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  1. 19 Sep, 2014 2 commits
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    • Linus Torvalds's avatar
      Merge branches 'locking-urgent-for-linus' and 'timers-urgent-for-linus' of... · 1536340e
      Linus Torvalds authored 
      Merge branches 'locking-urgent-for-linus' and 'timers-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull futex and timer fixes from Thomas Gleixner:
 "A oneliner bugfix for the jinxed futex code:

   - Drop hash bucket lock in the error exit path.  I really could slap
     myself for intruducing that bug while fixing all the other horror
     in that code three month ago ...

  and the timer department is not too proud about the following fixes:

   - Deal with a long standing rounding bug in the timeval to jiffies
     conversion.  It's a real issue and this fix fell through the cracks
     for quite some time.

   - Another round of alarmtimer fixes.  Finally this code gets used
     more widely and the subtle issues hidden for quite some time are
     noticed and fixed.  Nothing really exciting, just the itty bitty
     details which bite the serious users here and there"

* 'locking-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  futex: Unlock hb->lock in futex_wait_requeue_pi() error path

* 'timers-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  alarmtimer: Lock k_itimer during timer callback
  alarmtimer: Do not signal SIGEV_NONE timers
  alarmtimer: Return relative times in timer_gettime
  jiffies: Fix timeval conversion to jiffies
      1536340e
    • Guy Martin's avatar
      parisc: Implement new LWS CAS supporting 64 bit operations. · 89206491
      Guy Martin authored 
      The current LWS cas only works correctly for 32bit. The new LWS allows
for CAS operations of variable size.
Signed-off-by: default avatarGuy Martin <gmsoft@tuxicoman.be>
Cc: <stable@vger.kernel.org> # 3.13+
Signed-off-by: default avatarHelge Deller <deller@gmx.de>
      89206491
    • Linus Torvalds's avatar
      vfs: fix bad hashing of dentries · 99d263d4
      Linus Torvalds authored 
      Josef Bacik found a performance regression between 3.2 and 3.10 and
narrowed it down to commit bfcfaa77 ("vfs: use 'unsigned long'
accesses for dcache name comparison and hashing"). He reports:

 "The test case is essentially

      for (i = 0; i < 1000000; i++)
              mkdir("a$i");

  On xfs on a fio card this goes at about 20k dir/sec with 3.2, and 12k
  dir/sec with 3.10.  This is because we spend waaaaay more time in
  __d_lookup on 3.10 than in 3.2.

  The new hashing function for strings is suboptimal for <
  sizeof(unsigned long) string names (and hell even > sizeof(unsigned
  long) string names that I've tested).  I broke out the old hashing
  function and the new one into a userspace helper to get real numbers
  and this is what I'm getting:

      Old hash table had 1000000 entries, 0 dupes, 0 max dupes
      New hash table had 12628 entries, 987372 dupes, 900 max dupes
      We had 11400 buckets with a p50 of 30 dupes, p90 of 240 dupes, p99 of 567 dupes for the new hash

  My test does the hash, and then does the d_hash into a integer pointer
  array the same size as the dentry hash table on my system, and then
  just increments the value at the address we got to see how many
  entries we overlap with.

  As you can see the old hash function ended up with all 1 million
  entries in their own bucket, whereas the new one they are only
  distributed among ~12.5k buckets, which is why we're using so much
  more CPU in __d_lookup".

The reason for this hash regression is two-fold:

 - On 64-bit architectures the down-mixing of the original 64-bit
   word-at-a-time hash into the final 32-bit hash value is very
   simplistic and suboptimal, and just adds the two 32-bit parts
   together.

   In particular, because there is no bit shuffling and the mixing
   boundary is also a byte boundary, similar character patterns in the
   low and high word easily end up just canceling each other out.

 - the old byte-at-a-time hash mixed each byte into the final hash as it
   hashed the path component name, resulting in the low bits of the hash
   generally being a good source of hash data.  That is not true for the
   word-at-a-time case, and the hash data is distributed among all the
   bits.

The fix is the same in both cases: do a better job of mixing the bits up
and using as much of the hash data as possible.  We already have the
"hash_32|64()" functions to do that.
Reported-by: default avatarJosef Bacik <jbacik@fb.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Chris Mason <clm@fb.com>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
      99d263d4
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