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

Robin Holt
2013-07-23T11:15:51Z

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@zytor.com> 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