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>

1 files changed, 25 insertions, 19 deletions

diff --git a/arch/x86/include/asm/cacheflush.h b/arch/x86/include/asm/cacheflush.h
index 634c40a739a..c70068d05f7 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; }