sync: don't block the flusher thread waiting on IO

    When sync does it's WB_SYNC_ALL writeback, it issues data Io and
    then immediately waits for IO completion. This is done in the
    context of the flusher thread, and hence completely ties up the
    flusher thread for the backing device until all the dirty inodes
    have been synced. On filesystems that are dirtying inodes constantly
    and quickly, this means the flusher thread can be tied up for
    minutes per sync call and hence badly affect system level write IO
    performance as the page cache cannot be cleaned quickly.
    We already have a wait loop for IO completion for sync(2), so cut
    this out of the flusher thread and delegate it to wait_sb_inodes().
    Hence we can do rapid IO submission, and then wait for it all to
    complete.
    Effect of sync on fsmark before the patch:
    FSUse%        Count         Size    Files/sec     App Overhead
    .....
         0       640000         4096      35154.6          1026984
         0       720000         4096      36740.3          1023844
         0       800000         4096      36184.6           916599
         0       880000         4096       1282.7          1054367
         0       960000         4096       3951.3           918773
         0      1040000         4096      40646.2           996448
         0      1120000         4096      43610.1           895647
         0      1200000         4096      40333.1           921048
    And a single sync pass took:
      real    0m52.407s
      user    0m0.000s
      sys     0m0.090s
    After the patch, there is no impact on fsmark results, and each
    individual sync(2) operation run concurrently with the same fsmark
    workload takes roughly 7s:
      real    0m6.930s
      user    0m0.000s
      sys     0m0.039s
    IOWs, sync is 7-8x faster on a busy filesystem and does not have an
    adverse impact on ongoing async data write operations.
    Signed-off-by: Dave Chinner <dchinner@redhat.com>
    Reviewed-by: Jan Kara <jack@suse.cz>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>