On 3/6/18 12:45 PM, Andrew Morton wrote:
On Tue, 27 Feb 2018 08:25:47 +0800 Yang Shi <yang....@linux.alibaba.com> wrote:

When running vm-scalability with large memory (> 300GB), the below hung
task issue happens occasionally.

INFO: task ps:14018 blocked for more than 120 seconds.
        Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  ps              D    0 14018      1 0x00000004
   ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
   ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
   00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
  Call Trace:
   [<ffffffff817154d0>] ? __schedule+0x250/0x730
   [<ffffffff817159e6>] schedule+0x36/0x80
   [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
   [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
   [<ffffffff81717db0>] down_read+0x20/0x40
   [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0
   [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
   [<ffffffff81241d87>] __vfs_read+0x37/0x150
   [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
   [<ffffffff81242266>] vfs_read+0x96/0x130
   [<ffffffff812437b5>] SyS_read+0x55/0xc0
   [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5

When manipulating a large mapping, the process may hold the mmap_sem for
long time, so reading /proc/<pid>/cmdline may be blocked in
uninterruptible state for long time.
We already have killable version APIs for semaphore, here use 
to improve the responsiveness.

Maybe I'm missing something, but I don't see how this solves the
problem.  Yes, the read of /proc/pid/cmdline will be abandoned if
someone interrupts that process.  But if nobody does that, the read
will still just sit there for 2 minutes and the watchdog warning will
still come out?

No, the warning will not come out since down_read_killable() puts the task into TASK_KILLABLE state instead of TASK_UNINTERRUPTIBLE state. The hung task check will skip TASK_KILLABLE tasks, please see the below code in (kernel/hung_task.c):

/* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
                if (t->state == TASK_UNINTERRUPTIBLE)
                        check_hung_task(t, timeout);

It just mitigates the hung task warning, can't resolve the mmap_sem scalability issue. Furthermore, waiting on pure uninterruptible state for reading /proc sounds unnecessary. It doesn't wait for I/O completion.

Where the heck are we holding mmap_sem for so long?  Can that be fixed?

The mmap_sem is held for unmapping a large map which has every single page mapped. This is not a issue in real production code. Just found it by running vm-scalability on a machine with ~600GB memory.

AFAIK, I don't see any easy fix for the mmap_sem scalability issue. I saw range locking patches (https://lwn.net/Articles/723648/) were floating around. But, it may not help too much on the case that a large map with every single page mapped.


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