mutex: Move mutex spinning code from sched/core.c back to mutex.c

As mentioned by Ingo, the SCHED_FEAT_OWNER_SPIN scheduler
feature bit was really just an early hack to make with/without
mutex-spinning testable. So it is no longer necessary.

This patch removes the SCHED_FEAT_OWNER_SPIN feature bit and
move the mutex spinning code from kernel/sched/core.c back to
kernel/mutex.c which is where they should belong.

Signed-off-by: Waiman Long <Waiman.Long@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Chandramouleeswaran Aswin <aswin@hp.com>
Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>
Cc: Norton Scott J <scott.norton@hp.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
mutex: Make more scalable by doing less atomic operations

In the __mutex_lock_common() function, an initial entry into
the lock slow path will cause two atomic_xchg instructions to be
issued. Together with the atomic decrement in the fast path, a
total of three atomic read-modify-write instructions will be
issued in rapid succession. This can cause a lot of cache
bouncing when many tasks are trying to acquire the mutex at the
same time.

This patch will reduce the number of atomic_xchg instructions
used by checking the counter value first before issuing the
instruction. The atomic_read() function is just a simple memory
read. The atomic_xchg() function, on the other hand, can be up
to 2 order of magnitude or even more in cost when compared with
atomic_read(). By using atomic_read() to check the value first
before calling atomic_xchg(), we can avoid a lot of unnecessary
cache coherency traffic. The only downside with this change is
that a task on the slow path will have a tiny bit less chance of
getting the mutex when competing with another task in the fast
path.

The same is true for the atomic_cmpxchg() function in the
mutex-spin-on-owner loop. So an atomic_read() is also performed
before calling atomic_cmpxchg().

The mutex locking and unlocking code for the x86 architecture
can allow any negative number to be used in the mutex count to
indicate that some tasks are waiting for the mutex. I am not so
sure if that is the case for the other architectures. So the
default is to avoid atomic_xchg() if the count has already been
set to -1. For x86, the check is modified to include all
negative numbers to cover a larger case.

The following table shows the jobs per minutes (JPM) scalability
data on an 8-node 80-core Westmere box with a 3.7.10 kernel. The
numactl command is used to restrict the running of the
high_systime workloads to 1/2/4/8 nodes with hyperthreading on
and off.

+-----------------+-----------+------------+----------+
|  Configuration  | Mean JPM  |  Mean JPM  | % Change |
|		  | w/o patch | with patch |	      |
+-----------------+-----------------------------------+
|		  |      User Range 1100 - 2000	      |
+-----------------+-----------------------------------+
| 8 nodes, HT on  |    36980   |   148590  | +301.8%  |
| 8 nodes, HT off |    42799   |   145011  | +238.8%  |
| 4 nodes, HT on  |    61318   |   118445  |  +51.1%  |
| 4 nodes, HT off |   158481   |   158592  |   +0.1%  |
| 2 nodes, HT on  |   180602   |   173967  |   -3.7%  |
| 2 nodes, HT off |   198409   |   198073  |   -0.2%  |
| 1 node , HT on  |   149042   |   147671  |   -0.9%  |
| 1 node , HT off |   126036   |   126533  |   +0.4%  |
+-----------------+-----------------------------------+
|		  |       User Range 200 - 1000	      |
+-----------------+-----------------------------------+
| 8 nodes, HT on  |   41525    |   122349  | +194.6%  |
| 8 nodes, HT off |   49866    |   124032  | +148.7%  |
| 4 nodes, HT on  |   66409    |   106984  |  +61.1%  |
| 4 nodes, HT off |  119880    |   130508  |   +8.9%  |
| 2 nodes, HT on  |  138003    |   133948  |   -2.9%  |
| 2 nodes, HT off |  132792    |   131997  |   -0.6%  |
| 1 node , HT on  |  116593    |   115859  |   -0.6%  |
| 1 node , HT off |  104499    |   104597  |   +0.1%  |
+-----------------+------------+-----------+----------+

At low user range 10-100, the JPM differences were within +/-1%.
So they are not that interesting.

AIM7 benchmark run has a pretty large run-to-run variance due to
random nature of the subtests executed. So a difference of less
than +-5% may not be really significant.

This patch improves high_systime workload performance at 4 nodes
and up by maintaining transaction rates without significant
drop-off at high node count.  The patch has practically no
impact on 1 and 2 nodes system.

The table below shows the percentage time (as reported by perf
record -a -s -g) spent on the __mutex_lock_slowpath() function
by the high_systime workload at 1500 users for 2/4/8-node
configurations with hyperthreading off.

+---------------+-----------------+------------------+---------+
| Configuration | %Time w/o patch | %Time with patch | %Change |
+---------------+-----------------+------------------+---------+
|    8 nodes    |      65.34%     |      0.69%       |  -99%   |
|    4 nodes    |       8.70%	  |      1.02%	     |  -88%   |
|    2 nodes    |       0.41%     |      0.32%       |  -22%   |
+---------------+-----------------+------------------+---------+

It is obvious that the dramatic performance improvement at 8
nodes was due to the drastic cut in the time spent within the
__mutex_lock_slowpath() function.

The table below show the improvements in other AIM7 workloads
(at 8 nodes, hyperthreading off).

+--------------+---------------+----------------+-----------------+
|   Workload   | mean % change | mean % change  | mean % change   |
|              | 10-100 users  | 200-1000 users | 1100-2000 users |
+--------------+---------------+----------------+-----------------+
| alltests     |     +0.6%     |   +104.2%      |   +185.9%       |
| five_sec     |     +1.9%     |     +0.9%      |     +0.9%       |
| fserver      |     +1.4%     |     -7.7%      |     +5.1%       |
| new_fserver  |     -0.5%     |     +3.2%      |     +3.1%       |
| shared       |    +13.1%     |   +146.1%      |   +181.5%       |
| short        |     +7.4%     |     +5.0%      |     +4.2%       |
+--------------+---------------+----------------+-----------------+

Signed-off-by: Waiman Long <Waiman.Long@hp.com>
Reviewed-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Chandramouleeswaran Aswin <aswin@hp.com>
Cc: Norton: Scott J <scott.norton@hp.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1366226594-5506-3-git-send-email-Waiman.Long@hp.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
mutex: Queue mutex spinners with MCS lock to reduce cacheline contention

The current mutex spinning code (with MUTEX_SPIN_ON_OWNER option
turned on) allow multiple tasks to spin on a single mutex
concurrently. A potential problem with the current approach is
that when the mutex becomes available, all the spinning tasks
will try to acquire the mutex more or less simultaneously. As a
result, there will be a lot of cacheline bouncing especially on
systems with a large number of CPUs.

This patch tries to reduce this kind of contention by putting
the mutex spinners into a queue so that only the first one in
the queue will try to acquire the mutex. This will reduce
contention and allow all the tasks to move forward faster.

The queuing of mutex spinners is done using an MCS lock based
implementation which will further reduce contention on the mutex
cacheline than a similar ticket spinlock based implementation.
This patch will add a new field into the mutex data structure
for holding the MCS lock. This expands the mutex size by 8 bytes
for 64-bit system and 4 bytes for 32-bit system. This overhead
will be avoid if the MUTEX_SPIN_ON_OWNER option is turned off.

The following table shows the jobs per minute (JPM) scalability
data on an 8-node 80-core Westmere box with a 3.7.10 kernel. The
numactl command is used to restrict the running of the fserver
workloads to 1/2/4/8 nodes with hyperthreading off.

+-----------------+-----------+-----------+-------------+----------+
|  Configuration  | Mean JPM  | Mean JPM  |  Mean JPM   | % Change |
|                 | w/o patch | patch 1   | patches 1&2 |  1->1&2  |
+-----------------+------------------------------------------------+
|                 |              User Range 1100 - 2000            |
+-----------------+------------------------------------------------+
| 8 nodes, HT off |  227972   |  227237   |   305043    |  +34.2%  |
| 4 nodes, HT off |  393503   |  381558   |   394650    |   +3.4%  |
| 2 nodes, HT off |  334957   |  325240   |   338853    |   +4.2%  |
| 1 node , HT off |  198141   |  197972   |   198075    |   +0.1%  |
+-----------------+------------------------------------------------+
|                 |              User Range 200 - 1000             |
+-----------------+------------------------------------------------+
| 8 nodes, HT off |  282325   |  312870   |   332185    |   +6.2%  |
| 4 nodes, HT off |  390698   |  378279   |   393419    |   +4.0%  |
| 2 nodes, HT off |  336986   |  326543   |   340260    |   +4.2%  |
| 1 node , HT off |  197588   |  197622   |   197582    |    0.0%  |
+-----------------+-----------+-----------+-------------+----------+

At low user range 10-100, the JPM differences were within +/-1%.
So they are not that interesting.

The fserver workload uses mutex spinning extensively. With just
the mutex change in the first patch, there is no noticeable
change in performance.  Rather, there is a slight drop in
performance. This mutex spinning patch more than recovers the
lost performance and show a significant increase of +30% at high
user load with the full 8 nodes. Similar improvements were also
seen in a 3.8 kernel.

The table below shows the %time spent by different kernel
functions as reported by perf when running the fserver workload
at 1500 users with all 8 nodes.

+-----------------------+-----------+---------+-------------+
|        Function       |  % time   | % time  |   % time    |
|                       | w/o patch | patch 1 | patches 1&2 |
+-----------------------+-----------+---------+-------------+
| __read_lock_failed    |  34.96%   | 34.91%  |   29.14%    |
| __write_lock_failed   |  10.14%   | 10.68%  |    7.51%    |
| mutex_spin_on_owner   |   3.62%   |  3.42%  |    2.33%    |
| mspin_lock            |    N/A    |   N/A   |    9.90%    |
| __mutex_lock_slowpath |   1.46%   |  0.81%  |    0.14%    |
| _raw_spin_lock        |   2.25%   |  2.50%  |    1.10%    |
+-----------------------+-----------+---------+-------------+

The fserver workload for an 8-node system is dominated by the
contention in the read/write lock. Mutex contention also plays a
role. With the first patch only, mutex contention is down (as
shown by the __mutex_lock_slowpath figure) which help a little
bit. We saw only a few percents improvement with that.

By applying patch 2 as well, the single mutex_spin_on_owner
figure is now split out into an additional mspin_lock figure.
The time increases from 3.42% to 11.23%. It shows a great
reduction in contention among the spinners leading to a 30%
improvement. The time ratio 9.9/2.33=4.3 indicates that there
are on average 4+ spinners waiting in the spin_lock loop for
each spinner in the mutex_spin_on_owner loop. Contention in
other locking functions also go down by quite a lot.

The table below shows the performance change of both patches 1 &
2 over patch 1 alone in other AIM7 workloads (at 8 nodes,
hyperthreading off).

+--------------+---------------+----------------+-----------------+
|   Workload   | mean % change | mean % change  | mean % change   |
|              | 10-100 users  | 200-1000 users | 1100-2000 users |
+--------------+---------------+----------------+-----------------+
| alltests     |      0.0%     |     -0.8%      |     +0.6%       |
| five_sec     |     -0.3%     |     +0.8%      |     +0.8%       |
| high_systime |     +0.4%     |     +2.4%      |     +2.1%       |
| new_fserver  |     +0.1%     |    +14.1%      |    +34.2%       |
| shared       |     -0.5%     |     -0.3%      |     -0.4%       |
| short        |     -1.7%     |     -9.8%      |     -8.3%       |
+--------------+---------------+----------------+-----------------+

The short workload is the only one that shows a decline in
performance probably due to the spinner locking and queuing
overhead.

[myfluxi: On our 3.4 base, this patch series gives another
reproducable 3.5% advantage which is significantly above the +- 1-2% standard variation.

 Performance counter stats for 'hackbench 10  ' (10 runs):

       2885.888073 task-clock                #    3.565 CPUs utilized
 ( +-  0.75% )
             16429 context-switches          #    0.006 M/sec
 ( +-  7.47% )
              4234 CPU-migrations            #    0.001 M/sec
 ( +- 11.08% )
             21541 page-faults               #    0.007 M/sec
 ( +-  0.02% )
        5959743473 cycles                    #    2.065 GHz
 ( +-  0.70% ) [85.81%]
                 0 stalled-cycles-frontend   #    0.00% frontend cycles idle
 ( +-  0.00% ) [86.55%]
                 0 stalled-cycles-backend    #    0.00% backend  cycles idle
 ( +-  0.00% ) [88.53%]
        2018815336 instructions              #    0.34  insns per cycle
 ( +-  0.57% ) [88.31%]
         195542833 branches                  #   67.968 M/sec
 ( +-  0.80% ) [89.30%]
           4178524 branch-misses             #    2.14% of all branches
 ( +-  2.39% ) [76.06%]

       0.807441968 seconds time elapsed
 ( +-  1.86% )]

Signed-off-by: Waiman Long <Waiman.Long@hp.com>
Reviewed-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Chandramouleeswaran Aswin <aswin@hp.com>
Cc: Norton Scott J <scott.norton@hp.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
mutex: Back out architecture specific check for negative mutex count

Linus suggested that probably all the supported architectures can
allow a negative mutex count without incorrect behavior, so we can
then back out the architecture specific change and allow the
mutex count to go to any negative number. That should further
reduce contention for non-x86 architecture.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Waiman Long <Waiman.Long@hp.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Chandramouleeswaran Aswin <aswin@hp.com>
Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>
Cc: Norton Scott J <scott.norton@hp.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
writeback: fix occasional slow sync(1)

In case when system contains no dirty pages, wakeup_flusher_threads() will
submit WB_SYNC_NONE writeback for 0 pages so wb_writeback() exits
immediately without doing anything, even though there are dirty inodes in
the system.  Thus sync(1) will write all the dirty inodes from a
WB_SYNC_ALL writeback pass which is slow.

Fix the problem by using get_nr_dirty_pages() in wakeup_flusher_threads()
instead of calculating number of dirty pages manually.  That function also
takes number of dirty inodes into account.

Signed-off-by: Jan Kara <jack@suse.cz>
Reported-by: Paul Taysom <taysom@chromium.org>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
writeback: fix race that cause writeback hung

There is a race between mark inode dirty and writeback thread, see the
following scenario.  In this case, writeback thread will not run though
there is dirty_io.

__mark_inode_dirty()                                          bdi_writeback_workfn()
	...                                                       	...
	spin_lock(&inode->i_lock);
	...
	if (bdi_cap_writeback_dirty(bdi)) {
	    <<< assume wb has dirty_io, so wakeup_bdi is false.
	    <<< the following inode_dirty also have wakeup_bdi false.
	    if (!wb_has_dirty_io(&bdi->wb))
		    wakeup_bdi = true;
	}
	spin_unlock(&inode->i_lock);
	                                                            <<< assume last dirty_io is removed here.
	                                                            pages_written = wb_do_writeback(wb);
	                                                            ...
	                                                            <<< work_list empty and wb has no dirty_io,
	                                                            <<< delayed_work will not be queued.
	                                                            if (!list_empty(&bdi->work_list) ||
	                                                                (wb_has_dirty_io(wb) && dirty_writeback_interval))
	                                                                queue_delayed_work(bdi_wq, &wb->dwork,
	                                                                    msecs_to_jiffies(dirty_writeback_interval * 10));
	spin_lock(&bdi->wb.list_lock);
	inode->dirtied_when = jiffies;
	<<< new dirty_io is added.
	list_move(&inode->i_wb_list, &bdi->wb.b_dirty);
	spin_unlock(&bdi->wb.list_lock);

	<<< though there is dirty_io, but wakeup_bdi is false,
	<<< so writeback thread will not be waked up and
	<<< the new dirty_io will not be flushed.
	if (wakeup_bdi)
	    bdi_wakeup_thread_delayed(bdi);

Writeback will run until there is a new flush work queued.  This may cause
a lot of dirty pages stay in memory for a long time.

Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
writeback: fix writeback cache thrashing

Consider Process A: huge I/O on sda
        doing heavy write operation - dirty memory becomes more
        than dirty_background_ratio
        on HDD - flusher thread flush-8:0

Consider Process B: small I/O on sdb
        doing while [1]; read 1024K + rewrite 1024K + sleep 2sec
        on Flash device - flusher thread flush-8:16

As Process A is a heavy dirtier, dirty memory becomes more
than dirty_background_thresh. Due to this, below check becomes
true(checking global_page_state in over_bground_thresh)
for all bdi devices(even for very small dirtied bdi - sdb):

In this case, even small cached data on 'sdb' is forced to flush
and writeback cache thrashing happens.

When we added debug prints inside above 'if' condition and ran
above Process A(heavy dirtier on bdi with flush-8:0) and
Process B(1024K frequent read/rewrite on bdi with flush-8:16)
we got below prints:

[Test setup: ARM dual core CPU, 512 MB RAM]

[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  56064 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  56704 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 84720 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 94720 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   384 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   960 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =    64 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 92160 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   256 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   768 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =    64 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   256 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   320 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =     0 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 92032 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 91968 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   192 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =  1024 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =    64 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   192 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   576 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =     0 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 84352 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   192 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   512 KB
[over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =     0 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 92608 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 92544 KB

As mentioned in above log, when global dirty memory > global background_thresh
small cached data is also forced to flush by flush-8:16.
If removing global background_thresh checking code, we can reduce cache
thrashing of frequently used small data.
And It will be great if we can reserve a portion of writeback cache using
min_ratio.

After applying patch:
$ echo 5 > /sys/block/sdb/bdi/min_ratio
$ cat /sys/block/sdb/bdi/min_ratio
5
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  56064 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  56704 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  84160 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  96960 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  94080 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  93120 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  93120 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  91520 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  89600 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  93696 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  93696 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  72960 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  90624 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  90624 KB
[over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  90688 KB

As mentioned in the above logs, once cache is reserved for Process B,
and patch is applied there is less writeback cache thrashing on sdb
by frequent forced writeback by flush-8:16 in over_bground_thresh.

After all, small cached data will be flushed by periodic writeback
once every dirty_writeback_interval.

Suggested-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Vivek Trivedi <t.vivek@samsung.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
writeback: increase bdi_min_ratio to 5

Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
CPU hotplug, writeback: Don't call writeback_set_ratelimit() too often during hotplug

The CPU hotplug callback related to writeback calls writeback_set_ratelimit()
during every state change in the hotplug sequence. This is unnecessary
since num_online_cpus() changes only once during the entire hotplug operation.

So invoke the function only once per hotplug, thereby avoiding the
unnecessary repetition of those costly calculations.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>