[Crash-utility] [PATCH] task.c: avoid unnecessary cpu cycles during init

[lijiang]

在 2020年11月17日 01:00, crash-utility-request at redhat.com 写道:
>> While stkptr_to_task does the job of trying to match a stack pointer
>> to a task, it runs through each task's stack to find whether the given
>> SP falls into its range. This can be a very expensive operation, if
>> the vmcore is from a system running too many tasks. It can get even
>> worse when the total number of CPUs on the system is in the order of
>> thousands. Given the expensive nature of the operation, it must be
>> optimized as much as possible. Possible options to optimize:
>>
>>   1) Get min & max of the stack range in first pass and use these
>>      values against the given SP to decide whether or not to proceed
>>      with stack lookup.
>>   2) Use multithreading to parallely update irq_tasks.
>>   3) Skip stkptr_to_task() when SP is 0
>>
>> Though option 3 is a low hanging fruit, it significantly improved the
>> time taken between starting crash utility & reaching crash prompt.
>> Implement option 3 to optimize while listing the other two options
>> as TODO items for follow-up.
> It looks like on x86_64 the stkptr_to_task() is not called when starting
> session and I cannot test, but the patch looks safe enough.
> 
> Acked-by: Kazuhito Hagio <k-hagio-ab at nec.com>
> 
> Lianbo, Bhupesh, this is not a fix but huge improvement for some
> machines or situations, so I'd like to pick it up for crash-7.2.9.

Agree with you, this improvement looks good.

> Could you ack this? and my patch for the compiler warnings?
> 
Yes. After applying your patch, it doesn't reproduce any more.

Good findings, Kazu.

Thanks.
Lianbo

> Thanks,
> Kazu
>