1. 13 Dec, 2021 1 commit
    • Linus Torvalds's avatar
      fget: clarify and improve __fget_files() implementation · e386dfc5
      Linus Torvalds authored
      Commit 054aa8d4 ("fget: check that the fd still exists after getting
      a ref to it") fixed a race with getting a reference to a file just as it
      was being closed.  It was a fairly minimal patch, and I didn't think
      re-checking the file pointer lookup would be a measurable overhead,
      since it was all right there and cached.
      
      But I was wrong, as pointed out by the kernel test robot.
      
      The 'poll2' case of the will-it-scale.per_thread_ops benchmark regressed
      quite noticeably.  Admittedly it seems to be a very artificial test:
      doing "poll()" system calls on regular files in a very tight loop in
      multiple threads.
      
      That means that basically all the time is spent just looking up file
      descriptors without ever doing anything useful with them (not that doing
      'poll()' on a regular file is useful to begin with).  And as a result it
      shows the extra "re-check fd" cost as a sore thumb.
      
      Happily, the regression is fixable by just writing the code to loook up
      the fd to be better and clearer.  There's still a cost to verify the
      file pointer, but now it's basically in the noise even for that
      benchmark that does nothing else - and the code is more understandable
      and has better comments too.
      
      [ Side note: this patch is also a classic case of one that looks very
        messy with the default greedy Myers diff - it's much more legible with
        either the patience of histogram diff algorithm ]
      
      Link: https://lore.kernel.org/lkml/20211210053743.GA36420@xsang-OptiPlex-9020/
      Link: https://lore.kernel.org/lkml/20211213083154.GA20853@linux.intel.com/Reported-by: default avatarkernel test robot <oliver.sang@intel.com>
      Tested-by: default avatarCarel Si <beibei.si@intel.com>
      Cc: Jann Horn <jannh@google.com>
      Cc: Miklos Szeredi <mszeredi@redhat.com>
      Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      e386dfc5
  2. 12 Dec, 2021 14 commits
  3. 11 Dec, 2021 25 commits