From: Ravikiran G Thirumalai <kiran@in.ibm.com>

The routine disk_round_stats showed up considerably under oprofile for high
disk io load (four processes doing dd to the same disk (different
partitions) on a 4 way).

This is because the counter in_flight which is per-cpu right now gets read
every time disk_round_stats gets called.  Per cpu counters like disk
statistics improve write speed, but reads are slow (since all cpus' local
counter values have to be read and summed up).  Considering the fact that
in_flight counter is modified post disk_round_stats (which reads the
in_flight counter) it is better not to per-cpu this counter.

Following patch does just that.  Below is the profile comparison before and
after the change.  This was on a 4 way PIII Xeon, 1G ram, 2.6.0-test4-mm2. 

Before:
c010aa60 2910109  92.2249     poll_idle
c0275340 23208    0.73549     __copy_to_user_ll
c02753b0 11191    0.354657    __copy_from_user_ll
c0114aa0 7168     0.227163    mark_offset_tsc
c011ad10 6767     0.214455    schedule
c011a2b0 6741     0.213631    load_balance
c0138890 6710     0.212648    __generic_file_aio_write_nolock
c011d302 4683     0.14841     .text.lock.sched
c02e4b50 4533     0.143656    ahc_linux_isr
c029cec0 3582     0.113518    disk_round_stats
c0119b40 3509     0.111205    try_to_wake_up
c029d320 3306     0.104771    __make_request
c01567d0 3300     0.104581    __block_write_full_page
c0156c00 3299     0.104549    __block_prepare_write

After:
c010aa60 2777940  92.1302     poll_idle
c0275340 23479    0.778679    __copy_to_user_ll
c02753b0 10943    0.362924    __copy_from_user_ll
c0114aa0 7022     0.232884    mark_offset_tsc
c0138890 6988     0.231757    __generic_file_aio_write_nolock
c011ad10 6607     0.219121    schedule
c011d302 5771     0.191395    .text.lock.sched
c02e4a60 4458     0.147849    ahc_linux_isr
c011a2b0 3921     0.13004     load_balance
c01567d0 3569     0.118366    __block_write_full_page
c029d2a0 3540     0.117404    __make_request
...
c029ceb0 311      0.0103143   disk_round_stats
c011d5b0 299      0.00991631  remove_wait_queue