Re: [PATCH][RFC] 4K stacks default, not a debug thing any more...?

Rene Herman
2007-07-14T22:20:20Z

On 07/14/2007 09:17 PM, Matt Mackall wrote:

> On Fri, Jul 13, 2007 at 03:20:54PM +0200, Rene Herman wrote:

>> As far as I'm aware, the actual reason for 4K stacks is that after the
>> system has been up and running for some time getting "1 physically
>> contiguous pages" becomes significantly easier than 2 which wouldn't be
>> arbitrary.
>
> If there are exactly two free pages in the system, the odds of them
> being buddies (ie adjacent AND properly aligned) is quite small. The
> available page pool has to grow quite a bit before the availability of
> order-1 page pairs approaches 100%.
>
> So if we fail to allocate an 8k stack when we could have allocated a
> 4k stack, we're almost certainly failing significantly prematurely.

Quite. Ofcourse, saying "our stacks are 1 page" would be the by far easiest
solution to that. Personally, I've been running with 4K stacks exclusively
on a variety of machines for quite some time now, but I can't say I'm all
too adventurous with respect to filesystems (especially) so I'm not sure how
many problems remain with 4K stacks. I did recently see Andrew Morton say
that problems _do_ still exist. If it's just XFS -- well, heck...

Moreover though, rather than 4K, the issue is "single page" stacks meaning a
larger (soft-) pagesize would seem to fix things nicely. I've been reading
about that on this list off and on for some time -- no idea where that
stands though.

> As I've pointed out before, it's fairly easy to make our stack
> growable with a trampoline in the troublesome paths. Something like:
>
> int growstack(int headroom, int func, void *data)
> {
> void *new_stack;
> int ret;
>
> if (likely(available_stack() > headroom))
> return func(data);
>
> #ifdef CONFIG_GROWSTACK_STATS
> /* gather statistics about stack-heavy paths */
> #endif
> /* warn/abort if we're recursing too deeply */
>
> new_stack = get_free_page();
> switch_to_new_stack(new_stack);
> ret = func(data);
> cleanup_stack(new_stack);
> return ret;
> }

This would also need something to tell func() where its current_thread_info
is now at. Which might not be much of a problem. Can't think of much else
either but it's the kind of thing you'd _like_ to be a problem just to have
an excuse to shoot down an icky notion like that...

Would you intend this just as a "make this path work until we fix it
properly" kind of thing?

Rene.