hi，
I have encountered kernel hung task when running stability and stress test.
test scenarios:
1)the kernel hungtask settings are following:
hung_task_panic = 1
hung_task_timeout_secs = 120
2)the rootfs type is squashfs(read-only)
what the test does is to fork many child process and each process will alloc
memory.
when there is no free memory in the system, OOM killer is triggerred. and then
the kernel triggers hung task(after about five minutes) .
the reason for hung task is that some process keep D states for 120 seconds.
if there is no free memory in the system, many process state is D, they enter
into D state by kernel path `squashfs_cache_get()--->wait_event()`.
the backtrace is:
[ 313.950118] [<c02d2014>] (__schedule+0x448/0x5cc) from [<c014e510>]
(squashfs_cache_get+0x120/0x3ec)
[ 314.059660] [<c014e510>] (squashfs_cache_get+0x120/0x3ec) from [<c014fd1c>]
(squashfs_readpage+0x748/0xa2c)
[ 314.176497] [<c014fd1c>] (squashfs_readpage+0x748/0xa2c) from [<c00b7be0>]
(__do_page_cache_readahead+0x1ac/0x200)
[ 314.300621] [<c00b7be0>] (__do_page_cache_readahead+0x1ac/0x200) from
[<c00b7e98>] (ra_submit+0x24/0x28)
[ 314.414325] [<c00b7e98>] (ra_submit+0x24/0x28) from [<c00b043c>]
(filemap_fault+0x16c/0x3f0)
[ 314.515521] [<c00b043c>] (filemap_fault+0x16c/0x3f0) from [<c00c94e0>]
(__do_fault+0xc0/0x570)
[ 314.618802] [<c00c94e0>] (__do_fault+0xc0/0x570) from [<c00cbdc4>]
(handle_pte_fault+0x47c/0x1048)
[ 314.726250] [<c00cbdc4>] (handle_pte_fault+0x47c/0x1048) from [<c00cd928>]
(handle_mm_fault+0x164/0x218)
[ 314.839959] [<c00cd928>] (handle_mm_fault+0x164/0x218) from [<c02d4878>]
(do_page_fault.part.7+0x108/0x360)
[ 314.956788] [<c02d4878>] (do_page_fault.part.7+0x108/0x360) from
[<c02d4afc>] (do_page_fault+0x2c/0x70)
[ 315.069442] [<c02d4afc>] (do_page_fault+0x2c/0x70) from [<c00084cc>]
(do_PrefetchAbort+0x2c/0x90)
[ 315.175850] [<c00084cc>] (do_PrefetchAbort+0x2c/0x90) from [<c02d3674>]
(ret_from_exception+0x0/0x10)
when a task is already exiting because of OOM killer,the next time OOM killer
will kill the same task.
so, if the first time of OOM killer select a task(A) that in D state (the task
ingore exit signal beacuse of D state).
then the next time of OOM killer will also kill task A. In this scenario, oom
killer will not free memory.
with no free memory, many process sleep in function squashfs_cache_get. about 2
minutes, the system hung task and panic.
because of OOM feature and squashfs, on heavy system, This problem is easily
reproduce.
Is this a problem about squashfs or about the OOM killer. Can anyone give me
some good ideas about this?
Best Regards
Wang Long
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