Subject: Re: [PATCH V4 0/2] mm: FAULT_AROUND_ORDER patchset performance data for powerpc

From: "Kirill A. Shutemov" <[email-masked]>
Date: Tue, 20 May 2014 13:27:38 +0300 (EEST)

Rusty Russell wrote:
> "Kirill A. Shutemov" <[email-masked]> writes:
> > Andrew Morton wrote:
> >> On Mon, 19 May 2014 16:23:07 -0700 (PDT) Hugh Dickins <[email-masked]> wrote:
> >> 
> >> > Shouldn't FAULT_AROUND_ORDER and fault_around_order be changed to be
> >> > the order of the fault-around size in bytes, and fault_around_pages()
> >> > use 1UL << (fault_around_order - PAGE_SHIFT)
> >> 
> >> Yes.  And shame on me for missing it (this time!) at review.
> >> 
> >> There's still time to fix this.  Patches, please.
> >
> > Here it is. Made at 3.30 AM, build tested only.
> 
> Prefer on top of Maddy's patch which makes it always a variable, rather
> than CONFIG_DEBUG_FS.  It's got enough hair as it is.

Something like this?

From: "Kirill A. Shutemov" <[email-masked]>
Date: Tue, 20 May 2014 13:02:03 +0300
Subject: [PATCH] mm: nominate faultaround area in bytes rather then page order

There are evidences that faultaround feature is less relevant on
architectures with page size bigger then 4k. Which makes sense since
page fault overhead per byte of mapped area should be less there.

Let's rework the feature to specify faultaround area in bytes instead of
page order. It's 64 kilobytes for now.

The patch effectively disables faultaround on architectures with
page size >= 64k (like ppc64).

It's possible that some other size of faultaround area is relevant for a
platform. We can expose `fault_around_bytes' variable to arch-specific
code once such platforms will be found.

Signed-off-by: Kirill A. Shutemov <[email-masked]>
---
 mm/memory.c | 62 +++++++++++++++++++++++--------------------------------------
 1 file changed, 23 insertions(+), 39 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 037b812a9531..252b319e8cdf 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3402,63 +3402,47 @@ void do_set_pte(struct vm_area_struct *vma, unsigned long address,
 	update_mmu_cache(vma, address, pte);
 }
 
-#define FAULT_AROUND_ORDER 4
+static unsigned long fault_around_bytes = 65536;
+
+static inline unsigned long fault_around_pages(void)
+{
+	return rounddown_pow_of_two(fault_around_bytes) / PAGE_SIZE;
+}
+
+static inline unsigned long fault_around_mask(void)
+{
+	return ~(rounddown_pow_of_two(fault_around_bytes) - 1) & PAGE_MASK;
+}
 
-#ifdef CONFIG_DEBUG_FS
-static unsigned int fault_around_order = FAULT_AROUND_ORDER;
 
-static int fault_around_order_get(void *data, u64 *val)
+#ifdef CONFIG_DEBUG_FS
+static int fault_around_bytes_get(void *data, u64 *val)
 {
-	*val = fault_around_order;
+	*val = fault_around_bytes;
 	return 0;
 }
 
-static int fault_around_order_set(void *data, u64 val)
+static int fault_around_bytes_set(void *data, u64 val)
 {
-	BUILD_BUG_ON((1UL << FAULT_AROUND_ORDER) > PTRS_PER_PTE);
-	if (1UL << val > PTRS_PER_PTE)
+	if (val / PAGE_SIZE > PTRS_PER_PTE)
 		return -EINVAL;
-	fault_around_order = val;
+	fault_around_bytes = val;
 	return 0;
 }
-DEFINE_SIMPLE_ATTRIBUTE(fault_around_order_fops,
-		fault_around_order_get, fault_around_order_set, "%llu\n");
+DEFINE_SIMPLE_ATTRIBUTE(fault_around_bytes_fops,
+		fault_around_bytes_get, fault_around_bytes_set, "%llu\n");
 
 static int __init fault_around_debugfs(void)
 {
 	void *ret;
 
-	ret = debugfs_create_file("fault_around_order",	0644, NULL, NULL,
-			&fault_around_order_fops);
+	ret = debugfs_create_file("fault_around_bytes", 0644, NULL, NULL,
+			&fault_around_bytes_fops);
 	if (!ret)
-		pr_warn("Failed to create fault_around_order in debugfs");
+		pr_warn("Failed to create fault_around_bytes in debugfs");
 	return 0;
 }
 late_initcall(fault_around_debugfs);
-
-static inline unsigned long fault_around_pages(void)
-{
-	return 1UL << fault_around_order;
-}
-
-static inline unsigned long fault_around_mask(void)
-{
-	return ~((1UL << (PAGE_SHIFT + fault_around_order)) - 1);
-}
-#else
-static inline unsigned long fault_around_pages(void)
-{
-	unsigned long nr_pages;
-
-	nr_pages = 1UL << FAULT_AROUND_ORDER;
-	BUILD_BUG_ON(nr_pages > PTRS_PER_PTE);
-	return nr_pages;
-}
-
-static inline unsigned long fault_around_mask(void)
-{
-	return ~((1UL << (PAGE_SHIFT + FAULT_AROUND_ORDER)) - 1);
-}
 #endif
 
 static void do_fault_around(struct vm_area_struct *vma, unsigned long address,
@@ -3515,7 +3499,7 @@ static int do_read_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	 * if page by the offset is not ready to be mapped (cold cache or
 	 * something).
 	 */
-	if (vma->vm_ops->map_pages) {
+	if (vma->vm_ops->map_pages && fault_around_pages() > 1) {
 		pte = pte_offset_map_lock(mm, pmd, address, &ptl);
 		do_fault_around(vma, address, pte, pgoff, flags);
 		if (!pte_same(*pte, orig_pte))
-- 
 Kirill A. Shutemov