Re: [RFC 4/4] Sparse initialization of struct page array.

[Robin Holt]

I think the other critical path which is affected is in expand().
There, we just call ensure_page_is_initialized() blindly which does
the check against the other page.  The below is a nearly zero addition.
Sorry for the confusion.  My morning coffee has not kicked in yet.

Robin

On Tue, Jul 23, 2013 at 06:09:47AM -0500, Robin Holt wrote:
> On Tue, Jul 23, 2013 at 10:32:11AM +0200, Ingo Molnar wrote:
> > 
> > * H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> > 
> > > On 07/15/2013 11:26 AM, Robin Holt wrote:
> > >
> > > > Is there a fairly cheap way to determine definitively that the struct 
> > > > page is not initialized?
> > > 
> > > By definition I would assume no.  The only way I can think of would be 
> > > to unmap the memory associated with the struct page in the TLB and 
> > > initialize the struct pages at trap time.
> > 
> > But ... the only fastpath impact I can see of delayed initialization right 
> > now is this piece of logic in prep_new_page():
> > 
> > @@ -903,6 +964,10 @@ static int prep_new_page(struct page *page, int order, gfp_t gfp_flags)
> > 
> >         for (i = 0; i < (1 << order); i++) {
> >                 struct page *p = page + i;
> > +
> > +               if (PageUninitialized2Mib(p))
> > +                       expand_page_initialization(page);
> > +
> >                 if (unlikely(check_new_page(p)))
> >                         return 1;
> > 
> > That is where I think it can be made zero overhead in the 
> > already-initialized case, because page-flags are already used in 
> > check_new_page():
> 
> The problem I see here is that the page flags we need to check for the
> uninitialized flag are in the "other" page for the page aligned at the
> 2MiB virtual address, not the page currently being referenced.
> 
> Let me try a version of the patch where we set the PG_unintialized_2m
> flag on all pages, including the aligned pages and see what that does
> to performance.
> 
> Robin
> 
> > 
> > static inline int check_new_page(struct page *page)
> > {
> >         if (unlikely(page_mapcount(page) |
> >                 (page->mapping != NULL)  |
> >                 (atomic_read(&page->_count) != 0)  |
> >                 (page->flags & PAGE_FLAGS_CHECK_AT_PREP) |
> >                 (mem_cgroup_bad_page_check(page)))) {
> >                 bad_page(page);
> >                 return 1;
> > 
> > see that PAGE_FLAGS_CHECK_AT_PREP flag? That always gets checked for every 
> > struct page on allocation.
> > 
> > We can micro-optimize that low overhead to zero-overhead, by integrating 
> > the PageUninitialized2Mib() check into check_new_page(). This can be done 
> > by adding PG_uninitialized2mib to PAGE_FLAGS_CHECK_AT_PREP and doing:
> > 
> > 
> > 	if (unlikely(page->flags & PAGE_FLAGS_CHECK_AT_PREP)) {
> > 		if (PageUninitialized2Mib(p))
> > 			expand_page_initialization(page);
> > 		...
> > 	}
> > 
> >         if (unlikely(page_mapcount(page) |
> >                 (page->mapping != NULL)  |
> >                 (atomic_read(&page->_count) != 0)  |
> >                 (mem_cgroup_bad_page_check(page)))) {
> >                 bad_page(page);
> > 
> >                 return 1;
> > 
> > this will result in making it essentially zero-overhead, the 
> > expand_page_initialization() logic is now in a slowpath.
> > 
> > Am I missing anything here?
> > 
> > Thanks,
> > 
> > 	Ingo
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