Hey all,
this is a V2 of attempting something that has been discussed when Minchan
proposed to expand the x86 kernel stack [1], namely the reduction of huge
number of parameters that the alloc_pages* family and get_page_from_freelist()
functions have.
The result is this series, ordered in the subjective importance of the patches.
Note that it's only build-tested and considered RFC. I would like some feedback
whether this is worth finishing and posting properly, and I welcome testing
with different configs/arches/gcc versions. I also welcome feedback and
suggestions on the evaluation methodology as this probably doesn't tell the
whole picture.
The series is based on mmotm-2014-12-02-15-55 and I use gcc 4.8.3 20140627 on
openSUSE 13.2. Config includess NUMA and COMPACTION, I can post whole if
needed.
The core is a new struct alloc_context, which looks like this:
struct alloc_context {
struct zonelist *zonelist;
nodemask_t *nodemask;
struct zone *preferred_zone;
unsigned int order;
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 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.
First, let's look at the code size savings by bloat-o-meter, as the patches
stack up:
Patch 1 (vs mmotm):
function old new delta
get_page_from_freelist 2554 2490 -64
Patch 2:
function old new delta
__alloc_pages_nodemask 571 2220 +1649
get_page_from_freelist 2490 2560 +70
__alloc_pages_direct_compact 332 302 -30
__alloc_pages_slowpath 1878 - -1878
Here gcc decided to inline _slowpath, so let's try comparing with Patch 1
plus forced inline of _slowpath:
add/remove: 0/0 grow/shrink: 1/2 up/down: 70/-428 (-358)
function old new delta
get_page_from_freelist 2490 2560 +70
__alloc_pages_direct_compact 332 302 -30
__alloc_pages_nodemask 2618 2220 -398
Looks like get_page_from_freelist() did benefit from getting the parameters
separately, but overal it's a win.
Patch 3:
__alloc_pages_direct_compact 302 254 -48
try_to_compact_pages 582 598 +16
A tiny overal win.
Patch 4:
function old new delta
__alloc_pages_nodemask 2220 2217 -3
nr_free_zone_pages 129 115 -14
get_page_from_freelist 2560 2521 -39
try_to_compact_pages 598 592 -6
Small but clear win. A few more object files should be also affected
but were not tested.
Now stack sizes per ./scripts/checkstack.pl:
mmotm P1 P2
__alloc_pages_slowpath 176 176 -
get_page_from_freelist: 160 152 160
__alloc_pages_nodemask 104 104 168
__alloc_pages_direct_c 32 32 24
Patch 1 saves a bit, Patch 2 result muddled by inlining.
Again, let's use Patch 1 + forced inline as baseline for the rest:
P1i P2 P3 P4
__alloc_pages_nodemask 240 168 168 168
get_page_from_freelist: 152 160 160 160
try_to_compact_pages 64 64 64 64
__alloc_pages_direct_c 32 24 - -
Again, Patch 2 bloats get_page_from_freelist(), but overal is a win. The
rest doesn't affect stack usage.
[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 | 14 ++-
include/linux/mm.h | 11 ++
include/linux/mmzone.h | 12 +--
mm/compaction.c | 16 +--
mm/mmzone.c | 4 +-
mm/page_alloc.c | 256 ++++++++++++++++++---------------------------
6 files changed, 136 insertions(+), 177 deletions(-)
--
2.1.2
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