Re: [linux-next: Tree for Jun 1] __khugepaged_exit rwsem_down_write_failed lockup

[Michal Hocko]

[CCing Andrea]

On Thu 02-06-16 10:48:35, Sergey Senozhatsky wrote:
> On (06/01/16 13:11), Stephen Rothwell wrote:
> > Hi all,
> > 
> > Changes since 20160531:
> > 
> > My fixes tree contains:
> > 
> >   of: silence warnings due to max() usage
> > 
> > The arm tree gained a conflict against Linus' tree.
> > 
> > Non-merge commits (relative to Linus' tree): 1100
> >  936 files changed, 38159 insertions(+), 17475 deletions(-)
> 
> Hello,
> 
> the cc1 process ended up in DN state during kernel -j4 compilation.
> 
> ...
> [ 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...

I am trying to wrap my head around the synchronization here and I
suspect it is unnecessarily complex. We should be able to go without
down_write in the exit path... The following patch would only workaround
the issue you are seeing but I guess it is worth considering this
approach.

Andrea, does the following look reasonable to you? I haven't tested it
and I might be missing some subtle details. The code is really not
trivial...
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