On Tue, May 19, 2015 at 05:27:10PM +0200, Michal Hocko wrote:

On Tue 19-05-15 16:13:02, Mel Gorman wrote:
[...]

quoted

               :ffffffff811c160f:       je     ffffffff811c1630 <mem_cgroup_try_charge+0x40>
               :ffffffff811c1611:       xor    %eax,%eax
               :ffffffff811c1613:       xor    %ebx,%ebx
     1 1.7e-05 :ffffffff811c1615:       mov    %rbx,(%r12)
     7 1.2e-04 :ffffffff811c1619:       add    $0x10,%rsp
  1211  0.0203 :ffffffff811c161d:       pop    %rbx
     5 8.4e-05 :ffffffff811c161e:       pop    %r12
     5 8.4e-05 :ffffffff811c1620:       pop    %r13
  1249  0.0210 :ffffffff811c1622:       pop    %r14
     7 1.2e-04 :ffffffff811c1624:       pop    %rbp
     5 8.4e-05 :ffffffff811c1625:       retq   
               :ffffffff811c1626:       nopw   %cs:0x0(%rax,%rax,1)
   295  0.0050 :ffffffff811c1630:       mov    (%rdi),%rax
160703  2.6973 :ffffffff811c1633:       mov    %edx,%r13d

Huh, what? Even if this was off by one and the preceding instruction has
consumed the time. This would be reading from page->flags but the page
should be hot by the time we got here, no?

I would have expected so but it's not the first time I've seen cases where
examining the flags was a costly instruction. I suspect it's due to an
ordering issue or more likely, a frequent branch mispredict that is being
accounted for against this instruction.

-- 
Mel Gorman
SUSE Labs