Re: [PATCH v5 04/13] mm/khugepaged: make hugepage allocation context-specific

["Zach O'Keefe" <zokeefe@google.com>]

On Wed, May 25, 2022 at 10:59 AM Yang Shi <shy828301@gmail.com> wrote:
>
> On Tue, May 17, 2022 at 3:35 PM Zach O'Keefe <zokeefe@google.com> wrote:
> >
> > On Tue, May 17, 2022 at 10:51 AM Yang Shi <shy828301@gmail.com> wrote:
> > >
> > > On Fri, May 13, 2022 at 4:56 PM Zach O'Keefe <zokeefe@google.com> wrote:
> > > >
> > > > On Fri, May 13, 2022 at 4:17 PM Yang Shi <shy828301@gmail.com> wrote:
> > > > >
> > > > > On Fri, May 13, 2022 at 4:05 PM Zach O'Keefe <zokeefe@google.com> wrote:
> > > > > >
> > > > > > Thanks for the review, David!
> > > > > >
> > > > > > On Thu, May 12, 2022 at 1:02 PM David Rientjes <rientjes@google.com> wrote:
> > > > > > >
> > > > > > > On Wed, 4 May 2022, Zach O'Keefe wrote:
> > > > > > >
> > > > > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > > > > > > index c94bc43dff3e..6095fcb3f07c 100644
> > > > > > > > --- a/mm/khugepaged.c
> > > > > > > > +++ b/mm/khugepaged.c
> > > > > > > > @@ -92,6 +92,10 @@ struct collapse_control {
> > > > > > > >
> > > > > > > >       /* Last target selected in khugepaged_find_target_node() */
> > > > > > > >       int last_target_node;
> > > > > > > > +
> > > > > > > > +     struct page *hpage;
> > > > > > > > +     int (*alloc_charge_hpage)(struct mm_struct *mm,
> > > > > > > > +                               struct collapse_control *cc);
> > > > > > > >  };
> > > > > > > >
> > > > > > > >  /**
> > > > > > >
> > > > > > > Embedding this function pointer into collapse_contol seems like it would
> > > > > > > need some pretty strong rationale.  Not to say that it should be a
> > > > > > > non-starter, but I think the changelog needs to clearly indicate why this
> > > > > > > is better/cleaner than embedding the needed info for a single allocation
> > > > > > > and charge function to use.  If the callbacks would truly be so different
> > > > > > > that unifying them would be more complex, I think this makes sense.
> > > > > >
> > > > > > Mostly, this boils down to khugepaged having different a allocation
> > > > > > pattern for NUMA/UMA ; the former scans the pages first to determine
> > > > > > the right node, the latter preallocates before scanning. khugepaged
> > > > > > has the luxury on UMA systems of just holding onto a hugepage
> > > > > > indefinitely for the next collapse target.
> > > > > >
> > > > > > For MADV_COLLAPSE, we never preallocate, and so its pattern doesn't
> > > > > > depend on NUMA or UMA configs. Trying to avoid "if (khugepaged) ...
> > > > > > else" casing, defining this as a context-defined operation seemed
> > > > > > appropriate.
> > > > > >
> > > > > > Collapsing both alloc and charging together was mostly a code
> > > > > > cleanliness decision resulting from not wanting to embed a ->gfp()
> > > > > > hook (gfp flags are used both by allocation and memcg charging).
> > > > > > Alternatively, a .gfp member could exist - it would just need to be
> > > > > > refreshed periodically in the khugepaged codepath.
> > > > > >
> > > > > > That all said - let me take another crack at seeing if I can make this
> > > > > > work without the need for a function pointer here.
> > > > >
> > > > > I had a patch that removed UMA allocation, please refer to
> > > > > https://lore.kernel.org/linux-mm/20210817202146.3218-1-shy828301@gmail.com/#t
> > > > >
> > > > > It was not made upstream due to some requests for further cleanup, but
> > > > > unfortunately I haven't got time to look into it yet.
> > > > >
> > > > > If this page were merged, would that make your life easier?
> > > >
> > > > Hey Yang,
> > > >
> > > > First, sorry for missing that patch in the first place. I actually
> > > > have some patches queued up that do a similar cleanup of
> > > > khugepaged_prealloc_page() that was mentioned, but decided to not
> > > > include them here.
> > > >
> > > > Second, removing the NUMA/UMA story does make this patch easier, I
> > > > think (esp since the sched change was dropped for now). This is
> > > > something I wanted while writing this series, but without the larger
> > > > context referenced in your patch (most users don't build NUMA=n even
> > > > on single node systems, and the pcp hugepage lists optimization)
> > > > couldn't justify myself.
> > >
> > > Thanks, it would be better to add this patch into your series as a
> > > prerequisite so that you could make the MADV_COLLAPSE easier.
> > >
> > > I don't think I will be able to find time to rework the patch and
> > > solve all the review comments at any time soon. If you'd like to take
> > > it, please do it.
> > >
> >
> > Sounds good, Yang. I'll take a crack at it for v6.
>
> Hi Zach,
>
> Fortunately I got some time, if you haven't spent too much effort in
> this, I could rework the patch then resubmit. Just let me know.
>

Hey Yang,

Thanks for letting me know. In my v6 series, I actually ended up
incorporating this work *afterwards*. Reason being there are some
patches in my series, like "mm/khugepaged: pipe enum scan_result codes
back to callers" which makes cleanup of khugepaged_scan_mm_slot() and
khugepaged_scan_mm_slot() easier, IMO[1].

I'll privately send what I have planned for v6 and we can discuss.

Thanks,
Zach

[1] https://lore.kernel.org/linux-mm/20220504214437.2850685-6-zokeefe@google.com/


> >
> > Thanks for taking the time,
> > Zach
> >
> > > >
> > > > Best,
> > > > Zach