Re: 2.6.8.1-mm2

[Hans Reiser]

Andrew Morton wrote:

> Hans Reiser <reiser@xxxxxxxxxxx> wrote:
>  
>
> > John Cherry wrote:
> >
> >    
> >
> > > The new "errors" are from reiser4 code and they all appear to be...
> > >
> > > fs/reiser4/reiser4.h:18:2: #error "Please turn 4k stack off"
> > >
> > >
> > >
> > >      
> > zam, can you or Mr. Demidov work on using kmalloc to reduce stack usage?
> >
> > Andrew suggested that for statically sized objects kmalloc is quite fast 
> > (one instruction I think he said), so my objection to kmallocing a lot 
> > has faded.
> >    
>
> err, not that quick - but it's pretty quick.
>
> With a kmalloc with a constant size and, preferably, a constant gfp mask
> we'll jump directly into __cache_alloc() and in the common case we'll pluck
> an entry directly out of the cpu-local head array:
>
> So the kmalloc fastpath is, effectively:
>
> 	local_irq_save(save_flags);
> 	ac = ac_data(cachep);
> 	if (likely(ac->avail)) {
> 		ac->touched = 1;
> 		objp = ac_entry(ac)[--ac->avail];
> 	}
> 	local_irq_restore(save_flags);
> 	return objp;
>
>
> Not bad...
>
>
>  
but not trivial.  Sigh.  It means determining whether we can get below 
4k without performance loss requires detailed code examination to 
determine what is using up the stack in practice, and discussion.

Well, maybe zam has a comment.

Hans
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/