Re: Re: [PATCH] ksm : use checksum and memcmp for rb_tree

[Kyeongdon Kim]

Sorry, re-send this email because of the Delivery failed message (to 
linux-kernel)


On 2017-10-30 오후 10:22, Timofey Titovets wrote:

2017-10-30 15:03 GMT+03:00 Kyeongdon Kim :
> The current ksm is using memcmp to insert and search 'rb_tree'.
> It does cause very expensive computation cost.
> In order to reduce the time of this operation,
> we have added a checksum to traverse before memcmp operation.
>
> Nearly all 'rb_node' in stable_tree_insert() function
> can be inserted as a checksum, most of it is possible
> in unstable_tree_search_insert() function.
> In stable_tree_search() function, the checksum may be an additional.
> But, checksum check duration is extremely small.
> Considering the time of the whole cmp_and_merge_page() function,
> it requires very little cost on average.
>
> Using this patch, we compared the time of ksm_do_scan() function
> by adding kernel trace at the start-end position of operation.
> (ARM 32bit target android device,
> over 1000 sample time gap stamps average)
>
> On original KSM scan avg duration = 0.0166893 sec
> 24991.975619 : ksm_do_scan_start: START: ksm_do_scan
> 24991.990975 : ksm_do_scan_end: END: ksm_do_scan
> 24992.008989 : ksm_do_scan_start: START: ksm_do_scan
> 24992.016839 : ksm_do_scan_end: END: ksm_do_scan
> ...
>
> On patch KSM scan avg duration = 0.0041157 sec
> 41081.461312 : ksm_do_scan_start: START: ksm_do_scan
> 41081.466364 : ksm_do_scan_end: END: ksm_do_scan
> 41081.484767 : ksm_do_scan_start: START: ksm_do_scan
> 41081.487951 : ksm_do_scan_end: END: ksm_do_scan
> ...
>
> We have tested randomly so many times for the stability
> and couldn't see any abnormal issue until now.
> Also, we found out this patch can make some good advantage
> for the power consumption than KSM default enable.
>
> Signed-off-by: Kyeongdon Kim 
> ---
> mm/ksm.c | 49 +
> 1 file changed, 45 insertions(+), 4 deletions(-)
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index be8f457..66ab4f4 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -150,6 +150,7 @@ struct stable_node {
> struct hlist_head hlist;
> union {
> unsigned long kpfn;
> + u32 oldchecksum;
> unsigned long chain_prune_time;
> };
> /*

May be just checksum? i.e. that's can be "old", where checksum can 
change,

in stable tree, checksum also stable.

Also, as checksum are stable, may be that make a sense to move it out
of union? (I'm afraid of clashes)

Also, you miss update comment above struct stable_node, about checksum 
var.



Thanks for your comment, and we may change those lines like below :

+ * @oldchecksum: previous checksum of the page about a stable_node
  * @nid: NUMA node id of stable tree in which linked (may not match kpfn)
  */
 struct stable_node {
@@ -159,6 +160,7 @@ struct stable_node {
 */
 #define STABLE_NODE_CHAIN -1024
    int rmap_hlist_len;
+   u32 oldchecksum;
 #ifdef CONFIG_NUMA

And I think if checksum are matched,
then we can use original memcmp logic in stable tree.
the worst case that I imagine is no page merging(just in that moment).
But, in my humble opinion, there will be no critical memory issue. but 
just return.

(as I said, we tested a lot to check some abnormal memory operation,
but so far, so good - only performance improvement)
> @@ -1522,7 +1523,7 @@ static __always_inline struct page 
*chain(struct stable_node **s_n_d,
> * This function returns the stable tree node of identical content if 
found,

> * NULL otherwise.
> */
> -static struct page *stable_tree_search(struct page *page)
> +static struct page *stable_tree_search(struct page *page, u32 
checksum)

> {
> int nid;
> struct rb_root *root;
> @@ -1540,6 +1541,8 @@ static struct page *stable_tree_search(struct 
page *page)

>
> nid = get_kpfn_nid(page_to_pfn(page));
> root = root_stable_tree + nid;
> + if (!checksum)
> + return NULL;

That's not a pointer, and 0x0 - is a valid checksum.
Also, jhash2 not so collision free, i.e.:
jhash2((uint32_t *) , 2, 17);

Example of collisions, where hash = 0x0:
hash: 0x0 - num: 610041898
hash: 0x0 - num: 4893164379
hash: 0x0 - num: 16423540221
hash: 0x0 - num: 29036382188

You also compare values, so hash = 0, is a acceptable checksum.


well, if then, I can remove this check line.


Thanks,
anyway in general idea looks good.

Reviewed-by: Timofey Titovets 

--
Have a nice day,
Timofey.

Thanks a lot :)
Actually, our organization want to use this KSM feature in general,
but, current logic needs too high cost.
So I wish to change more light version.
Please kindly give your opinion on this idea.

Thanks,
Kyeongdon Kim

Re: Re: [PATCH] ksm : use checksum and memcmp for rb_tree

[Kyeongdon Kim]

Sorry, re-send this email because of the Delivery failed message (to 
linux-kernel)


On 2017-10-30 오후 10:22, Timofey Titovets wrote:

2017-10-30 15:03 GMT+03:00 Kyeongdon Kim :
> The current ksm is using memcmp to insert and search 'rb_tree'.
> It does cause very expensive computation cost.
> In order to reduce the time of this operation,
> we have added a checksum to traverse before memcmp operation.
>
> Nearly all 'rb_node' in stable_tree_insert() function
> can be inserted as a checksum, most of it is possible
> in unstable_tree_search_insert() function.
> In stable_tree_search() function, the checksum may be an additional.
> But, checksum check duration is extremely small.
> Considering the time of the whole cmp_and_merge_page() function,
> it requires very little cost on average.
>
> Using this patch, we compared the time of ksm_do_scan() function
> by adding kernel trace at the start-end position of operation.
> (ARM 32bit target android device,
> over 1000 sample time gap stamps average)
>
> On original KSM scan avg duration = 0.0166893 sec
> 24991.975619 : ksm_do_scan_start: START: ksm_do_scan
> 24991.990975 : ksm_do_scan_end: END: ksm_do_scan
> 24992.008989 : ksm_do_scan_start: START: ksm_do_scan
> 24992.016839 : ksm_do_scan_end: END: ksm_do_scan
> ...
>
> On patch KSM scan avg duration = 0.0041157 sec
> 41081.461312 : ksm_do_scan_start: START: ksm_do_scan
> 41081.466364 : ksm_do_scan_end: END: ksm_do_scan
> 41081.484767 : ksm_do_scan_start: START: ksm_do_scan
> 41081.487951 : ksm_do_scan_end: END: ksm_do_scan
> ...
>
> We have tested randomly so many times for the stability
> and couldn't see any abnormal issue until now.
> Also, we found out this patch can make some good advantage
> for the power consumption than KSM default enable.
>
> Signed-off-by: Kyeongdon Kim 
> ---
> mm/ksm.c | 49 +
> 1 file changed, 45 insertions(+), 4 deletions(-)
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index be8f457..66ab4f4 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -150,6 +150,7 @@ struct stable_node {
> struct hlist_head hlist;
> union {
> unsigned long kpfn;
> + u32 oldchecksum;
> unsigned long chain_prune_time;
> };
> /*

May be just checksum? i.e. that's can be "old", where checksum can 
change,

in stable tree, checksum also stable.

Also, as checksum are stable, may be that make a sense to move it out
of union? (I'm afraid of clashes)

Also, you miss update comment above struct stable_node, about checksum 
var.



Thanks for your comment, and we may change those lines like below :

+ * @oldchecksum: previous checksum of the page about a stable_node
  * @nid: NUMA node id of stable tree in which linked (may not match kpfn)
  */
 struct stable_node {
@@ -159,6 +160,7 @@ struct stable_node {
 */
 #define STABLE_NODE_CHAIN -1024
    int rmap_hlist_len;
+   u32 oldchecksum;
 #ifdef CONFIG_NUMA

And I think if checksum are matched,
then we can use original memcmp logic in stable tree.
the worst case that I imagine is no page merging(just in that moment).
But, in my humble opinion, there will be no critical memory issue. but 
just return.

(as I said, we tested a lot to check some abnormal memory operation,
but so far, so good - only performance improvement)
> @@ -1522,7 +1523,7 @@ static __always_inline struct page 
*chain(struct stable_node **s_n_d,
> * This function returns the stable tree node of identical content if 
found,

> * NULL otherwise.
> */
> -static struct page *stable_tree_search(struct page *page)
> +static struct page *stable_tree_search(struct page *page, u32 
checksum)

> {
> int nid;
> struct rb_root *root;
> @@ -1540,6 +1541,8 @@ static struct page *stable_tree_search(struct 
page *page)

>
> nid = get_kpfn_nid(page_to_pfn(page));
> root = root_stable_tree + nid;
> + if (!checksum)
> + return NULL;

That's not a pointer, and 0x0 - is a valid checksum.
Also, jhash2 not so collision free, i.e.:
jhash2((uint32_t *) , 2, 17);

Example of collisions, where hash = 0x0:
hash: 0x0 - num: 610041898
hash: 0x0 - num: 4893164379
hash: 0x0 - num: 16423540221
hash: 0x0 - num: 29036382188

You also compare values, so hash = 0, is a acceptable checksum.


well, if then, I can remove this check line.


Thanks,
anyway in general idea looks good.

Reviewed-by: Timofey Titovets 

--
Have a nice day,
Timofey.

Thanks a lot :)
Actually, our organization want to use this KSM feature in general,
but, current logic needs too high cost.
So I wish to change more light version.
Please kindly give your opinion on this idea.

Thanks,
Kyeongdon Kim

Re: [PATCH] ksm : use checksum and memcmp for rb_tree

[Timofey Titovets]

2017-10-30 15:03 GMT+03:00 Kyeongdon Kim :
> The current ksm is using memcmp to insert and search 'rb_tree'.
> It does cause very expensive computation cost.
> In order to reduce the time of this operation,
> we have added a checksum to traverse before memcmp operation.
>
> Nearly all 'rb_node' in stable_tree_insert() function
> can be inserted as a checksum, most of it is possible
> in unstable_tree_search_insert() function.
> In stable_tree_search() function, the checksum may be an additional.
> But, checksum check duration is extremely small.
> Considering the time of the whole cmp_and_merge_page() function,
> it requires very little cost on average.
>
> Using this patch, we compared the time of ksm_do_scan() function
> by adding kernel trace at the start-end position of operation.
> (ARM 32bit target android device,
> over 1000 sample time gap stamps average)
>
> On original KSM scan avg duration = 0.0166893 sec
> 24991.975619 : ksm_do_scan_start: START: ksm_do_scan
> 24991.990975 : ksm_do_scan_end: END: ksm_do_scan
> 24992.008989 : ksm_do_scan_start: START: ksm_do_scan
> 24992.016839 : ksm_do_scan_end: END: ksm_do_scan
> ...
>
> On patch KSM scan avg duration = 0.0041157 sec
> 41081.461312 : ksm_do_scan_start: START: ksm_do_scan
> 41081.466364 : ksm_do_scan_end: END: ksm_do_scan
> 41081.484767 : ksm_do_scan_start: START: ksm_do_scan
> 41081.487951 : ksm_do_scan_end: END: ksm_do_scan
> ...
>
> We have tested randomly so many times for the stability
> and couldn't see any abnormal issue until now.
> Also, we found out this patch can make some good advantage
> for the power consumption than KSM default enable.
>
> Signed-off-by: Kyeongdon Kim 
> ---
>  mm/ksm.c | 49 +
>  1 file changed, 45 insertions(+), 4 deletions(-)
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index be8f457..66ab4f4 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -150,6 +150,7 @@ struct stable_node {
> struct hlist_head hlist;
> union {
> unsigned long kpfn;
> +   u32 oldchecksum;
> unsigned long chain_prune_time;
> };
> /*

May be just checksum? i.e. that's can be "old", where checksum can change,
in stable tree, checksum also stable.

Also, as checksum are stable, may be that make a sense to move it out
of union? (I'm afraid of clashes)

Also, you miss update comment above struct stable_node, about checksum var.

> @@ -1522,7 +1523,7 @@ static __always_inline struct page *chain(struct 
> stable_node **s_n_d,
>   * This function returns the stable tree node of identical content if found,
>   * NULL otherwise.
>   */
> -static struct page *stable_tree_search(struct page *page)
> +static struct page *stable_tree_search(struct page *page, u32 checksum)
>  {
> int nid;
> struct rb_root *root;
> @@ -1540,6 +1541,8 @@ static struct page *stable_tree_search(struct page 
> *page)
>
> nid = get_kpfn_nid(page_to_pfn(page));
> root = root_stable_tree + nid;
> +   if (!checksum)
> +   return NULL;

That's not a pointer, and 0x0 - is a valid checksum.
Also, jhash2 not so collision free, i.e.:
jhash2((uint32_t *) , 2, 17);

Example of collisions, where hash = 0x0:
hash: 0x0 - num:610041898
hash: 0x0 - num:4893164379
hash: 0x0 - num:16423540221
hash: 0x0 - num:29036382188

You also compare values, so hash = 0, is a acceptable checksum.

>  again:
> new = >rb_node;
> parent = NULL;
> @@ -1550,6 +1553,18 @@ static struct page *stable_tree_search(struct page 
> *page)
>
> cond_resched();
> stable_node = rb_entry(*new, struct stable_node, node);
> +
> +   /* first make rb_tree by checksum */
> +   if (checksum < stable_node->oldchecksum) {
> +   parent = *new;
> +   new = >rb_left;
> +   continue;
> +   } else if (checksum > stable_node->oldchecksum) {
> +   parent = *new;
> +   new = >rb_right;
> +   continue;
> +   }
> +
> stable_node_any = NULL;
> tree_page = chain_prune(_node_dup, _node, root);
> /*
> @@ -1768,7 +1783,7 @@ static struct page *stable_tree_search(struct page 
> *page)
>   * This function returns the stable tree node just allocated on success,
>   * NULL otherwise.
>   */
> -static struct stable_node *stable_tree_insert(struct page *kpage)
> +static struct stable_node *stable_tree_insert(struct page *kpage, u32 
> checksum)
>  {
> int nid;
> unsigned long kpfn;
> @@ -1792,6 +1807,18 @@ static struct stable_node *stable_tree_insert(struct 
> page *kpage)
> cond_resched();
> stable_node = rb_entry(*new, struct stable_node, node);
> stable_node_any = NULL;
> +
> +   

Re: [PATCH] ksm : use checksum and memcmp for rb_tree

[Timofey Titovets]

2017-10-30 15:03 GMT+03:00 Kyeongdon Kim :
> The current ksm is using memcmp to insert and search 'rb_tree'.
> It does cause very expensive computation cost.
> In order to reduce the time of this operation,
> we have added a checksum to traverse before memcmp operation.
>
> Nearly all 'rb_node' in stable_tree_insert() function
> can be inserted as a checksum, most of it is possible
> in unstable_tree_search_insert() function.
> In stable_tree_search() function, the checksum may be an additional.
> But, checksum check duration is extremely small.
> Considering the time of the whole cmp_and_merge_page() function,
> it requires very little cost on average.
>
> Using this patch, we compared the time of ksm_do_scan() function
> by adding kernel trace at the start-end position of operation.
> (ARM 32bit target android device,
> over 1000 sample time gap stamps average)
>
> On original KSM scan avg duration = 0.0166893 sec
> 24991.975619 : ksm_do_scan_start: START: ksm_do_scan
> 24991.990975 : ksm_do_scan_end: END: ksm_do_scan
> 24992.008989 : ksm_do_scan_start: START: ksm_do_scan
> 24992.016839 : ksm_do_scan_end: END: ksm_do_scan
> ...
>
> On patch KSM scan avg duration = 0.0041157 sec
> 41081.461312 : ksm_do_scan_start: START: ksm_do_scan
> 41081.466364 : ksm_do_scan_end: END: ksm_do_scan
> 41081.484767 : ksm_do_scan_start: START: ksm_do_scan
> 41081.487951 : ksm_do_scan_end: END: ksm_do_scan
> ...
>
> We have tested randomly so many times for the stability
> and couldn't see any abnormal issue until now.
> Also, we found out this patch can make some good advantage
> for the power consumption than KSM default enable.
>
> Signed-off-by: Kyeongdon Kim 
> ---
>  mm/ksm.c | 49 +
>  1 file changed, 45 insertions(+), 4 deletions(-)
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index be8f457..66ab4f4 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -150,6 +150,7 @@ struct stable_node {
> struct hlist_head hlist;
> union {
> unsigned long kpfn;
> +   u32 oldchecksum;
> unsigned long chain_prune_time;
> };
> /*

May be just checksum? i.e. that's can be "old", where checksum can change,
in stable tree, checksum also stable.

Also, as checksum are stable, may be that make a sense to move it out
of union? (I'm afraid of clashes)

Also, you miss update comment above struct stable_node, about checksum var.

> @@ -1522,7 +1523,7 @@ static __always_inline struct page *chain(struct 
> stable_node **s_n_d,
>   * This function returns the stable tree node of identical content if found,
>   * NULL otherwise.
>   */
> -static struct page *stable_tree_search(struct page *page)
> +static struct page *stable_tree_search(struct page *page, u32 checksum)
>  {
> int nid;
> struct rb_root *root;
> @@ -1540,6 +1541,8 @@ static struct page *stable_tree_search(struct page 
> *page)
>
> nid = get_kpfn_nid(page_to_pfn(page));
> root = root_stable_tree + nid;
> +   if (!checksum)
> +   return NULL;

That's not a pointer, and 0x0 - is a valid checksum.
Also, jhash2 not so collision free, i.e.:
jhash2((uint32_t *) , 2, 17);

Example of collisions, where hash = 0x0:
hash: 0x0 - num:610041898
hash: 0x0 - num:4893164379
hash: 0x0 - num:16423540221
hash: 0x0 - num:29036382188

You also compare values, so hash = 0, is a acceptable checksum.

>  again:
> new = >rb_node;
> parent = NULL;
> @@ -1550,6 +1553,18 @@ static struct page *stable_tree_search(struct page 
> *page)
>
> cond_resched();
> stable_node = rb_entry(*new, struct stable_node, node);
> +
> +   /* first make rb_tree by checksum */
> +   if (checksum < stable_node->oldchecksum) {
> +   parent = *new;
> +   new = >rb_left;
> +   continue;
> +   } else if (checksum > stable_node->oldchecksum) {
> +   parent = *new;
> +   new = >rb_right;
> +   continue;
> +   }
> +
> stable_node_any = NULL;
> tree_page = chain_prune(_node_dup, _node, root);
> /*
> @@ -1768,7 +1783,7 @@ static struct page *stable_tree_search(struct page 
> *page)
>   * This function returns the stable tree node just allocated on success,
>   * NULL otherwise.
>   */
> -static struct stable_node *stable_tree_insert(struct page *kpage)
> +static struct stable_node *stable_tree_insert(struct page *kpage, u32 
> checksum)
>  {
> int nid;
> unsigned long kpfn;
> @@ -1792,6 +1807,18 @@ static struct stable_node *stable_tree_insert(struct 
> page *kpage)
> cond_resched();
> stable_node = rb_entry(*new, struct stable_node, node);
> stable_node_any = NULL;
> +
> +   /* first make rb_tree by checksum */
> +  

[PATCH] ksm : use checksum and memcmp for rb_tree

[Kyeongdon Kim]

The current ksm is using memcmp to insert and search 'rb_tree'.
It does cause very expensive computation cost.
In order to reduce the time of this operation,
we have added a checksum to traverse before memcmp operation.

Nearly all 'rb_node' in stable_tree_insert() function
can be inserted as a checksum, most of it is possible
in unstable_tree_search_insert() function.
In stable_tree_search() function, the checksum may be an additional.
But, checksum check duration is extremely small.
Considering the time of the whole cmp_and_merge_page() function,
it requires very little cost on average.

Using this patch, we compared the time of ksm_do_scan() function
by adding kernel trace at the start-end position of operation.
(ARM 32bit target android device,
over 1000 sample time gap stamps average)

On original KSM scan avg duration = 0.0166893 sec
24991.975619 : ksm_do_scan_start: START: ksm_do_scan
24991.990975 : ksm_do_scan_end: END: ksm_do_scan
24992.008989 : ksm_do_scan_start: START: ksm_do_scan
24992.016839 : ksm_do_scan_end: END: ksm_do_scan
...

On patch KSM scan avg duration = 0.0041157 sec
41081.461312 : ksm_do_scan_start: START: ksm_do_scan
41081.466364 : ksm_do_scan_end: END: ksm_do_scan
41081.484767 : ksm_do_scan_start: START: ksm_do_scan
41081.487951 : ksm_do_scan_end: END: ksm_do_scan
...

We have tested randomly so many times for the stability
and couldn't see any abnormal issue until now.
Also, we found out this patch can make some good advantage
for the power consumption than KSM default enable.

Signed-off-by: Kyeongdon Kim 
---
 mm/ksm.c | 49 +
 1 file changed, 45 insertions(+), 4 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index be8f457..66ab4f4 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -150,6 +150,7 @@ struct stable_node {
struct hlist_head hlist;
union {
unsigned long kpfn;
+   u32 oldchecksum;
unsigned long chain_prune_time;
};
/*
@@ -1522,7 +1523,7 @@ static __always_inline struct page *chain(struct 
stable_node **s_n_d,
  * This function returns the stable tree node of identical content if found,
  * NULL otherwise.
  */
-static struct page *stable_tree_search(struct page *page)
+static struct page *stable_tree_search(struct page *page, u32 checksum)
 {
int nid;
struct rb_root *root;
@@ -1540,6 +1541,8 @@ static struct page *stable_tree_search(struct page *page)
 
nid = get_kpfn_nid(page_to_pfn(page));
root = root_stable_tree + nid;
+   if (!checksum)
+   return NULL;
 again:
new = >rb_node;
parent = NULL;
@@ -1550,6 +1553,18 @@ static struct page *stable_tree_search(struct page *page)
 
cond_resched();
stable_node = rb_entry(*new, struct stable_node, node);
+
+   /* first make rb_tree by checksum */
+   if (checksum < stable_node->oldchecksum) {
+   parent = *new;
+   new = >rb_left;
+   continue;
+   } else if (checksum > stable_node->oldchecksum) {
+   parent = *new;
+   new = >rb_right;
+   continue;
+   }
+
stable_node_any = NULL;
tree_page = chain_prune(_node_dup, _node, root);
/*
@@ -1768,7 +1783,7 @@ static struct page *stable_tree_search(struct page *page)
  * This function returns the stable tree node just allocated on success,
  * NULL otherwise.
  */
-static struct stable_node *stable_tree_insert(struct page *kpage)
+static struct stable_node *stable_tree_insert(struct page *kpage, u32 checksum)
 {
int nid;
unsigned long kpfn;
@@ -1792,6 +1807,18 @@ static struct stable_node *stable_tree_insert(struct 
page *kpage)
cond_resched();
stable_node = rb_entry(*new, struct stable_node, node);
stable_node_any = NULL;
+
+   /* first make rb_tree by checksum */
+   if (checksum < stable_node->oldchecksum) {
+   parent = *new;
+   new = >rb_left;
+   continue;
+   } else if (checksum > stable_node->oldchecksum) {
+   parent = *new;
+   new = >rb_right;
+   continue;
+   }
+
tree_page = chain(_node_dup, stable_node, root);
if (!stable_node_dup) {
/*
@@ -1850,6 +1877,7 @@ static struct stable_node *stable_tree_insert(struct page 
*kpage)
 
INIT_HLIST_HEAD(_node_dup->hlist);
stable_node_dup->kpfn = kpfn;
+   stable_node_dup->oldchecksum = checksum;
set_page_stable_node(kpage, stable_node_dup);
stable_node_dup->rmap_hlist_len = 0;
DO_NUMA(stable_node_dup->nid = nid);
@@ -1907,6 +1935,19 @@ struct rmap_item 

[PATCH] ksm : use checksum and memcmp for rb_tree

[Kyeongdon Kim]

The current ksm is using memcmp to insert and search 'rb_tree'.
It does cause very expensive computation cost.
In order to reduce the time of this operation,
we have added a checksum to traverse before memcmp operation.

Nearly all 'rb_node' in stable_tree_insert() function
can be inserted as a checksum, most of it is possible
in unstable_tree_search_insert() function.
In stable_tree_search() function, the checksum may be an additional.
But, checksum check duration is extremely small.
Considering the time of the whole cmp_and_merge_page() function,
it requires very little cost on average.

Using this patch, we compared the time of ksm_do_scan() function
by adding kernel trace at the start-end position of operation.
(ARM 32bit target android device,
over 1000 sample time gap stamps average)

On original KSM scan avg duration = 0.0166893 sec
24991.975619 : ksm_do_scan_start: START: ksm_do_scan
24991.990975 : ksm_do_scan_end: END: ksm_do_scan
24992.008989 : ksm_do_scan_start: START: ksm_do_scan
24992.016839 : ksm_do_scan_end: END: ksm_do_scan
...

On patch KSM scan avg duration = 0.0041157 sec
41081.461312 : ksm_do_scan_start: START: ksm_do_scan
41081.466364 : ksm_do_scan_end: END: ksm_do_scan
41081.484767 : ksm_do_scan_start: START: ksm_do_scan
41081.487951 : ksm_do_scan_end: END: ksm_do_scan
...

We have tested randomly so many times for the stability
and couldn't see any abnormal issue until now.
Also, we found out this patch can make some good advantage
for the power consumption than KSM default enable.

Signed-off-by: Kyeongdon Kim 
---
 mm/ksm.c | 49 +
 1 file changed, 45 insertions(+), 4 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index be8f457..66ab4f4 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -150,6 +150,7 @@ struct stable_node {
struct hlist_head hlist;
union {
unsigned long kpfn;
+   u32 oldchecksum;
unsigned long chain_prune_time;
};
/*
@@ -1522,7 +1523,7 @@ static __always_inline struct page *chain(struct 
stable_node **s_n_d,
  * This function returns the stable tree node of identical content if found,
  * NULL otherwise.
  */
-static struct page *stable_tree_search(struct page *page)
+static struct page *stable_tree_search(struct page *page, u32 checksum)
 {
int nid;
struct rb_root *root;
@@ -1540,6 +1541,8 @@ static struct page *stable_tree_search(struct page *page)
 
nid = get_kpfn_nid(page_to_pfn(page));
root = root_stable_tree + nid;
+   if (!checksum)
+   return NULL;
 again:
new = >rb_node;
parent = NULL;
@@ -1550,6 +1553,18 @@ static struct page *stable_tree_search(struct page *page)
 
cond_resched();
stable_node = rb_entry(*new, struct stable_node, node);
+
+   /* first make rb_tree by checksum */
+   if (checksum < stable_node->oldchecksum) {
+   parent = *new;
+   new = >rb_left;
+   continue;
+   } else if (checksum > stable_node->oldchecksum) {
+   parent = *new;
+   new = >rb_right;
+   continue;
+   }
+
stable_node_any = NULL;
tree_page = chain_prune(_node_dup, _node, root);
/*
@@ -1768,7 +1783,7 @@ static struct page *stable_tree_search(struct page *page)
  * This function returns the stable tree node just allocated on success,
  * NULL otherwise.
  */
-static struct stable_node *stable_tree_insert(struct page *kpage)
+static struct stable_node *stable_tree_insert(struct page *kpage, u32 checksum)
 {
int nid;
unsigned long kpfn;
@@ -1792,6 +1807,18 @@ static struct stable_node *stable_tree_insert(struct 
page *kpage)
cond_resched();
stable_node = rb_entry(*new, struct stable_node, node);
stable_node_any = NULL;
+
+   /* first make rb_tree by checksum */
+   if (checksum < stable_node->oldchecksum) {
+   parent = *new;
+   new = >rb_left;
+   continue;
+   } else if (checksum > stable_node->oldchecksum) {
+   parent = *new;
+   new = >rb_right;
+   continue;
+   }
+
tree_page = chain(_node_dup, stable_node, root);
if (!stable_node_dup) {
/*
@@ -1850,6 +1877,7 @@ static struct stable_node *stable_tree_insert(struct page 
*kpage)
 
INIT_HLIST_HEAD(_node_dup->hlist);
stable_node_dup->kpfn = kpfn;
+   stable_node_dup->oldchecksum = checksum;
set_page_stable_node(kpage, stable_node_dup);
stable_node_dup->rmap_hlist_len = 0;
DO_NUMA(stable_node_dup->nid = nid);
@@ -1907,6 +1935,19 @@ struct rmap_item