In some swap scalability test, it is found that there are heavy lock
contention on swap cache even if we have split one swap cache radix tree
per swap device to one swap cache radix tree every 64 MB trunk in commit
4b3ef9da ("mm/swap: split swap cache into 64MB trunks").

The reason is as follow.  After the swap device becomes fragmented so
that there's no free swap cluster, the swap device will be scanned
linearly to find the free swap slots.  swap_info_struct->cluster_next is
the next scanning base that is shared by all CPUs.  So nearby free swap
slots will be allocated for different CPUs.  The probability for
multiple CPUs to operate on the same 64 MB trunk is high.  This causes
the lock contention on the swap cache.

To solve the issue, in this patch, for SSD swap device, a percpu version
next scanning base (cluster_next_cpu) is added.  Every CPU will use its
own per-cpu next scanning base.  And after finishing scanning a 64MB
trunk, the per-cpu scanning base will be changed to the beginning of
another randomly selected 64MB trunk.  In this way, the probability for
multiple CPUs to operate on the same 64 MB trunk is reduced greatly.
Thus the lock contention is reduced too.  For HDD, because sequential
access is more important for IO performance, the original shared next
scanning base is used.

To test the patch, we have run 16-process pmbench memory benchmark on a
2-socket server machine with 48 cores.  One ram disk is configured as the
swap device per socket.  The pmbench working-set size is much larger than
the available memory so that swapping is triggered.  The memory read/write
ratio is 80/20 and the accessing pattern is random.  In the original
implementation, the lock contention on the swap cache is heavy.  The perf
profiling data of the lock contention code path is as following,

 _raw_spin_lock_irq.add_to_swap_cache.add_to_swap.shrink_page_list:      7.91
 _raw_spin_lock_irqsave.__remove_mapping.shrink_page_list:               7.11
 _raw_spin_lock.swapcache_free_entries.free_swap_slot.__swap_entry_free: 2.51
 _raw_spin_lock_irqsave.swap_cgroup_record.mem_cgroup_uncharge_swap:     1.66
 _raw_spin_lock_irq.shrink_inactive_list.shrink_lruvec.shrink_node:      1.29
 _raw_spin_lock.free_pcppages_bulk.drain_pages_zone.drain_pages:         1.03
 _raw_spin_lock_irq.shrink_active_list.shrink_lruvec.shrink_node:        0.93

After applying this patch, it becomes,

 _raw_spin_lock.swapcache_free_entries.free_swap_slot.__swap_entry_free: 3.58
 _raw_spin_lock_irq.shrink_inactive_list.shrink_lruvec.shrink_node:      2.3
 _raw_spin_lock_irqsave.swap_cgroup_record.mem_cgroup_uncharge_swap:     2.26
 _raw_spin_lock_irq.shrink_active_list.shrink_lruvec.shrink_node:        1.8
 _raw_spin_lock.free_pcppages_bulk.drain_pages_zone.drain_pages:         1.19

The lock contention on the swap cache is almost eliminated.

And the pmbench score increases 18.5%.  The swapin throughput increases
18.7% from 2.96 GB/s to 3.51 GB/s.  While the swapout throughput increases
18.5% from 2.99 GB/s to 3.54 GB/s.

We need really fast disk to show the benefit.  I have tried this on 2
Intel P3600 NVMe disks.  The performance improvement is only about 1%.
The improvement should be better on the faster disks, such as Intel Optane
disk.

[ying.huang@intel.com: fix cluster_next_cpu allocation and freeing, per Daniel]
  Link: http://lkml.kernel.org/r/20200525002648.336325-1-ying.huang@intel.com
[ying.huang@intel.com: v4]
  Link: http://lkml.kernel.org/r/20200529010840.928819-1-ying.huang@intel.comSigned-off-by: "Huang, Ying" <ying.huang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Link: http://lkml.kernel.org/r/20200520031502.175659-1-ying.huang@intel.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>