Re: [PATCH V6] Eliminate task stack trace duplication

[David Rientjes <rientjes@google.com>]

On Thu, 1 Dec 2011, Ying Han wrote:

> The problem with small dmesg ring buffer like 512k is that only limited number
> of task traces will be logged. Sometimes we lose important information only
> because of too many duplicated stack traces. This problem occurs when dumping
> lots of stacks in a single operation, such as sysrq-T.
> 
> This patch tries to reduce the duplication of task stack trace in the dump
> message by hashing the task stack. The hashtable is a 32k pre-allocated buffer
> during bootup. Each time if we find the identical task trace in the task stack,
> we dump only the pid of the task which has the task trace dumped. So it is easy
> to back track to the full stack with the pid.
> 
> When we do the hashing, we eliminate garbage entries from stack traces. Those
> entries are still being printed in the dump to provide more debugging
> informations.
> 
> [   58.469730] kworker/0:0     S 0000000000000000     0     4      2 0x00000000
> [   58.469735]  ffff88082fcfde80 0000000000000046 ffff88082e9d8000 ffff88082fcfc010
> [   58.469739]  ffff88082fce9860 0000000000011440 ffff88082fcfdfd8 ffff88082fcfdfd8
> [   58.469743]  0000000000011440 0000000000000000 ffff88082fcee180 ffff88082fce9860
> [   58.469747] Call Trace:
> [   58.469751]  [<ffffffff8108525a>] worker_thread+0x24b/0x250
> [   58.469754]  [<ffffffff8108500f>] ? manage_workers+0x192/0x192
> [   58.469757]  [<ffffffff810885bd>] kthread+0x82/0x8a
> [   58.469760]  [<ffffffff8141aed4>] kernel_thread_helper+0x4/0x10
> [   58.469763]  [<ffffffff8108853b>] ? kthread_worker_fn+0x112/0x112
> [   58.469765]  [<ffffffff8141aed0>] ? gs_change+0xb/0xb
> [   58.469768] kworker/u:0     S 0000000000000004     0     5      2 0x00000000
> [   58.469773]  ffff88082fcffe80 0000000000000046 ffff880800000000 ffff88082fcfe010
> [   58.469777]  ffff88082fcea080 0000000000011440 ffff88082fcfffd8 ffff88082fcfffd8
> [   58.469781]  0000000000011440 0000000000000000 ffff88082fd4e9a0 ffff88082fcea080
> [   58.469785] Call Trace:
> [   58.469786] <Same stack as pid 4>
> [   58.470235] kworker/0:1     S 0000000000000000     0    13      2 0x00000000
> [   58.470255]  ffff88082fd3fe80 0000000000000046 ffff880800000000 ffff88082fd3e010
> [   58.470279]  ffff88082fcee180 0000000000011440 ffff88082fd3ffd8 ffff88082fd3ffd8
> [   58.470301]  0000000000011440 0000000000000000 ffffffff8180b020 ffff88082fcee180
> [   58.470325] Call Trace:
> [   58.470332] <Same stack as pid 4>
> 
> changelog v6..v5:
> 1. clear saved stack trace before printing a set of stacks. this ensures the printed
> stack traces are not omitted messages.
> 2. add log level in printing duplicate stack.
> 3. remove the show_stack() API change, and non-x86 arch won't need further change.
> 4. add more inline documentations.
> 
> changelog v5..v4:
> 1. removed changes to Kconfig file
> 2. changed hashtable to keep only hash value and length of stack
> 3. simplified hashtable lookup
> 
> changelog v4..v3:
> 1. improve de-duplication by eliminating garbage entries from stack traces.
> with this change 793/825 stack traces were recognized as duplicates. in v3
> only 482/839 were duplicates.
> 
> changelog v3..v2:
> 1. again better documentation on the patch description.
> 2. make the stack_hash_table to be allocated at compile time.
> 3. have better name of variable index
> 4. move save_dup_stack_trace() in kernel/stacktrace.c
> 
> changelog v2..v1:
> 1. better documentation on the patch description
> 2. move the spinlock inside the hash lockup, so reducing the holding time.
> 
> Note:
> 1. with pid namespace, we might have same pid number for different processes. i
> wonder how the stack trace (w/o dedup) handles the case, it uses tsk->pid as well
> as far as I checked.
> 2. the core functionality is in x86-specific code, this could be moved out to
> support other architectures.
> 3. Andrew made the suggestion of doing appending to stack_hash_table[].
> 
> Signed-off-by: Ying Han <yinghan@google.com>
> Signed-off-by: Greg Thelen <gthelen@google.com>
> ---
>  arch/x86/include/asm/stacktrace.h |   11 +++-
>  arch/x86/kernel/dumpstack.c       |   24 ++++++-
>  arch/x86/kernel/dumpstack_32.c    |    7 +-
>  arch/x86/kernel/dumpstack_64.c    |    7 +-
>  arch/x86/kernel/stacktrace.c      |  123 +++++++++++++++++++++++++++++++++++++
>  include/linux/sched.h             |    3 +
>  include/linux/stacktrace.h        |    4 +
>  kernel/sched.c                    |   32 +++++++++-
>  kernel/stacktrace.c               |   15 +++++
>  9 files changed, 211 insertions(+), 15 deletions(-)
> 

Looks like something that would go through x86/debug?  Probably best to cc 
Ingo, Peter, and Thomas.

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