[PATCH 2/2] lib/core/fiber: Allow to extend default stack size

[Vladimir Davydov]

On Tue, Mar 19, 2019 at 10:38:45PM +0300, Cyrill Gorcunov wrote:
> Recently we found that 64K stack we've been using
> for years is not longer enough due to third party
> libraries, so we extended its size.
> 
> Still there is no guarantee that even new size will
> lasts forever and users might face same problem again,
> waiting for new build where stack capacity increased.
> 
> Instead of this ping-pong game lets allow a user to
> setup default stack size via FIBER_STACK_SIZE_DEFAULT
> environment variable.
> 
> Note we fetch this value once on program startup and
> because it is early init stage we can't use box
> config engine.
> ---
>  src/lib/core/fiber.c | 78 ++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 68 insertions(+), 10 deletions(-)
> 
> diff --git a/src/lib/core/fiber.c b/src/lib/core/fiber.c
> index 922a0bfe8..d3cb18a15 100644
> --- a/src/lib/core/fiber.c
> +++ b/src/lib/core/fiber.c
> @@ -107,18 +107,25 @@ pthread_t main_thread_id;
>  static size_t page_size;
>  static bool stack_growsdown;
>  
> +/** Indices in fiber stack sizes (FSS) */
>  enum {
> +	FSS_IDX_MINIMAL,
> +	FSS_IDX_WATERMARK,
> +	FSS_IDX_DEFAULT,
> +};
> +
> +/** Fiber stack sizes */
> +static size_t fiber_stack_size[] = {
>  	/* The minimum allowable fiber stack size in bytes */
> -	FIBER_STACK_SIZE_MINIMAL = 16384,
> +	[FSS_IDX_MINIMAL]	= 16384,
> +	/* Stack size watermark in bytes */
> +	[FSS_IDX_WATERMARK]	= 65536,
>  	/* Default fiber stack size in bytes */
> -	FIBER_STACK_SIZE_DEFAULT = 524288,
> -	/* Stack size watermark in bytes. */
> -	FIBER_STACK_SIZE_WATERMARK = 65536,
> +	[FSS_IDX_DEFAULT]	= 524288,
>  };

Please avoid unrelated changes. If you want to rename or refactor
something, do it in a separate patch to ease review.

Anyway, I don't see much point in this change - in Tarantool code it's
common to use a enum for storing constants.

>  
>  /** Default fiber attributes */
> -static const struct fiber_attr fiber_attr_default = {
> -       .stack_size = FIBER_STACK_SIZE_DEFAULT,
> +static struct fiber_attr fiber_attr_default = {
>         .flags = FIBER_DEFAULT_FLAGS
>  };
>  
> @@ -172,13 +179,13 @@ fiber_attr_delete(struct fiber_attr *fiber_attr)
>  int
>  fiber_attr_setstacksize(struct fiber_attr *fiber_attr, size_t stack_size)
>  {
> -	if (stack_size < FIBER_STACK_SIZE_MINIMAL) {
> +	if (stack_size < fiber_stack_size[FSS_IDX_MINIMAL]) {
>  		errno = EINVAL;
>  		diag_set(SystemError, "stack size is too small");
>  		return -1;
>  	}
>  	fiber_attr->stack_size = stack_size;
> -	if (stack_size != FIBER_STACK_SIZE_DEFAULT) {
> +	if (stack_size != fiber_stack_size[FSS_IDX_DEFAULT]) {
>  		fiber_attr->flags |= FIBER_CUSTOM_STACK;
>  	} else {
>  		fiber_attr->flags &= ~FIBER_CUSTOM_STACK;
> @@ -844,11 +851,11 @@ fiber_stack_watermark_create(struct fiber *fiber)
>  	size_t offset = rand() % POISON_OFF * sizeof(poison_pool[0]);
>  	if (stack_growsdown) {
>  		fiber->stack_watermark  = fiber->stack + fiber->stack_size;
> -		fiber->stack_watermark -= FIBER_STACK_SIZE_WATERMARK;
> +		fiber->stack_watermark -= fiber_stack_size[FSS_IDX_WATERMARK];
>  		fiber->stack_watermark += offset;
>  	} else {
>  		fiber->stack_watermark  = fiber->stack;
> -		fiber->stack_watermark += FIBER_STACK_SIZE_WATERMARK;
> +		fiber->stack_watermark += fiber_stack_size[FSS_IDX_WATERMARK];
>  		fiber->stack_watermark -= page_size;
>  		fiber->stack_watermark += offset;
>  	}
> @@ -1383,6 +1390,56 @@ cord_slab_cache(void)
>  	return &cord()->slabc;
>  }
>  
> +/**
> + * Initialize early parameters of the stack
> + * and setup default variables.
> + */
> +static void
> +fiber_stack_init(void)
> +{
> +	static const char default_size_name[] = "FIBER_STACK_SIZE_DEFAULT";

There's a risk it may conflict with another environment variable.
At the very least, we should prefix it with TARANTOOL_.

However, I don't think that using an environmental variable is a proper
way to go. After all, Tarantool is an application server so we typically
set system variables right from Lua code.

Can we introduce a function in the fiber module for this? The tricky
part is that it should change the stack size of the calling fiber,
I guess.