x86, pat: Update the page flags for memtype atomically instead of using memtype_lock

While testing an application using the xpmem (out of kernel) driver, we
noticed a significant page fault rate reduction of x86_64 with respect
to ia64.  For one test running with 32 cpus, one thread per cpu, it
took 01:08 for each of the threads to vm_insert_pfn 2GB worth of pages.
For the same test running on 256 cpus, one thread per cpu, it took 14:48
to vm_insert_pfn 2 GB worth of pages.

The slowdown was tracked to lookup_memtype which acquires the
spinlock memtype_lock.  This heavily contended lock was slowing down
vm_insert_pfn().

With the cmpxchg on page->flags method, both the 32 cpu and 256 cpu
cases take approx 00:01.3 seconds to complete.

Signed-off-by: Robin Holt <holt@sgi.com>
LKML-Reference: <20100423153627.751194346@gulag1.americas.sgi.com>
Cc: Venkatesh Pallipadi <venkatesh.pallipadi@gmail.com>
Cc: Rafael Wysocki <rjw@novell.com>
Reviewed-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

2 files changed, 25 insertions, 27 deletions

diff --git a/arch/x86/include/asm/cacheflush.h b/arch/x86/include/asm/cacheflush.h
index 634c40a739a6..c70068d05f70 100644
--- a/arch/x86/include/asm/cacheflush.h
+++ b/arch/x86/include/asm/cacheflush.h
@@ -44,9 +44,6 @@ static inline void copy_from_user_page(struct vm_area_struct *vma,
         memcpy(dst, src, len);
 }
 
-#define PG_WC                           PG_arch_1
-PAGEFLAG(WC, WC)
-
 #ifdef CONFIG_X86_PAT
 /*
  * X86 PAT uses page flags WC and Uncached together to keep track of
@@ -55,16 +52,24 @@ PAGEFLAG(WC, WC)
  * _PAGE_CACHE_UC_MINUS and fourth state where page's memory type has not
  * been changed from its default (value of -1 used to denote this).
  * Note we do not support _PAGE_CACHE_UC here.
- *
- * Caller must hold memtype_lock for atomicity.
  */
+
+#define _PGMT_DEFAULT           0
+#define _PGMT_WC                (1UL << PG_arch_1)
+#define _PGMT_UC_MINUS          (1UL << PG_uncached)
+#define _PGMT_WB                (1UL << PG_uncached | 1UL << PG_arch_1)
+#define _PGMT_MASK              (1UL << PG_uncached | 1UL << PG_arch_1)
+#define _PGMT_CLEAR_MASK        (~_PGMT_MASK)
+
 static inline unsigned long get_page_memtype(struct page *pg)
 {
-        if (!PageUncached(pg) && !PageWC(pg))
+        unsigned long pg_flags = pg->flags & _PGMT_MASK;
+
+        if (pg_flags == _PGMT_DEFAULT)
                 return -1;
-        else if (!PageUncached(pg) && PageWC(pg))
+        else if (pg_flags == _PGMT_WC)
                 return _PAGE_CACHE_WC;
-        else if (PageUncached(pg) && !PageWC(pg))
+        else if (pg_flags == _PGMT_UC_MINUS)
                 return _PAGE_CACHE_UC_MINUS;
         else
                 return _PAGE_CACHE_WB;
@@ -72,25 +77,26 @@ static inline unsigned long get_page_memtype(struct page *pg)
 
 static inline void set_page_memtype(struct page *pg, unsigned long memtype)
 {
+        unsigned long memtype_flags = _PGMT_DEFAULT;
+        unsigned long old_flags;
+        unsigned long new_flags;
+
         switch (memtype) {
         case _PAGE_CACHE_WC:
-                ClearPageUncached(pg);
-                SetPageWC(pg);
+                memtype_flags = _PGMT_WC;
                 break;
         case _PAGE_CACHE_UC_MINUS:
-                SetPageUncached(pg);
-                ClearPageWC(pg);
+                memtype_flags = _PGMT_UC_MINUS;
                 break;
         case _PAGE_CACHE_WB:
-                SetPageUncached(pg);
-                SetPageWC(pg);
-                break;
-        default:
-        case -1:
-                ClearPageUncached(pg);
-                ClearPageWC(pg);
+                memtype_flags = _PGMT_WB;
                 break;
         }
+
+        do {
+                old_flags = pg->flags;
+                new_flags = (old_flags & _PGMT_CLEAR_MASK) | memtype_flags;
+        } while (cmpxchg(&pg->flags, old_flags, new_flags) != old_flags);
 }
 #else
 static inline unsigned long get_page_memtype(struct page *pg) { return -1; }
diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index 951011166ef5..501fc60e5e4d 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -190,8 +190,6 @@ static int pat_pagerange_is_ram(unsigned long start, unsigned long end)
  * Here we do two pass:
  * - Find the memtype of all the pages in the range, look for any conflicts
  * - In case of no conflicts, set the new memtype for pages in the range
- *
- * Caller must hold memtype_lock for atomicity.
  */
 static int reserve_ram_pages_type(u64 start, u64 end, unsigned long req_type,
                                   unsigned long *new_type)
@@ -297,9 +295,7 @@ int reserve_memtype(u64 start, u64 end, unsigned long req_type,
         is_range_ram = pat_pagerange_is_ram(start, end);
         if (is_range_ram == 1) {
 
-                spin_lock(&memtype_lock);
                 err = reserve_ram_pages_type(start, end, req_type, new_type);
-                spin_unlock(&memtype_lock);
 
                 return err;
         } else if (is_range_ram < 0) {
@@ -351,9 +347,7 @@ int free_memtype(u64 start, u64 end)
         is_range_ram = pat_pagerange_is_ram(start, end);
         if (is_range_ram == 1) {
 
-                spin_lock(&memtype_lock);
                 err = free_ram_pages_type(start, end);
-                spin_unlock(&memtype_lock);
 
                 return err;
         } else if (is_range_ram < 0) {
@@ -394,10 +388,8 @@ static unsigned long lookup_memtype(u64 paddr)
 
         if (pat_pagerange_is_ram(paddr, paddr + PAGE_SIZE)) {
                 struct page *page;
-                spin_lock(&memtype_lock);
                 page = pfn_to_page(paddr >> PAGE_SHIFT);
                 rettype = get_page_memtype(page);
-                spin_unlock(&memtype_lock);
                 /*
                  * -1 from get_page_memtype() implies RAM page is in its
                  * default state and not reserved, and hence of type WB