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

[HAGIO KAZUHITO(萩尾 一仁)]

Hi Hari,

-----Original Message-----
> 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.
Could you ack this? and my patch for the compiler warnings?

Thanks,
Kazu

> 
> Signed-off-by: Hari Bathini <hbathini at linux.ibm.com>
> ---
> 
> On a system with about 1500 CPUs 165K running tasks, it was taking
> about a day to get to the crash prompt without this patch, while it
> takes only about 5-10 mins with this change..
> 
> 
>  task.c |    8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/task.c b/task.c
> index 8dd2b96..423cd45 100644
> --- a/task.c
> +++ b/task.c
> @@ -713,6 +713,7 @@ irqstacks_init(void)
>  	} else
>  		error(WARNING, "cannot determine hardirq_ctx addresses\n");
> 
> +	/* TODO: Use multithreading to parallely update irq_tasks. */
>  	for (i = 0; i < NR_CPUS; i++) {
>  		if (!(tt->hardirq_ctx[i]))
>  			continue;
> @@ -5005,6 +5006,10 @@ pid_exists(ulong pid)
>  /*
>   *  Translate a stack pointer to a task, dealing with possible split.
>   *  If that doesn't work, check the hardirq_stack and softirq_stack.
> + *
> + * TODO: This function can be optimized by getting 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.
>   */
>  ulong
>  stkptr_to_task(ulong sp)
> @@ -5013,6 +5018,9 @@ stkptr_to_task(ulong sp)
>          struct task_context *tc;
>  	struct bt_info bt_info, *bt;
> 
> +	if (!sp)
> +		return NO_TASK;
> +
>  	bt = &bt_info;
>          tc = FIRST_CONTEXT();
>          for (i = 0; i < RUNNING_TASKS(); i++, tc++) {
>