On Fri 15-12-17 03:53:23, Yang Shi wrote: > In the current design, khugepaged need acquire mmap_sem before scanning > mm, but in some corner case, khugepaged may scan the current running > process which might be modifying memory mapping, so khugepaged might > block in uninterruptible state. But, the process might hold the mmap_sem > for long time when modifying a huge memory space, then 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: > [<ffffffff817079a5>] ? __schedule+0x235/0x6e0 > [<ffffffff81707e86>] schedule+0x36/0x80 > [<ffffffff8170a970>] rwsem_down_read_failed+0xf0/0x150 > [<ffffffff81384998>] call_rwsem_down_read_failed+0x18/0x30 > [<ffffffff8170a1c0>] down_read+0x20/0x40 > [<ffffffff81226836>] khugepaged+0x476/0x11d0 > [<ffffffff810c9d0e>] ? idle_balance+0x1ce/0x300 > [<ffffffff810d0850>] ? prepare_to_wait_event+0x100/0x100 > [<ffffffff812263c0>] ? collapse_shmem+0xbf0/0xbf0 > [<ffffffff810a8d46>] kthread+0xe6/0x100 > [<ffffffff810a8c60>] ? kthread_park+0x60/0x60 > [<ffffffff8170cd15>] ret_from_fork+0x25/0x30 I am definitely interested in what the holder of the write lock does here for such a long time. > So, it sounds pointless to just block for waiting for the semaphore for > khugepaged, here replace down_read() to down_read_trylock() to move to > scan 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 finish the > current round full_scan. > > And, it soudns the change can improve khugepaged efficiency a little > bit. > > The below is the test result with 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 > > Signed-off-by: Yang Shi <yang.s@xxxxxxxxxxxxxxx> > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> > Cc: Michal Hocko <mhocko@xxxxxxxx> > Cc: Hugh Dickins <hughd@xxxxxxxxxx> > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> The patch makes sense to me Acked-by: Michal Hocko <mhocko@xxxxxxxx> > --- > mm/khugepaged.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index ea4ff25..ecc2b68 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -1674,7 +1674,12 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, > spin_unlock(&khugepaged_mm_lock); > > mm = mm_slot->mm; > - down_read(&mm->mmap_sem); > + /* > + * Not wait for semaphore to avoid long time waiting, just move > + * to the next mm on the list. > + */ > + if (unlikely(!down_read_trylock(&mm->mmap_sem))) > + goto breakouterloop_mmap_sem; > if (unlikely(khugepaged_test_exit(mm))) > vma = NULL; > else > -- > 1.8.3.1 > -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
