• Nick Piggin's avatar
    fs: icache RCU free inodes · fa0d7e3d
    Nick Piggin authored
    RCU free the struct inode. This will allow:
    
    - Subsequent store-free path walking patch. The inode must be consulted for
      permissions when walking, so an RCU inode reference is a must.
    - sb_inode_list_lock to be moved inside i_lock because sb list walkers who want
      to take i_lock no longer need to take sb_inode_list_lock to walk the list in
      the first place. This will simplify and optimize locking.
    - Could remove some nested trylock loops in dcache code
    - Could potentially simplify things a bit in VM land. Do not need to take the
      page lock to follow page->mapping.
    
    The downsides of this is the performance cost of using RCU. In a simple
    creat/unlink microbenchmark, performance drops by about 10% due to inability to
    reuse cache-hot slab objects. As iterations increase and RCU freeing starts
    kicking over, this increases to about 20%.
    
    In cases where inode lifetimes are longer (ie. many inodes may be allocated
    during the average life span of a single inode), a lot of this cache reuse is
    not applicable, so the regression caused by this patch is smaller.
    
    The cache-hot regression could largely be avoided by using SLAB_DESTROY_BY_RCU,
    however this adds some complexity to list walking and store-free path walking,
    so I prefer to implement this at a later date, if it is shown to be a win in
    real situations. I haven't found a regression in any non-micro benchmark so I
    doubt it will be a problem.
    Signed-off-by: default avatarNick Piggin <npiggin@kernel.dk>
    fa0d7e3d
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