Re: kjournald hang on ext3 to ext3 copy
On Jun 16, 2007 08:17 -0500, John Marconi wrote:
I am running into a situation in which one of my ext3 filesystems is
getting hung during normal usage. There are three ext3 filesystems on a
CompactFLASH. One is mounted as / and one as /tmp. In my test, I am
copying a 100 MB file from /root to /tmp repeatedly. While doing this
test, I eventually see the copying stop, and any attempts to access /tmp
fail - if I even do ls /tmp the command will hang.
I suspect kjournald because of the following ps output:
PID PPID WCHAN:20 PCPU %MEM PSR COMM
884799 start_this_handle1.1 0.0 28 pdflush
885399 schedule_timeout 0.2 0.0 7 pdflush
188 1 kswapd 0.0 0.0 19 kswapd0
8051 1 mtd_blktrans_thread 0.0 0.0 22 mtdblockd
8243 1 kjournald0.0 0.0 0 kjournald
8305 1 schedule_timeout0.0 0.0 2 udevd
8378 1 kjournald0.0 0.0 0 kjournald
8379 1 journal_commit_trans 16.6 0.0 0 kjournald
8437 1 schedule_timeout 0.0 0.0 0 evlogd
8527 1 syslog0.0 0.0 1 klogd
8534 1 schedule_timeout 0.0 0.0 0 portmap
8569 1 schedule_timeout 0.0 0.0 0 rngd
8639 1 schedule_timeout 0.1 0.0 24 sshd
8741 8639 schedule_timeout0.0 0.0 0 sshd
8743 8741 wait0.0 0.0 9 bash
8857 8743 schedule_timeout4.9 0.0 7 cp
8664 1 schedule_timeout 0.0 0.0 0 xinetd
8679 1 schedule_timeout 0.0 0.0 0 evlnotifyd
8689 1 schedule_timeout 0.0 0.0 0 evlactiond
8704 1 wait 0.0 0.0 1 bash
8882 8704 -0.0 0.0 2 ps
If I run ps repeatedly, I always see process 8379 in
journal_commit_transaction, and it is always taking between 12% and 20%
of processor 0 up. This process never completes. I also see process
8847 in start_this_handle forever as well - so I believe they are related.
This system is using a 2.6.14 kernel.
Please try to reproduce with a newer kernel, as this kind of problem
might have been fixed already.
Two tips for debugging this kind of issue:
- you need to have detailed stack traces (e.g. sysrq-t) of all the
interesting processes
- if a process is stuck inside a large function (e.g. 8379 in example)
you need to provide the exact line number. this can be found by compiling
the kernel with CONFIG_DEBUG_INFO (-g flag to gcc) and then doing
gdb vmlinux and p *(journal_commit_transaction+{offset}), where the
byte offset is printed in the sysrq-t output, and then include the code
surrounding that line from the source file
- a process stuck in start_this_handle() is often just an innocent
bystander. It is waiting for the currently committing transaction to
complete before it can start a new filesystem-modifying operation (handle).
That said, the journal handle acts like a lock and has been the cause of
many deadlock problems (e.g. process 1 holds lock, waits for handle;
process 2 holds transaction open waiting for lock). pdflush might be one
of the process 1 kind of tasks, and some other process is holding the
transaction open preventing it from completing.
Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.
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Re: kjournald hang on ext3 to ext3 copy
On Jun 18, 2007 22:53 -0500, John Marconi wrote:
Andreas Dilger wrote:
Two tips for debugging this kind of issue:
- you need to have detailed stack traces (e.g. sysrq-t) of all the
interesting processes
- if a process is stuck inside a large function (e.g. 8379 in example)
you need to provide the exact line number. this can be found by
compiling
the kernel with CONFIG_DEBUG_INFO (-g flag to gcc) and then doing
gdb vmlinux and p *(journal_commit_transaction+{offset}), where the
byte offset is printed in the sysrq-t output, and then include the code
surrounding that line from the source file
- a process stuck in start_this_handle() is often just an innocent
bystander. It is waiting for the currently committing transaction to
complete before it can start a new filesystem-modifying operation
(handle).
That said, the journal handle acts like a lock and has been the cause of
many deadlock problems (e.g. process 1 holds lock, waits for handle;
process 2 holds transaction open waiting for lock). pdflush might be one
of the process 1 kind of tasks, and some other process is holding the
transaction open preventing it from completing.
I am not able to update the entire kernel to a new version for a variety
of reasons, however I can update certain parts in my system (such as the
filesystem). I did a diff of the 2.6.16 kernel against my kernel, and
the changes to jbd were minimal. I plan on looking at the latest
versions of the kernel to determine if anything has changed since 2.6.16.
The problem may also be in the ext3 layer and not jbd.
I took a look at the place that kjournald was stuck - it is in the
journal_commit_transaction while (comiit_transaction-t_updates) loop
and it is trying to spin_lock(journal-j_state_lock). When I look at
pdflush, it is also trying to take the journal-j_state_lock. Do you
have any tips on finding out which process might own journal-j_state_lock?
You can enable CONFIG_DEBUG_SPINLOCK in newer kernels and it appears the
spinlock will set the owner field to the task struct. You still need
to get access to this via e.g. crash or lkcd or something.
Hmm, it seems this is only set for ppc and s390??? That is how I would
debug this in any case. The other way (I've done this too many times
in the past) is to look through all of the stack traces and figure out
which ones are in a filesystem context, then check if any of them are
blocked on locks while holding transactions open. Needs a detailed
understanding of kernel callpaths.
Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.
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