mm: vmscan: do not iterate all mem cgroups for global direct reclaim

In current implementation, both kswapd and direct reclaim has to iterate
all mem cgroups.  It is not a problem before offline mem cgroups could
be iterated.  But, currently with iterating offline mem cgroups, it
could be very time consuming.  In our workloads, we saw over 400K mem
cgroups accumulated in some cases, only a few hundred are online memcgs.
Although kswapd could help out to reduce the number of memcgs, direct
reclaim still get hit with iterating a number of offline memcgs in some
cases.  We experienced the responsiveness problems due to this
occassionally.

A simple test with pref shows it may take around 220ms to iterate 8K
memcgs in direct reclaim:
             dd 13873 [011]   578.542919: vmscan:mm_vmscan_direct_reclaim_begin
             dd 13873 [011]   578.758689: vmscan:mm_vmscan_direct_reclaim_end
So for 400K, it may take around 11 seconds to iterate all memcgs.

Here just break the iteration once it reclaims enough pages as what
memcg direct reclaim does.  This may hurt the fairness among memcgs.
But the cached iterator cookie could help to achieve the fairness more
or less.

Link: http://lkml.kernel.org/r/1548799877-10949-1-git-send-email-yang.shi@linux.alibaba.com
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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