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  • Vivek Goyal's avatar
    blk-cgroup: Allow creation of hierarchical cgroups · bdc85df7
    Vivek Goyal authored
    
    
    o Allow hierarchical cgroup creation for blkio controller
    
    o Currently we disallow it as both the io controller policies (throttling
      as well as proportion bandwidth) do not support hierarhical accounting
      and control. But the flip side is that blkio controller can not be used with
      libvirt as libvirt creates a cgroup hierarchy deeper than 1 level.
    
      <top-level-cgroup-dir>/<controller>/libvirt/qemu/<virtual-machine-groups>
    
    o So this patch will allow creation of cgroup hierarhcy but at the backend
      everything will be treated as flat. So if somebody created a an hierarchy
      like as follows.
    
    			root
    			/  \
    		     test1 test2
    			|
    		     test3
    
      CFQ and throttling will practically treat all groups at same level.
    
    				pivot
    			     /  |   \  \
    			root  test1 test2  test3
    
    o Once we have actual support for hierarchical accounting and control
      then we can introduce another cgroup tunable file "blkio.use_hierarchy"
      which will be 0 by default but if user wants to enforce hierarhical
      control then it can be set to 1. This way there should not be any
      ABI problems down the line.
    
    o The only not so pretty part is introduction of extra file "use_hierarchy"
      down the line. Kame-san had mentioned that hierarhical accounting is
      expensive in memory controller hence they keep it off by default. I
      suspect same will be the case for IO controller also as for each IO
      completion we shall have to account IO through hierarchy up to the root.
      if yes, then it probably is not a very bad idea to introduce this extra
      file so that it will be used only when somebody needs it and some people
      might enable hierarchy only in part of the hierarchy.
    
    o This is how basically memory controller also uses "use_hierarhcy" and
      they also allowed creation of hierarchies when actual backend support
      was not available.
    
    Signed-off-by: default avatarVivek Goyal <vgoyal@redhat.com>
    Acked-by: default avatarBalbir Singh <balbir@linux.vnet.ibm.com>
    Reviewed-by: default avatarGui Jianfeng <guijianfeng@cn.fujitsu.com>
    Reviewed-by: default avatarCiju Rajan K <ciju@linux.vnet.ibm.com>
    Tested-by: default avatarCiju Rajan K <ciju@linux.vnet.ibm.com>
    Signed-off-by: default avatarJens Axboe <jaxboe@fusionio.com>
    bdc85df7