[PATCH] mm: split page table lock
Christoph Lameter demonstrated very poor scalability on the SGI 512-way, with
a many-threaded application which concurrently initializes different parts of
a large anonymous area.
This patch corrects that, by using a separate spinlock per page table page, to
guard the page table entries in that page, instead of using the mm's single
page_table_lock. (But even then, page_table_lock is still used to guard page
table allocation, and anon_vma allocation.)
In this implementation, the spinlock is tucked inside the struct page of the
page table page: with a BUILD_BUG_ON in case it overflows - which it would in
the case of 32-bit PA-RISC with spinlock debugging enabled.
Splitting the lock is not quite for free: another cacheline access. Ideally,
I suppose we would use split ptlock only for multi-threaded processes on
multi-cpu machines; but deciding that dynamically would have its own costs.
So for now enable it by config, at some number of cpus - since the Kconfig
language doesn't support inequalities, let preprocessor compare that with
NR_CPUS. But I don't think it's worth being user-configurable: for good
testing of both split and unsplit configs, split now at 4 cpus, and perhaps
change that to 8 later.
There is a benefit even for singly threaded processes: kswapd can be attacking
one part of the mm while another part is busy faulting.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
diff --git a/mm/shmem.c b/mm/shmem.c
index 37777f4..dc25565 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -71,9 +71,6 @@
/* Pretend that each entry is of this size in directory's i_size */
#define BOGO_DIRENT_SIZE 20
-/* Keep swapped page count in private field of indirect struct page */
-#define nr_swapped private
-
/* Flag allocation requirements to shmem_getpage and shmem_swp_alloc */
enum sgp_type {
SGP_QUICK, /* don't try more than file page cache lookup */
@@ -324,8 +321,10 @@
entry->val = value;
info->swapped += incdec;
- if ((unsigned long)(entry - info->i_direct) >= SHMEM_NR_DIRECT)
- kmap_atomic_to_page(entry)->nr_swapped += incdec;
+ if ((unsigned long)(entry - info->i_direct) >= SHMEM_NR_DIRECT) {
+ struct page *page = kmap_atomic_to_page(entry);
+ set_page_private(page, page_private(page) + incdec);
+ }
}
/*
@@ -368,9 +367,8 @@
spin_unlock(&info->lock);
page = shmem_dir_alloc(mapping_gfp_mask(inode->i_mapping) | __GFP_ZERO);
- if (page) {
- page->nr_swapped = 0;
- }
+ if (page)
+ set_page_private(page, 0);
spin_lock(&info->lock);
if (!page) {
@@ -561,7 +559,7 @@
diroff = 0;
}
subdir = dir[diroff];
- if (subdir && subdir->nr_swapped) {
+ if (subdir && page_private(subdir)) {
size = limit - idx;
if (size > ENTRIES_PER_PAGE)
size = ENTRIES_PER_PAGE;
@@ -572,10 +570,10 @@
nr_swaps_freed += freed;
if (offset)
spin_lock(&info->lock);
- subdir->nr_swapped -= freed;
+ set_page_private(subdir, page_private(subdir) - freed);
if (offset)
spin_unlock(&info->lock);
- BUG_ON(subdir->nr_swapped > offset);
+ BUG_ON(page_private(subdir) > offset);
}
if (offset)
offset = 0;
@@ -743,7 +741,7 @@
dir = shmem_dir_map(subdir);
}
subdir = *dir;
- if (subdir && subdir->nr_swapped) {
+ if (subdir && page_private(subdir)) {
ptr = shmem_swp_map(subdir);
size = limit - idx;
if (size > ENTRIES_PER_PAGE)