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>

Change-Id: I9e55d65f5ecb2305497711d4688f0647d9346035
timer: optimize apply_slack()

__fls(mask) is equivalent to find_last_bit(&mask, BITS_PER_LONG), but cheaper.
find_last_bit was showing up high on the list when I was profiling for stalls
on icache misses on a system with very small cache size (MIPS).

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: edoko <r_data@naver.com>

Change-Id: I8a5021a2fb2936c00ffd456663a76cb1b23e3100
net: flow: Prevent bringing up new CPUs during per-CPU initialization

In a rare race-condition, it's possible that a CPU will be brought
online between when the for_each_online_cpu() loop executes to
call flow_cache_cpu_prepare(), and when the hotplug notifier for
calling this same function is registered. If this happens,
flow_cache_cpu_prepare() will never be called for the new CPU,
resulting in crashes due to uninitialized per-cpu data.

Fix this by preventing CPUs from being added or removed during
this small but sensitive window.

Change-Id: Iafbbaa8a50e5c527392d130561874313720849d0
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>
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. 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.

Change-Id: I458027ae08d9a5a93202a7b97ace1f8da7a18a07
CC: stable@vger.kernel.org
Reported-by: Paul Taysom <taysom@chromium.org>
Signed-off-by: Jan Kara <jack@suse.cz>