lkml.org 
[lkml]   [2011]   [Apr]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH] mm: Check if PTE is already allocated during page fault
    On Fri, Apr 15, 2011 at 11:12:48AM +0100, Mel Gorman wrote:
    > With transparent hugepage support, handle_mm_fault() has to be careful
    > that a normal PMD has been established before handling a PTE fault. To
    > achieve this, it used __pte_alloc() directly instead of pte_alloc_map
    > as pte_alloc_map is unsafe to run against a huge PMD. pte_offset_map()
    > is called once it is known the PMD is safe.
    >
    > pte_alloc_map() is smart enough to check if a PTE is already present
    > before calling __pte_alloc but this check was lost. As a consequence,
    > PTEs may be allocated unnecessarily and the page table lock taken.
    > Thi useless PTE does get cleaned up but it's a performance hit which
    > is visible in page_test from aim9.
    >
    > This patch simply re-adds the check normally done by pte_alloc_map to
    > check if the PTE needs to be allocated before taking the page table
    > lock. The effect is noticable in page_test from aim9.
    >
    > AIM9
    > 2.6.38-vanilla 2.6.38-checkptenone
    > creat-clo 446.10 ( 0.00%) 424.47 (-5.10%)
    > page_test 38.10 ( 0.00%) 42.04 ( 9.37%)
    > brk_test 52.45 ( 0.00%) 51.57 (-1.71%)
    > exec_test 382.00 ( 0.00%) 456.90 (16.39%)
    > fork_test 60.11 ( 0.00%) 67.79 (11.34%)
    > MMTests Statistics: duration
    > Total Elapsed Time (seconds) 611.90 612.22
    >
    > (While this affects 2.6.38, it is a performance rather than a
    > functional bug and normally outside the rules -stable. While the big
    > performance differences are to a microbench, the difference in fork
    > and exec performance may be significant enough that -stable wants to
    > consider the patch)
    >
    > Reported-by: Raz Ben Yehuda <raziebe@gmail.com>
    > Signed-off-by: Mel Gorman <mgorman@suse.de>

    Acked-by: Johannes Weiner <hannes@cmpxchg.org>


    \
     
     \ /
      Last update: 2011-04-27 15:53    [W:0.026 / U:0.128 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site