[PATCH V4 0/4] Reducing parameters of alloc_pages* family of functions

[Vlastimil Babka]

Changes since v3:
o Moved struct alloc_context definition to mm/internal.h
o Rebased on latest -next and re-measured. Sadly, the code/stack size
  improvements are smaller with the new baseline.

The possibility of replacing the numerous parameters of alloc_pages* functions
with a single structure has been discussed when Minchan proposed to expand the
x86 kernel stack [1]. This series implements the change, along with few more
cleanups/microoptimizations.

The series is based on next-20150105 and I used gcc 4.8.3 20140627 on openSUSE
13.2 for compiling. Config includess NUMA and COMPACTION.

The core change is the introduction of a new struct alloc_context, which looks
like this:

struct alloc_context {
        struct zonelist *zonelist;
        nodemask_t *nodemask;
        struct zone *preferred_zone;
        int classzone_idx;
        int migratetype;
        enum zone_type high_zoneidx;
};

All the contents is mostly constant, except that __alloc_pages_slowpath()
changes preferred_zone, classzone_idx and potentially zonelist. But that's not
a problem in case control returns to retry_cpuset: in __alloc_pages_nodemask(),
those will be reset to initial values again (although it's a bit subtle).
On the other hand, gfp_flags and alloc_info mutate so much that it doesn't
make sense to put them into alloc_context. Still, the result is one parameter
instead of up to 7. This is all in Patch 2.

Patch 3 is a step to expand alloc_context usage out of page_alloc.c itself.
The function try_to_compact_pages() can also much benefit from the parameter
reduction, but it means the struct definition has to be moved to a shared
header.

Patch 1 should IMHO be included even if the rest is deemed not useful enough.
It improves maintainability and also has some code/stack reduction. Patch 4
is OTOH a tiny optimization.

Overall bloat-o-meter results:

add/remove: 0/1 grow/shrink: 1/3 up/down: 1587/-1941 (-354)
function                                     old     new   delta
__alloc_pages_nodemask                       589    2176   +1587
nr_free_zone_pages                           129     115     -14
__alloc_pages_direct_compact                 329     256     -73
get_page_from_freelist                      2670    2576     -94
__alloc_pages_slowpath                      1760       -   -1760
try_to_compact_pages                         582     579      -3

Overal bloat-o-meter with forced inline in baseline, for fair comparison:

add/remove: 0/0 grow/shrink: 0/4 up/down: 0/-512 (-512)
function                                     old     new   delta
nr_free_zone_pages                           129     115     -14
__alloc_pages_direct_compact                 329     256     -73
get_page_from_freelist                      2670    2576     -94
__alloc_pages_nodemask                      2507    2176    -331
try_to_compact_pages                         582     579      -3

Overall stack sizes per ./scripts/checkstack.pl:

                          old   new delta
__alloc_pages_slowpath    152     -  -152
get_page_from_freelist:   184   184     0
__alloc_pages_nodemask    120   184   +64
__alloc_pages_direct_c     40    40   -40  
try_to_compact_pages       72    72     0
                                     -128

Again with forced inline on baseline:

                          old   new delta
get_page_from_freelist:   184   184     0
__alloc_pages_nodemask    216   184   -32
__alloc_pages_direct_c     40     -   -40  
try_to_compact_pages       72    72     0
                                      -72

[1] http://marc.info/?l=linux-mm&m=140142462528257&w=2

Vlastimil Babka (4):
  mm: set page->pfmemalloc in prep_new_page()
  mm, page_alloc: reduce number of alloc_pages* functions' parameters
  mm: reduce try_to_compact_pages parameters
  mm: microoptimize zonelist operations

 include/linux/compaction.h |  17 ++--
 include/linux/mmzone.h     |  13 +--
 mm/compaction.c            |  23 ++---
 mm/internal.h              |  14 +++
 mm/mmzone.c                |   4 +-
 mm/page_alloc.c            | 245 +++++++++++++++++++--------------------------
 6 files changed, 144 insertions(+), 172 deletions(-)

-- 
2.1.2

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