• David S. Miller's avatar
    net: Implement read-only protection and COW'ing of metrics. · 62fa8a84
    David S. Miller authored
    Routing metrics are now copy-on-write.
    
    Initially a route entry points it's metrics at a read-only location.
    If a routing table entry exists, it will point there.  Else it will
    point at the all zero metric place-holder called 'dst_default_metrics'.
    
    The writeability state of the metrics is stored in the low bits of the
    metrics pointer, we have two bits left to spare if we want to store
    more states.
    
    For the initial implementation, COW is implemented simply via kmalloc.
    However future enhancements will change this to place the writable
    metrics somewhere else, in order to increase sharing.  Very likely
    this "somewhere else" will be the inetpeer cache.
    
    Note also that this means that metrics updates may transiently fail
    if we cannot COW the metrics successfully.
    
    But even by itself, this patch should decrease memory usage and
    increase cache locality especially for routing workloads.  In those
    cases the read-only metric copies stay in place and never get written
    to.
    
    TCP workloads where metrics get updated, and those rare cases where
    PMTU triggers occur, will take a very slight performance hit.  But
    that hit will be alleviated when the long-term writable metrics
    move to a more sharable location.
    
    Since the metrics storage went from a u32 array of RTAX_MAX entries to
    what is essentially a pointer, some retooling of the dst_entry layout
    was necessary.
    
    Most importantly, we need to preserve the alignment of the reference
    count so that it doesn't share cache lines with the read-mostly state,
    as per Eric Dumazet's alignment assertion checks.
    
    The only non-trivial bit here is the move of the 'flags' member into
    the writeable cacheline.  This is OK since we are always accessing the
    flags around the same moment when we made a modification to the
    reference count.
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    62fa8a84
dst.c 9.79 KB