On Thu 02-06-16 21:08:57, Sergey Senozhatsky wrote: > Hello Michal, > > On (06/02/16 11:21), Michal Hocko wrote: > [..] > > > [ 2856.323052] INFO: task cc1:4582 blocked for more than 21 seconds. > > > [ 2856.323055] Not tainted 4.7.0-rc1-next-20160601-dbg-00012-g52c180e-dirty #453 > > > [ 2856.323056] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. > > > [ 2856.323059] cc1 D ffff880057e9fd78 0 4582 4575 0x00000000 > > > [ 2856.323062] ffff880057e9fd78 ffff880057e08000 ffff880057e9fd90 ffff880057ea0000 > > > [ 2856.323065] ffff88005dc3dc68 ffffffff00000001 ffff880057e09500 ffff88005dc3dc80 > > > [ 2856.323067] ffff880057e9fd90 ffffffff81441e33 ffff88005dc3dc68 ffff880057e9fe00 > > > [ 2856.323068] Call Trace: > > > [ 2856.323074] [<ffffffff81441e33>] schedule+0x83/0x98 > > > [ 2856.323077] [<ffffffff81443d9b>] rwsem_down_write_failed+0x18e/0x1d3 > > > [ 2856.323080] [<ffffffff810a87cf>] ? unlock_page+0x2b/0x2d > > > [ 2856.323083] [<ffffffff811bdb77>] call_rwsem_down_write_failed+0x17/0x30 > > > [ 2856.323084] [<ffffffff811bdb77>] ? call_rwsem_down_write_failed+0x17/0x30 > > > [ 2856.323086] [<ffffffff81443630>] down_write+0x1f/0x2e > > > [ 2856.323089] [<ffffffff810ea4f3>] __khugepaged_exit+0x104/0x11a > > > [ 2856.323091] [<ffffffff8103702a>] mmput+0x29/0xc5 > > > [ 2856.323093] [<ffffffff8103bbd8>] do_exit+0x34c/0x894 > > > [ 2856.323095] [<ffffffff8102f9e0>] ? __do_page_fault+0x2f7/0x399 > > > [ 2856.323097] [<ffffffff8103c188>] do_group_exit+0x3c/0x98 > > > [ 2856.323099] [<ffffffff8103c1f3>] SyS_exit_group+0xf/0xf > > > [ 2856.323101] [<ffffffff81444cdb>] entry_SYSCALL_64_fastpath+0x13/0x8f > > > > down_write in the exit path is certainly not nice. It is hard to tell > > who is blocking the mmap_sem but it is clear that __khugepaged_exit > > waits for the khugepaged to release its mmap_sem. Do you hapen to have a > > trace of khugepaged? Note that the lock holder might be another writer > > which just hasn't pinned mm_users so khugepaged might be blocked on read > > lock as well. Or khugepaged might be just stuck somewhere... > > sorry, no. this is all I have. the kernel was compiled with almost no > debugging functionality enabled (no lockdep, no lock debug, nothing) > for zram performance testing purposes. > > I'll try to reproduce the problem; and give your patch some testing. > thanks. The patch will drop the down_write from the exit path which is, I believe the right thing to do, so it would paper over an existing problem when khugepaged could get stuck with mmap_sem held for read (if that is really a problem). So reproducing without the patch still makes some sense. Testing with the patch makes some sense as well, but I would like to hear from Andrea whether the approach is good because I am wondering why he hasn't done that before - it feels so much simpler than the current code. Anyway, thanks a lot for testing! -- 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>