• Nick Piggin's avatar
    fs: scale mntget/mntput · b3e19d92
    Nick Piggin authored
    The problem that this patch aims to fix is vfsmount refcounting scalability.
    We need to take a reference on the vfsmount for every successful path lookup,
    which often go to the same mount point.
    The fundamental difficulty is that a "simple" reference count can never be made
    scalable, because any time a reference is dropped, we must check whether that
    was the last reference. To do that requires communication with all other CPUs
    that may have taken a reference count.
    We can make refcounts more scalable in a couple of ways, involving keeping
    distributed counters, and checking for the global-zero condition less
    - check the global sum once every interval (this will delay zero detection
      for some interval, so it's probably a showstopper for vfsmounts).
    - keep a local count and only taking the global sum when local reaches 0 (this
      is difficult for vfsmounts, because we can't hold preempt off for the life of
      a reference, so a counter would need to be per-thread or tied strongly to a
      particular CPU which requires more locking).
    - keep a local difference of increments and decrements, which allows us to sum
      the total difference and hence find the refcount when summing all CPUs. Then,
      keep a single integer "long" refcount for slow and long lasting references,
      and only take the global sum of local counters when the long refcount is 0.
    This last scheme is what I implemented here. Attached mounts and process root
    and working directory references are "long" references, and everything else is
    a short reference.
    This allows scalable vfsmount references during path walking over mounted
    subtrees and unattached (lazy umounted) mounts with processes still running
    in them.
    This results in one fewer atomic op in the fastpath: mntget is now just a
    per-CPU inc, rather than an atomic inc; and mntput just requires a spinlock
    and non-atomic decrement in the common case. However code is otherwise bigger
    and heavier, so single threaded performance is basically a wash.
    Signed-off-by: default avatarNick Piggin <npiggin@kernel.dk>
perfmon.c 169 KB