Re: [PATCH] mm: sparse: pass section_nr to section_mark_present

[David Hildenbrand]

On 01.07.21 17:41, 권오훈 wrote:
> On Thu, Jul 01, 2021 at 04:34:13PM +0200, David Hildenbrand wrote:
> > On 01.07.21 15:55, 권오훈 wrote:
> > > With CONFIG_SPARSEMEM_EXTREME enabled, __section_nr() which converts
> > > mem_section to section_nr could be costly since it iterates all
> > > sections to check if the given mem_section is in its range.
> >   
> > It actually iterates all section roots.
> >   
> > > On the other hand, __nr_to_section which converts section_nr to
> > > mem_section can be done in O(1).
> > >
> > > Let's pass section_nr instead of mem_section ptr to section_mark_present
> > > in order to reduce needless iterations.
> >   
> > I'd expect this to be mostly noise, especially as we iterate section
> > roots and for most (smallish) machines we might just work on the lowest
> > section roots only.
> >   
> > Can you actually observe an improvement regarding boot times?
> >   
> > Anyhow, looks straight forward to me, although we might just reintroduce
> > similar patterns again easily if it's really just noise (see
> > find_memory_block() as used by). And it might allow for a nice cleanup
> > (see below).
> >   
> > Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
> >   
> >   
> > Can you send 1) a patch to convert find_memory_block() as well and 2) a
> > patch to rip out __section_nr() completely?
> >   
> > > Signed-off-by: Ohhoon Kwon <ohoono.kwon@xxxxxxxxxxx>
> > > ---
> > >    mm/sparse.c | 9 +++++----
> > >    1 file changed, 5 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/mm/sparse.c b/mm/sparse.c
> > > index 55c18aff3e42..4a2700e9a65f 100644
> > > --- a/mm/sparse.c
> > > +++ b/mm/sparse.c
> > > @@ -186,13 +186,14 @@ void __meminit mminit_validate_memmodel_limits(unsigned long *start_pfn,
> > >     * those loops early.
> > >     */
> > >    unsigned long __highest_present_section_nr;
> > > -static void section_mark_present(struct mem_section *ms)
> > > +static void section_mark_present(unsigned long section_nr)
> > >    {
> > > -        unsigned long section_nr = __section_nr(ms);
> > > +        struct mem_section *ms;
> > >    
> > >            if (section_nr > __highest_present_section_nr)
> > >                    __highest_present_section_nr = section_nr;
> > >    
> > > +        ms = __nr_to_section(section_nr);
> > >            ms->section_mem_map |= SECTION_MARKED_PRESENT;
> > >    }
> > >    
> > > @@ -279,7 +280,7 @@ static void __init memory_present(int nid, unsigned long start, unsigned long en
> > >                    if (!ms->section_mem_map) {
> > >                            ms->section_mem_map = sparse_encode_early_nid(nid) |
> > >                                                            SECTION_IS_ONLINE;
> > > -                        section_mark_present(ms);
> > > +                        section_mark_present(section);
> > >                    }
> > >            }
> > >    }
> > > @@ -933,7 +934,7 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
> > >    
> > >            ms = __nr_to_section(section_nr);
> > >            set_section_nid(section_nr, nid);
> > > -        section_mark_present(ms);
> > > +        section_mark_present(section_nr);
> > >    
> > >            /* Align memmap to section boundary in the subsection case */
> > >            if (section_nr_to_pfn(section_nr) != start_pfn)
> >   
> >   
> > --
> > Thanks,
> >   
> > David / dhildenb
> >   
> Dear David.
>
> I tried to check on time for memblocks_present, but when I tested with mobile
> phones with 8GB ram, the original binary took 0us either as well as the
> patched binary.
> I'm not sure how the results would differ on huge systems with bigger ram.
> I agree that it could turn out to be just a noise, as you expected.
>
> However as you also mentioned, the patches will be straight forward when all
> codes using __section_nr() are cleaned up nicely.
>
> Below are the two patches that you asked for.
> Please tell me if you need me to send the patches in separate e-mails.

Yes, please send them separately. Maybe sent all 3 patches combined in a 
single series, so Andrew can pick them easily, and reviewers can review 
more easily.

Thanks!

--
Thanks,

David / dhildenb