Re: [PATCH v7 0/5] Free the 2nd vmemmap page associated with each HugeTLB page

[Muchun Song]

On Wed, Jan 26, 2022 at 4:04 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>
> On Wed, Nov 24, 2021 at 11:09 AM Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Mon, 22 Nov 2021 12:21:32 +0800 Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> >
> > > On Wed, Nov 10, 2021 at 2:18 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> > > >
> > > > On Tue, Nov 9, 2021 at 3:33 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> > > > >
> > > > > On 11/8/21 12:16 AM, Muchun Song wrote:
> > > > > > On Mon, Nov 1, 2021 at 11:22 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> > > > > >>
> > > > > >> This series can minimize the overhead of struct page for 2MB HugeTLB pages
> > > > > >> significantly. It further reduces the overhead of struct page by 12.5% for
> > > > > >> a 2MB HugeTLB compared to the previous approach, which means 2GB per 1TB
> > > > > >> HugeTLB. It is a nice gain. Comments and reviews are welcome. Thanks.
> > > > > >>
> > > > > >
> > > > > > Hi,
> > > > > >
> > > > > > Ping guys. Does anyone have any comments or suggestions
> > > > > > on this series?
> > > > > >
> > > > > > Thanks.
> > > > > >
> > > > >
> > > > > I did look over the series earlier.  I have no issue with the hugetlb and
> > > > > vmemmap modifications as they are enhancements to the existing
> > > > > optimizations.  My primary concern is the (small) increased overhead
> > > > > for the helpers as outlined in your cover letter.  Since these helpers
> > > > > are not limited to hugetlb and used throughout the kernel, I would
> > > > > really like to get comments from others with a better understanding of
> > > > > the potential impact.
> > > >
> > > > Thanks Mike. I'd like to hear others' comments about this as well.
> > > > From my point of view, maybe the (small) overhead is acceptable
> > > > since it only affects the head page, however Matthew Wilcox's folio
> > > > series could reduce this situation as well.
> >
> > I think Mike was inviting you to run some tests to quantify the
> > overhead ;)
>
> Hi Andrew,
>
> Sorry for the late reply.
>
> Specific overhead figures are already in the cover letter. Also,
> I did some other tests, e.g. kernel compilation, sysbench. I didn't
> see any regressions.

The overhead is introduced by page_fixed_fake_head() which
has an "if" statement and an access to a possible cold cache line.
I think the main overhead is from the latter. However, probabilistically,
only 1/64 of the pages need to do the latter.  And
page_fixed_fake_head() is already simple (I mean the overhead
is small enough) and many performance bottlenecks in mm are
not in compound_head().  This also matches the tests I did.
I didn't see any regressions after enabling this feature.

I knew Mike's concern is the increased overhead to use cases
beyond HugeTLB. If we really want to avoid the access to
a possible cold cache line, we can introduce a new page
flag like PG_hugetlb and test if it is set in the page->flags,
if so, then return the read head page struct. Then
page_fixed_fake_head() looks like below.

static __always_inline const struct page *page_fixed_fake_head(const
struct page *page)
{
        if (!hugetlb_free_vmemmap_enabled())
                return page;

        if (test_bit(PG_hugetlb, &page->flags)) {
                unsigned long head = READ_ONCE(page[1].compound_head);

                if (likely(head & 1))
                        return (const struct page *)(head - 1);
        }
        return page;
}

But I don't think it's worth doing this.

Hi Mike and Andrew,

Since these helpers are not limited to hugetlb and used throughout the
kernel, I would really like to get comments from others with a better
understanding of the potential impact. Do you have any appropriate
reviewers to invite?

Thanks.
>
> >
> > > Ping guys.
> > >
> > > Hi Andrew,
> > >
> > > Do you have any suggestions on this series to move it on?
> > >
> >
> > I tossed it in there for some testing but yes please, additional
> > reviewing?
>
> It's already been in the next-tree (also in our ByteDance servers)
> for several months, and I didn't receive any negative feedback.
>
> Do you think it is ready for 5.17?
>
> Thanks.