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

Robin Holt
2013-07-23T11:41:52Z

On Tue, Jul 23, 2013 at 06:15:49AM -0500, Robin Holt wrote:
> 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.

I don't have access to the 16TiB system until Thursday unless the other
testing on it fails early. I did boot a 2TiB system with the a change
which set the Unitialized_2m flag on all pages in that 2MiB range
during memmap_init_zone. That makes the expand check test against
the referenced page instead of having to go back to the 2MiB page.
It appears to have added less than a second to the 2TiB boot so I hope
it has equally little impact to the 16TiB boot.

I will clean up this patch some more and resend the currently untested
set later today.

Thanks,
Robin

>
> 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