cfc09 but got
          reverted by 0c986253 due to the performance regression from
          the percpu_rwsem write down/up operations added to cgroup task
          migration path.  percpu_rwsem changes which alleviate the
          performance issue are pending for v4.4-rc1 merge window.
          Re-apply.
    
    The cgroup side of threadgroup locking uses signal_struct->group_rwsem
    to synchronize against threadgroup changes.  This per-process rwsem
    adds small overhead to thread creation, exit and exec paths, forces
    cgroup code paths to do lock-verify-unlock-retry dance in a couple
    places and makes it impossible to atomically perform operations across
    multiple processes.
    
    This patch replaces signal_struct->group_rwsem with a global
    percpu_rwsem cgroup_threadgroup_rwsem which is cheaper on the reader
    side and contained in cgroups proper.  This patch converts one-to-one.
    
    This does make writer side heavier and lower the granularity; however,
    cgroup process migration is a fairly cold path, we do want to optimize
    thread operations over it and cgroup migration operations don't take
    enough time for the lower granularity to matter.
    Signed-off-by: default avatarTejun Heo <tj@kernel.org>
    Link: http://lkml.kernel.org/g/55F8097A.7000206@de.ibm.com
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    1ed13287
fork.c 50.8 KB