On Tue, Jan 22, 2013 at 02:46:59PM -0800, Andrew Morton wrote:
> On Tue, 22 Jan 2013 17:12:41 +0000
> Mel Gorman <mgorman@xxxxxxx> wrote:
>
> > From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> >
> > page->_last_nid fits into page->flags on 64-bit. The unlikely 32-bit NUMA
> > configuration with NUMA Balancing will still need an extra page field.
> > As Peter notes "Completely dropping 32bit support for CONFIG_NUMA_BALANCING
> > would simplify things, but it would also remove the warning if we grow
> > enough 64bit only page-flags to push the last-cpu out."
>
> How much space remains in the 64-bit page->flags?
>
Good question.
There are 19 free bits in my configuration but it's related to
CONFIG_NODES_SHIFT which is 9 for me (512 nodes) and very heavily affected
by options such as CONFIG_SPARSEMEM_VMEMMAP. Memory hot-remove does not work
with CONFIG_SPARSEMEM_VMEMMAP and enterprise distribution configs may be
taking the performance hit to enable memory hot-remove. If I disable this
option to enable memory hot-remove then there are 0 free bits in page->flags.
Your milage will vary *considerably*.
In answering this question I remembered that mminit_loglevel is able to
answer these sort of questions but only if it's updated properly. I'll
post a follow-up patch.
> Was this the best possible use of the remaining space?
>
Another good question and I do not have a good answer. There is a definite
cost to having a larger struct page on large memory systems. The benefit
to saving flags on 64-bit page->flags for potential future use is more
intangiable.
> It's good that we can undo this later by flipping
> LAST_NID_NOT_IN_PAGE_FLAGS.
>
Yes and it generates a dirty warning if it's forced to use
LAST_NID_NOT_IN_PAGE_FLAGS.
> > [mgorman@xxxxxxx: Minor modifications]
> > Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
>
> Several of these patches are missing signoffs (Peter and Hugh).
>
In the case of Peter's patches, they changed enough that I couldn't preserve
the signed-off-by. This happened for the NUMA balancing patches too. I
preserved the "From" and I'm hoping he'll respond to add his Signed-off-by
to these patches if he's ok with them.
In Hugh's case he did not add his signed-off-by because he was not sure
whether there was a gremlin hidden in there. If there is, I was not able
to find it. It's up to him whether he wants to put his signed-off-by on
it but I preserved the "From:".
> >
> > ...
> >
> > +static inline int page_last_nid(struct page *page)
> > +{
> > + return (page->flags >> LAST_NID_PGSHIFT) & LAST_NID_MASK;
> > +}
> > +
> > +static inline int page_xchg_last_nid(struct page *page, int nid)
> > +{
> > + unsigned long old_flags, flags;
> > + int last_nid;
> > +
> > + do {
> > + old_flags = flags = page->flags;
> > + last_nid = page_last_nid(page);
> > +
> > + flags &= ~(LAST_NID_MASK << LAST_NID_PGSHIFT);
> > + flags |= (nid & LAST_NID_MASK) << LAST_NID_PGSHIFT;
> > + } while (unlikely(cmpxchg(&page->flags, old_flags, flags) != old_flags));
> > +
> > + return last_nid;
> > +}
> > +
> > +static inline void reset_page_last_nid(struct page *page)
> > +{
> > + page_xchg_last_nid(page, (1 << LAST_NID_SHIFT) - 1);
> > +}
>
> page_xchg_last_nid() and reset_page_last_nid() are getting nuttily
> large. Please investigate uninlining them?
>
Will do.
> reset_page_last_nid() is poorly named. page_reset_last_nid() would be
> better, and consistent.
>
Will fix.
--
Mel Gorman
SUSE Labs
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