[PATCH 1/4] mm, page_alloc: add likely macro to help compiler optimization

[PATCH 1/4] mm, page_alloc: add likely macro to help compiler optimization

[Joonsoo Kim]

We rarely allocate a page with ALLOC_NO_WATERMARKS and it is used
in slow path. For making fast path more faster, add likely macro to
help compiler optimization.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b100255..86ad44b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1901,7 +1901,7 @@ zonelist_scan:
 			goto this_zone_full;
 
 		BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
-		if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
+		if (likely(!(alloc_flags & ALLOC_NO_WATERMARKS))) {
 			unsigned long mark;
 			int ret;
 
-- 
1.7.9.5

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[PATCH 2/4] mm, migrate: allocation new page lazyily in unmap_and_move()

[Joonsoo Kim]

We don't need a new page and then go out immediately if some condition
is met. Allocation has overhead in comparison with some condition check,
so allocating lazyily is preferable solution.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

diff --git a/mm/migrate.c b/mm/migrate.c
index 6f0c244..86db87e 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -864,10 +864,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 {
 	int rc = 0;
 	int *result = NULL;
-	struct page *newpage = get_new_page(page, private, &result);
-
-	if (!newpage)
-		return -ENOMEM;
+	struct page *newpage = NULL;
 
 	if (page_count(page) == 1) {
 		/* page was freed from under us. So we are done. */
@@ -878,6 +875,10 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 		if (unlikely(split_huge_page(page)))
 			goto out;
 
+	newpage = get_new_page(page, private, &result);
+	if (!newpage)
+		return -ENOMEM;
+
 	rc = __unmap_and_move(page, newpage, force, mode);
 
 	if (unlikely(rc == MIGRATEPAGE_BALLOON_SUCCESS)) {
@@ -908,7 +909,8 @@ out:
 	 * Move the new page to the LRU. If migration was not successful
 	 * then this will free the page.
 	 */
-	putback_lru_page(newpage);
+	if (newpage)
+		putback_lru_page(newpage);
 	if (result) {
 		if (rc)
 			*result = rc;
-- 
1.7.9.5

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Re: [PATCH 2/4] mm, migrate: allocation new page lazyily in unmap_and_move()

[Johannes Weiner]

On Fri, Aug 02, 2013 at 11:07:57AM +0900, Joonsoo Kim wrote:
> We don't need a new page and then go out immediately if some condition
> is met. Allocation has overhead in comparison with some condition check,
> so allocating lazyily is preferable solution.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 6f0c244..86db87e 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -864,10 +864,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  {
>  	int rc = 0;
>  	int *result = NULL;
> -	struct page *newpage = get_new_page(page, private, &result);
> -
> -	if (!newpage)
> -		return -ENOMEM;
> +	struct page *newpage = NULL;
>  
>  	if (page_count(page) == 1) {
>  		/* page was freed from under us. So we are done. */
> @@ -878,6 +875,10 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  		if (unlikely(split_huge_page(page)))
>  			goto out;
>  
> +	newpage = get_new_page(page, private, &result);
> +	if (!newpage)
> +		return -ENOMEM;

get_new_page() sets up result to communicate error codes from the
following checks.  While the existing ones (page freed and thp split
failed) don't change rc, somebody else might add a condition whose
error code should be propagated back into *result but miss it.

Please leave get_new_page() where it is.  The win from this change is
not big enough to risk these problems.

Thanks!

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Re: [PATCH 2/4] mm, migrate: allocation new page lazyily in unmap_and_move()

[Joonsoo Kim]

> get_new_page() sets up result to communicate error codes from the
> following checks.  While the existing ones (page freed and thp split
> failed) don't change rc, somebody else might add a condition whose
> error code should be propagated back into *result but miss it.
> 
> Please leave get_new_page() where it is.  The win from this change is
> not big enough to risk these problems.

Hello, Johannes.

Okay. You are right. I will omit this patch next time.

Thanks.

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[PATCH 3/4] mm: move pgtable related functions to right place

[Joonsoo Kim]

pgtable related functions are mostly in pgtable-generic.c.
So move remaining functions from memory.c to pgtable-generic.c.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

diff --git a/mm/memory.c b/mm/memory.c
index 1ce2e2a..26bce51 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -374,30 +374,6 @@ void tlb_remove_table(struct mmu_gather *tlb, void *table)
 #endif /* CONFIG_HAVE_RCU_TABLE_FREE */
 
 /*
- * If a p?d_bad entry is found while walking page tables, report
- * the error, before resetting entry to p?d_none.  Usually (but
- * very seldom) called out from the p?d_none_or_clear_bad macros.
- */
-
-void pgd_clear_bad(pgd_t *pgd)
-{
-	pgd_ERROR(*pgd);
-	pgd_clear(pgd);
-}
-
-void pud_clear_bad(pud_t *pud)
-{
-	pud_ERROR(*pud);
-	pud_clear(pud);
-}
-
-void pmd_clear_bad(pmd_t *pmd)
-{
-	pmd_ERROR(*pmd);
-	pmd_clear(pmd);
-}
-
-/*
  * Note: this doesn't free the actual pages themselves. That
  * has been handled earlier when unmapping all the memory regions.
  */
diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index e1a6e4f..3929a40 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -10,6 +10,30 @@
 #include <asm/tlb.h>
 #include <asm-generic/pgtable.h>
 
+/*
+ * If a p?d_bad entry is found while walking page tables, report
+ * the error, before resetting entry to p?d_none.  Usually (but
+ * very seldom) called out from the p?d_none_or_clear_bad macros.
+ */
+
+void pgd_clear_bad(pgd_t *pgd)
+{
+	pgd_ERROR(*pgd);
+	pgd_clear(pgd);
+}
+
+void pud_clear_bad(pud_t *pud)
+{
+	pud_ERROR(*pud);
+	pud_clear(pud);
+}
+
+void pmd_clear_bad(pmd_t *pmd)
+{
+	pmd_ERROR(*pmd);
+	pmd_clear(pmd);
+}
+
 #ifndef __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS
 /*
  * Only sets the access flags (dirty, accessed), as well as write 
-- 
1.7.9.5

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[PATCH 4/4] swap: clean-up #ifdef in page_mapping()

[Joonsoo Kim]

PageSwapCache() is always false when !CONFIG_SWAP, so compiler
properly discard related code. Therefore, we don't need #ifdef explicitly.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

diff --git a/include/linux/swap.h b/include/linux/swap.h
index d95cde5..c638a71 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -414,6 +414,7 @@ mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent, bool swapout)
 
 #else /* CONFIG_SWAP */
 
+#define swap_address_space(entry)		(NULL)
 #define get_nr_swap_pages()			0L
 #define total_swap_pages			0L
 #define total_swapcache_pages()			0UL
diff --git a/mm/util.c b/mm/util.c
index 7441c41..eaf63fc2 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -388,15 +388,12 @@ struct address_space *page_mapping(struct page *page)
 	struct address_space *mapping = page->mapping;
 
 	VM_BUG_ON(PageSlab(page));
-#ifdef CONFIG_SWAP
 	if (unlikely(PageSwapCache(page))) {
 		swp_entry_t entry;
 
 		entry.val = page_private(page);
 		mapping = swap_address_space(entry);
-	} else
-#endif
-	if ((unsigned long)mapping & PAGE_MAPPING_ANON)
+	} else if ((unsigned long)mapping & PAGE_MAPPING_ANON)
 		mapping = NULL;
 	return mapping;
 }
-- 
1.7.9.5

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Re: [PATCH 4/4] swap: clean-up #ifdef in page_mapping()

[Johannes Weiner]

On Fri, Aug 02, 2013 at 11:07:59AM +0900, Joonsoo Kim wrote:
> PageSwapCache() is always false when !CONFIG_SWAP, so compiler
> properly discard related code. Therefore, we don't need #ifdef explicitly.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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Re: [PATCH 1/4] mm, page_alloc: add likely macro to help compiler optimization

[Michal Hocko]

On Fri 02-08-13 11:07:56, Joonsoo Kim wrote:
> We rarely allocate a page with ALLOC_NO_WATERMARKS and it is used
> in slow path. For making fast path more faster, add likely macro to
> help compiler optimization.

The code is different in mmotm tree (see mm: page_alloc: rearrange
watermark checking in get_page_from_freelist)

Besides that, make sure you provide numbers which prove your claims
about performance optimizations.

> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index b100255..86ad44b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1901,7 +1901,7 @@ zonelist_scan:
>  			goto this_zone_full;
>  
>  		BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
> -		if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
> +		if (likely(!(alloc_flags & ALLOC_NO_WATERMARKS))) {
>  			unsigned long mark;
>  			int ret;
>  
> -- 
> 1.7.9.5
> 
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-- 
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SUSE Labs

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Re: [PATCH 1/4] mm, page_alloc: add likely macro to help compiler optimization

[Johannes Weiner]

On Fri, Aug 02, 2013 at 06:27:22PM +0200, Michal Hocko wrote:
> On Fri 02-08-13 11:07:56, Joonsoo Kim wrote:
> > We rarely allocate a page with ALLOC_NO_WATERMARKS and it is used
> > in slow path. For making fast path more faster, add likely macro to
> > help compiler optimization.
> 
> The code is different in mmotm tree (see mm: page_alloc: rearrange
> watermark checking in get_page_from_freelist)

Yes, please rebase this on top.

> Besides that, make sure you provide numbers which prove your claims
> about performance optimizations.

Isn't that a bit overkill?  We know it's a likely path (we would
deadlock constantly if a sizable portion of allocations were to ignore
the watermarks).  Does he have to justify that likely in general makes
sense?

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Re: [PATCH 1/4] mm, page_alloc: add likely macro to help compiler optimization

[Michal Hocko]

On Fri 02-08-13 16:47:10, Johannes Weiner wrote:
> On Fri, Aug 02, 2013 at 06:27:22PM +0200, Michal Hocko wrote:
> > On Fri 02-08-13 11:07:56, Joonsoo Kim wrote:
> > > We rarely allocate a page with ALLOC_NO_WATERMARKS and it is used
> > > in slow path. For making fast path more faster, add likely macro to
> > > help compiler optimization.
> > 
> > The code is different in mmotm tree (see mm: page_alloc: rearrange
> > watermark checking in get_page_from_freelist)
> 
> Yes, please rebase this on top.
> 
> > Besides that, make sure you provide numbers which prove your claims
> > about performance optimizations.
> 
> Isn't that a bit overkill?  We know it's a likely path (we would
> deadlock constantly if a sizable portion of allocations were to ignore
> the watermarks).  Does he have to justify that likely in general makes
> sense?

That was more a generic comment. If there is a claim that something
would be faster it would be nice to back that claim by some numbers
(e.g. smaller hot path).

In this particular case, unlikely(alloc_flags & ALLOC_NO_WATERMARKS)
doesn't make any change to the generated code with gcc 4.8.1 resp.
4.3.4 I have here.
Maybe other versions of gcc would benefit from the hint but changelog
didn't tell us. I wouldn't add the anotation if it doesn't make any
difference for the resulting code.
-- 
Michal Hocko
SUSE Labs

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Re: [PATCH 1/4] mm, page_alloc: add likely macro to help compiler optimization

[Joonsoo Kim]

Hello, Michal.

On Fri, Aug 02, 2013 at 11:36:07PM +0200, Michal Hocko wrote:
> On Fri 02-08-13 16:47:10, Johannes Weiner wrote:
> > On Fri, Aug 02, 2013 at 06:27:22PM +0200, Michal Hocko wrote:
> > > On Fri 02-08-13 11:07:56, Joonsoo Kim wrote:
> > > > We rarely allocate a page with ALLOC_NO_WATERMARKS and it is used
> > > > in slow path. For making fast path more faster, add likely macro to
> > > > help compiler optimization.
> > > 
> > > The code is different in mmotm tree (see mm: page_alloc: rearrange
> > > watermark checking in get_page_from_freelist)
> > 
> > Yes, please rebase this on top.
> > 
> > > Besides that, make sure you provide numbers which prove your claims
> > > about performance optimizations.
> > 
> > Isn't that a bit overkill?  We know it's a likely path (we would
> > deadlock constantly if a sizable portion of allocations were to ignore
> > the watermarks).  Does he have to justify that likely in general makes
> > sense?
> 
> That was more a generic comment. If there is a claim that something
> would be faster it would be nice to back that claim by some numbers
> (e.g. smaller hot path).
> 
> In this particular case, unlikely(alloc_flags & ALLOC_NO_WATERMARKS)
> doesn't make any change to the generated code with gcc 4.8.1 resp.
> 4.3.4 I have here.
> Maybe other versions of gcc would benefit from the hint but changelog
> didn't tell us. I wouldn't add the anotation if it doesn't make any
> difference for the resulting code.

Hmm, Is there no change with gcc 4.8.1 and 4.3.4?

I found a change with gcc 4.6.3 and v3.10 kernel.

   text	   data	    bss	    dec	    hex	filename
     35683	   1461	    644	  37788	   939c	page_alloc_base.o
     35715	   1461	    644	  37820	   93bc	page_alloc_patch.o

Slightly larger (32 bytes) than before.
And assembly code looks different as I expected.

* Original code

 17126 .LVL1518:
 17127         .loc 2 1904 0 is_stmt 1                                                                
 17128         testb   $4, -116(%rbp)  #, %sfp
 17129         je      .L866   #,

(snip)

 17974 .L866:
 17975 .LBE6053:
 17976 .LBE6052:
 17977 .LBE6051:
 17978 .LBE6073:                                                                                      
 17979 .LBE6093:                                                                                      
 17980 .LBB6094:
 17981         .loc 2 1908 0 
 17982         movl    -116(%rbp), %r14d       # %sfp, D.42080
 17983         .loc 2 1909 0
 17984         movl    -116(%rbp), %r8d        # %sfp,
 17985         movq    %rbx, %rdi      # prephitmp.1723,
 17986         movl    -212(%rbp), %ecx        # %sfp,
 17987         movl    -80(%rbp), %esi # %sfp,
 17988         .loc 2 1908 0
 17989         andl    $3, %r14d       #, D.42080
 17990         movslq  %r14d, %rax     # D.42080, D.42080
 17991         movq    (%rbx,%rax,8), %r13     # prephitmp.1723_268->watermark, mark
 17992 .LVL1591:
 17993         .loc 2 1909 0
 17994         movq    %r13, %rdx      # mark,
 17995         call    zone_watermark_ok       #

On 17129 line, we check ALLOC_NO_WATERMARKS and if not matched, then jump to L866.
L866 is on 17981 line.

* Patched code

 17122 .L807:
 17123 .LVL1513:
 17124         .loc 2 1904 0 is_stmt 1
 17125         testb   $4, -88(%rbp)   #, %sfp
 17126         jne     .L811   #,
 17127 .LBB6092:
 17128         .loc 2 1908 0
 17129         movl    -88(%rbp), %r13d        # %sfp, D.42082
 17130         .loc 2 1909 0
 17131         movl    -88(%rbp), %r8d # %sfp,
 17132         movq    %rbx, %rdi      # prephitmp.1723,
 17133         movl    -160(%rbp), %ecx        # %sfp,
 17134         movl    -80(%rbp), %esi # %sfp,
 17135         .loc 2 1908 0
 17136         andl    $3, %r13d       #, D.42082
 17137         movslq  %r13d, %rax     # D.42082, D.42082
 17138         movq    (%rbx,%rax,8), %r12     # prephitmp.1723_270->watermark, mark
 17139 .LVL1514:
 17140         .loc 2 1909 0
 17141         movq    %r12, %rdx      # mark,
 17142         call    zone_watermark_ok       #

On 17124 line, we check ALLOC_NO_WATERMARKS (0x4) and if not matched,
execute following code without jumping. This is effect of 'likely' macro.
Isn't it reasonable?

Thanks.

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Re: [PATCH 1/4] mm, page_alloc: add likely macro to help compiler optimization

[Joonsoo Kim]

On Mon, Aug 05, 2013 at 05:10:08PM +0900, Joonsoo Kim wrote:
> Hello, Michal.
> 
> On Fri, Aug 02, 2013 at 11:36:07PM +0200, Michal Hocko wrote:
> > On Fri 02-08-13 16:47:10, Johannes Weiner wrote:
> > > On Fri, Aug 02, 2013 at 06:27:22PM +0200, Michal Hocko wrote:
> > > > On Fri 02-08-13 11:07:56, Joonsoo Kim wrote:
> > > > > We rarely allocate a page with ALLOC_NO_WATERMARKS and it is used
> > > > > in slow path. For making fast path more faster, add likely macro to
> > > > > help compiler optimization.
> > > > 
> > > > The code is different in mmotm tree (see mm: page_alloc: rearrange
> > > > watermark checking in get_page_from_freelist)
> > > 
> > > Yes, please rebase this on top.
> > > 
> > > > Besides that, make sure you provide numbers which prove your claims
> > > > about performance optimizations.
> > > 
> > > Isn't that a bit overkill?  We know it's a likely path (we would
> > > deadlock constantly if a sizable portion of allocations were to ignore
> > > the watermarks).  Does he have to justify that likely in general makes
> > > sense?
> > 
> > That was more a generic comment. If there is a claim that something
> > would be faster it would be nice to back that claim by some numbers
> > (e.g. smaller hot path).
> > 
> > In this particular case, unlikely(alloc_flags & ALLOC_NO_WATERMARKS)
> > doesn't make any change to the generated code with gcc 4.8.1 resp.
> > 4.3.4 I have here.
> > Maybe other versions of gcc would benefit from the hint but changelog
> > didn't tell us. I wouldn't add the anotation if it doesn't make any
> > difference for the resulting code.
> 
> Hmm, Is there no change with gcc 4.8.1 and 4.3.4?
> 
> I found a change with gcc 4.6.3 and v3.10 kernel.

Ah... I did a test on mmotm (Johannes's git) and found that this patch
doesn't make any effect. I guess, a change from Johannes ('rearrange
watermark checking in get_page_from_freelist') already makes better code
for !ALLOC_NO_WATERMARKS case. IMHO, although there is no effect, it is
better to add likely macro, because arrangement can be changed from time
to time without any consideration of assembly code generation. How about
your opinion, Johannes and Michal?

Thanks.

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Re: [PATCH 1/4] mm, page_alloc: add likely macro to help compiler optimization

[Michal Hocko]

On Mon 05-08-13 17:50:41, Joonsoo Kim wrote:
[...]
> IMHO, although there is no effect, it is better to add likely macro,
> because arrangement can be changed from time to time without any
> consideration of assembly code generation. How about your opinion,
> Johannes and Michal?

This is a matter of taste. I do not like new *likely annotations if they
do not make difference. But no strong objections if others like it.
-- 
Michal Hocko
SUSE Labs

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Re: [PATCH 1/4] mm, page_alloc: add likely macro to help compiler optimization

[Johannes Weiner]

On Mon, Aug 05, 2013 at 05:50:41PM +0900, Joonsoo Kim wrote:
> On Mon, Aug 05, 2013 at 05:10:08PM +0900, Joonsoo Kim wrote:
> > Hello, Michal.
> > 
> > On Fri, Aug 02, 2013 at 11:36:07PM +0200, Michal Hocko wrote:
> > > On Fri 02-08-13 16:47:10, Johannes Weiner wrote:
> > > > On Fri, Aug 02, 2013 at 06:27:22PM +0200, Michal Hocko wrote:
> > > > > On Fri 02-08-13 11:07:56, Joonsoo Kim wrote:
> > > > > > We rarely allocate a page with ALLOC_NO_WATERMARKS and it is used
> > > > > > in slow path. For making fast path more faster, add likely macro to
> > > > > > help compiler optimization.
> > > > > 
> > > > > The code is different in mmotm tree (see mm: page_alloc: rearrange
> > > > > watermark checking in get_page_from_freelist)
> > > > 
> > > > Yes, please rebase this on top.
> > > > 
> > > > > Besides that, make sure you provide numbers which prove your claims
> > > > > about performance optimizations.
> > > > 
> > > > Isn't that a bit overkill?  We know it's a likely path (we would
> > > > deadlock constantly if a sizable portion of allocations were to ignore
> > > > the watermarks).  Does he have to justify that likely in general makes
> > > > sense?
> > > 
> > > That was more a generic comment. If there is a claim that something
> > > would be faster it would be nice to back that claim by some numbers
> > > (e.g. smaller hot path).
> > > 
> > > In this particular case, unlikely(alloc_flags & ALLOC_NO_WATERMARKS)
> > > doesn't make any change to the generated code with gcc 4.8.1 resp.
> > > 4.3.4 I have here.
> > > Maybe other versions of gcc would benefit from the hint but changelog
> > > didn't tell us. I wouldn't add the anotation if it doesn't make any
> > > difference for the resulting code.
> > 
> > Hmm, Is there no change with gcc 4.8.1 and 4.3.4?
> > 
> > I found a change with gcc 4.6.3 and v3.10 kernel.
> 
> Ah... I did a test on mmotm (Johannes's git) and found that this patch
> doesn't make any effect. I guess, a change from Johannes ('rearrange
> watermark checking in get_page_from_freelist') already makes better code
> for !ALLOC_NO_WATERMARKS case. IMHO, although there is no effect, it is
> better to add likely macro, because arrangement can be changed from time
> to time without any consideration of assembly code generation. How about
> your opinion, Johannes and Michal?

I'm not against it.  It does not really matter if gcc gets it right by
accident right now and the annotation does not have an immediate
effect.  It's a compiler hint and we want gcc to know it so it doesn't
ever assume otherwise in the future.  Yes, nobody will re-evaluate if
the conditional still generates the jumps correctly after shifting the
code around.

Thanks!

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Re: [PATCH 1/4] mm, page_alloc: add likely macro to help compiler optimization

[Johannes Weiner]

On Fri, Aug 02, 2013 at 11:07:56AM +0900, Joonsoo Kim wrote:
> We rarely allocate a page with ALLOC_NO_WATERMARKS and it is used
> in slow path. For making fast path more faster, add likely macro to
> help compiler optimization.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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