[PATCH V3 0/4] Reducing parameters of alloc_pages* family of functions

From: Vlastimil Babka
Date: Fri Dec 12 2014 - 11:13:48 EST

Hey all,

this is a V3 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.

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. It also applies to

Changes since v2:
o Per Linus's suggestion, removed order from alloc_context due to poor
conceptual fit. Interestingly, this improved the stack/code size a bit.
o Attempted to merge migratetype into alloc_flags, but it was ugly and made
things only worse. So I didn't even attempt to merge order there.

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;
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

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 332 284 -48
__alloc_pages_nodemask 2618 2194 -424

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 284 256 -28
try_to_compact_pages 582 566 -16

A tiny overal win.

Patch 4:

function old new delta
__alloc_pages_nodemask 2194 2191 -3
nr_free_zone_pages 129 115 -14
get_page_from_freelist 2560 2505 -55
try_to_compact_pages 566 579 +13

Small overal 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 16

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 56 56
__alloc_pages_direct_c 32 16 - -

Again, Patch 2 bloats get_page_from_freelist(), but overal is a win. The
rest almost 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 | 15 ++-
include/linux/mm.h | 14 +++
include/linux/mmzone.h | 13 +--
mm/compaction.c | 23 ++---
mm/mmzone.c | 4 +-
mm/page_alloc.c | 250 +++++++++++++++++++--------------------------
6 files changed, 144 insertions(+), 175 deletions(-)


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