2.6.9-rc1: page_referenced_one() CPU consumption

[Nikita Danilov <nikita@clusterfs.com>]

Hello,

in 2.6.9-rc1 page_referenced_one() is among top CPU consumers (which
wasn't a case for 2.6.8-rc2) in the host kernel when running heavily
loaded UML. readprofile -b shows that time is spent in
spin_lock(&mm->page_table_lock), so, I reckon, recent "rmaplock: kill
page_map_lock" changes are probably not completely unrelated.

Without any deep investigation, one possible scenario is that multiple
threads are doing (as part of direct reclaim),

   refill_inactive_zone()
       page_referenced()
           page_referenced_file() /* (1) mapping->i_mmap_lock doesn't
                                     serialize them */
               page_referenced_one()
                   spin_lock(&mm->page_table_lock) /* (2) everybody is
                                                     serialized here */

(1) and (2) will be true if we have one huge address space with a lot of
VMAs, which seems to be exactly what UML does:

$ wc /proc/<UML-host-pid>/maps
4134 28931 561916

This didn't happen before, because page_referenced_one() used to
try-lock.

Nikita.