• Yang Shi's avatar
    mm: thp: use down_read_trylock() in khugepaged to avoid long block · 3b454ad3
    Yang Shi authored
    In the current design, khugepaged needs to acquire mmap_sem before
    scanning an mm.  But in some corner cases, khugepaged may scan a process
    which is modifying its memory mapping, so khugepaged blocks in
    uninterruptible state.  But the process might hold the mmap_sem for a
    long time when modifying a huge memory space and it may trigger the
    below khugepaged hung issue:
    
      INFO: task khugepaged:270 blocked for more than 120 seconds.
      Tainted: G E 4.9.65-006.ali3000.alios7.x86_64 #1
      "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
      khugepaged D 0 270 2 0x00000000 
      ffff883f3deae4c0 0000000000000000 ffff883f610596c0 ffff883f7d359440
      ffff883f63818000 ffffc90019adfc78 ffffffff817079a5 d67e5aa8c1860a64
      0000000000000246 ffff883f7d359440 ffffc90019adfc88 ffff883f610596c0
      Call Trace:
        schedule+0x36/0x80
        rwsem_down_read_failed+0xf0/0x150
        call_rwsem_down_read_failed+0x18/0x30
        down_read+0x20/0x40
        khugepaged+0x476/0x11d0
        kthread+0xe6/0x100
        ret_from_fork+0x25/0x30
    
    So it sounds pointless to just block khugepaged waiting for the
    semaphore so replace down_read() with down_read_trylock() to move to
    scan the next mm quickly instead of just blocking on the semaphore so
    that other processes can get more chances to install THP.  Then
    khugepaged can come back to scan the skipped mm when it has finished the
    current round full_scan.
    
    And it appears that the change can improve khugepaged efficiency a
    little bit.
    
    Below is the test result when running LTP on a 24 cores 4GB memory 2
    nodes NUMA VM:
    
                                        pristine          w/ trylock
      full_scan                         197               187
      pages_collapsed                   21                26
      thp_fault_alloc                   40818             44466
      thp_fault_fallback                18413             16679
      thp_collapse_alloc                21                150
      thp_collapse_alloc_failed         14                16
      thp_file_alloc                    369               369
    
    [akpm@linux-foundation.org: coding-style fixes]
    [akpm@linux-foundation.org: tweak comment]
    [arnd@arndb.de: avoid uninitialized variable use]
      Link: http://lkml.kernel.org/r/20171215125129.2948634-1-arnd@arndb.de
    Link: http://lkml.kernel.org/r/1513281203-54878-1-git-send-email-yang.s@alibaba-inc.comSigned-off-by: default avatarYang Shi <yang.s@alibaba-inc.com>
    Acked-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Acked-by: default avatarMichal Hocko <mhocko@suse.com>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Signed-off-by: default avatarArnd Bergmann <arnd@arndb.de>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    3b454ad3
khugepaged.c 48.6 KB