1. 23 1月, 2014 18 次提交
  2. 22 1月, 2014 1 次提交
    • H
      reciprocal_divide: update/correction of the algorithm · 809fa972
      Hannes Frederic Sowa 提交于
      Jakub Zawadzki noticed that some divisions by reciprocal_divide()
      were not correct [1][2], which he could also show with BPF code
      after divisions are transformed into reciprocal_value() for runtime
      invariance which can be passed to reciprocal_divide() later on;
      reverse in BPF dump ended up with a different, off-by-one K in
      some situations.
      
      This has been fixed by Eric Dumazet in commit aee636c4
      ("bpf: do not use reciprocal divide"). This follow-up patch
      improves reciprocal_value() and reciprocal_divide() to work in
      all cases by using Granlund and Montgomery method, so that also
      future use is safe and without any non-obvious side-effects.
      Known problems with the old implementation were that division by 1
      always returned 0 and some off-by-ones when the dividend and divisor
      where very large. This seemed to not be problematic with its
      current users, as far as we can tell. Eric Dumazet checked for
      the slab usage, we cannot surely say so in the case of flex_array.
      Still, in order to fix that, we propose an extension from the
      original implementation from commit 6a2d7a95 resp. [3][4],
      by using the algorithm proposed in "Division by Invariant Integers
      Using Multiplication" [5], Torbjörn Granlund and Peter L.
      Montgomery, that is, pseudocode for q = n/d where q, n, d is in
      u32 universe:
      
      1) Initialization:
      
        int l = ceil(log_2 d)
        uword m' = floor((1<<32)*((1<<l)-d)/d)+1
        int sh_1 = min(l,1)
        int sh_2 = max(l-1,0)
      
      2) For q = n/d, all uword:
      
        uword t = (n*m')>>32
        q = (t+((n-t)>>sh_1))>>sh_2
      
      The assembler implementation from Agner Fog [6] also helped a lot
      while implementing. We have tested the implementation on x86_64,
      ppc64, i686, s390x; on x86_64/haswell we're still half the latency
      compared to normal divide.
      
      Joint work with Daniel Borkmann.
      
        [1] http://www.wireshark.org/~darkjames/reciprocal-buggy.c
        [2] http://www.wireshark.org/~darkjames/set-and-dump-filter-k-bug.c
        [3] https://gmplib.org/~tege/division-paper.pdf
        [4] http://homepage.cs.uiowa.edu/~jones/bcd/divide.html
        [5] http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1.2556
        [6] http://www.agner.org/optimize/asmlib.zipReported-by: NJakub Zawadzki <darkjames-ws@darkjames.pl>
      Cc: Eric Dumazet <eric.dumazet@gmail.com>
      Cc: Austin S Hemmelgarn <ahferroin7@gmail.com>
      Cc: linux-kernel@vger.kernel.org
      Cc: Jesse Gross <jesse@nicira.com>
      Cc: Jamal Hadi Salim <jhs@mojatatu.com>
      Cc: Stephen Hemminger <stephen@networkplumber.org>
      Cc: Matt Mackall <mpm@selenic.com>
      Cc: Pekka Enberg <penberg@kernel.org>
      Cc: Christoph Lameter <cl@linux-foundation.org>
      Cc: Andy Gospodarek <andy@greyhouse.net>
      Cc: Veaceslav Falico <vfalico@redhat.com>
      Cc: Jay Vosburgh <fubar@us.ibm.com>
      Cc: Jakub Zawadzki <darkjames-ws@darkjames.pl>
      Signed-off-by: NDaniel Borkmann <dborkman@redhat.com>
      Signed-off-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
      Signed-off-by: NDavid S. Miller <davem@davemloft.net>
      809fa972
  3. 18 1月, 2014 2 次提交
  4. 04 1月, 2014 2 次提交
  5. 02 1月, 2014 2 次提交
  6. 30 12月, 2013 1 次提交
  7. 20 12月, 2013 5 次提交
  8. 18 12月, 2013 5 次提交
  9. 14 12月, 2013 4 次提交