Re: [PATCH v2] vmstat: Keep count of the maximum page reached by the kernel stack

[Andrew Morton]

On Thu, 14 Mar 2024 14:54:57 +0000 Pasha Tatashin <pasha.tatashin@xxxxxxxxxx> wrote:

> CONFIG_DEBUG_STACK_USAGE provides a mechanism to determine the minimum
> amount of memory left in a stack. Every time a new low-memory record is
> reached, a message is printed to the console.
> 
> However, this doesn't reveal how many pages within each stack were
> actually used. Introduce a mechanism that keeps count the number of
> times each of the stack's pages were reached:
> 
> 	$ grep kstack /proc/vmstat
> 	kstack_page_1 19974
> 	kstack_page_2 94
> 	kstack_page_3 0
> 	kstack_page_4 0
> 
> In the above example, out of 20,068 threads that exited on this
> machine, only 94 reached the second page of their stack, and none
> touched pages three or four.
> 
> In fleet environments with millions of machines, this data can help
> optimize kernel stack sizes.

We really should have somewhere to document vmstat things.

> --- a/include/linux/vm_event_item.h
> +++ b/include/linux/vm_event_item.h
> @@ -153,10 +153,39 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>  		VMA_LOCK_ABORT,
>  		VMA_LOCK_RETRY,
>  		VMA_LOCK_MISS,
> +#endif
> +#ifdef CONFIG_DEBUG_STACK_USAGE
> +		KSTACK_PAGE_1,
> +		KSTACK_PAGE_2,
> +#if THREAD_SIZE >= (4 * PAGE_SIZE)
> +		KSTACK_PAGE_3,
> +		KSTACK_PAGE_4,
> +#endif
> +#if THREAD_SIZE > (4 * PAGE_SIZE)
> +		KSTACK_PAGE_REST,
> +#endif
>  #endif
>  		NR_VM_EVENT_ITEMS
>  };

This seems a rather cumbersome way to produce a kind of histogram.  I
wonder if there should be a separate pseudo file for this.

And there may be a call for extending this.  For example I can forsee
people wanting to know "hey, which process did that", in which case
we'll want to record additional info.