[PATCH 0/3] migration/ram: Some modifications about ram_save_host_page()

[PATCH 0/3] migration/ram: Some modifications about ram_save_host_page()

[Kunkun Jiang]

Hi,

This series include patches as below:
Patch 1-2:
- modified the comment and code of ram_save_host_page() to make them match each other

Patch 3:
- optimized ram_save_host_page() by using migration_bitmap_find_dirty() to find dirty
pages

Best Regards

Kunkun Jiang

Kunkun Jiang (3):
  migration/ram: Modify the code comment of ram_save_host_page()
  migration/ram: Modify ram_save_host_page() to match the comment
  migration/ram: Optimize ram_save_host_page()

 migration/ram.c | 35 ++++++++++++++++++++---------------
 1 file changed, 20 insertions(+), 15 deletions(-)

-- 
2.23.0

[PATCH 1/3] migration/ram: Modify the code comment of ram_save_host_page()

[Kunkun Jiang]

The ram_save_host_page() has been modified several times
since its birth. But the comment hasn't been modified as it should
be. It'd better to modify the comment to explain ram_save_host_page()
more clearly.

Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
Signed-off-by: Kunkun Jiang <jiangkunkun@huawei.com>
---
 migration/ram.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index 72143da0ac..fc49c3f898 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1970,15 +1970,16 @@ static int ram_save_target_page(RAMState *rs, PageSearchStatus *pss,
 }
 
 /**
- * ram_save_host_page: save a whole host page
+ * ram_save_host_page: save a whole host page or the rest of a block
  *
- * Starting at *offset send pages up to the end of the current host
- * page. It's valid for the initial offset to point into the middle of
- * a host page in which case the remainder of the hostpage is sent.
- * Only dirty target pages are sent. Note that the host page size may
- * be a huge page for this block.
- * The saving stops at the boundary of the used_length of the block
- * if the RAMBlock isn't a multiple of the host page size.
+ * Starting at pss->page send pages up to the end of the current host
+ * page or the boundary of used_length of the block (if the RAMBlock
+ * isn't a multiple of the host page size). The min one is selected.
+ * Only dirty target pages are sent.
+ *
+ * Note that the host page size may be a huge page for this block, it's
+ * valid for the initial offset to point into the middle of a host page
+ * in which case the remainder of the hostpage is sent.
  *
  * Returns the number of pages written or negative on error
  *
-- 
2.23.0

Re: [PATCH 1/3] migration/ram: Modify the code comment of ram_save_host_page()

[David Edmondson]

On Tuesday, 2021-02-23 at 10:16:43 +08, Kunkun Jiang wrote:

> The ram_save_host_page() has been modified several times
> since its birth. But the comment hasn't been modified as it should
> be. It'd better to modify the comment to explain ram_save_host_page()
> more clearly.
>
> Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
> Signed-off-by: Kunkun Jiang <jiangkunkun@huawei.com>
> ---
>  migration/ram.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
>
> diff --git a/migration/ram.c b/migration/ram.c
> index 72143da0ac..fc49c3f898 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1970,15 +1970,16 @@ static int ram_save_target_page(RAMState *rs, PageSearchStatus *pss,
>  }
>  
>  /**
> - * ram_save_host_page: save a whole host page
> + * ram_save_host_page: save a whole host page or the rest of a block
>   *
> - * Starting at *offset send pages up to the end of the current host
> - * page. It's valid for the initial offset to point into the middle of
> - * a host page in which case the remainder of the hostpage is sent.
> - * Only dirty target pages are sent. Note that the host page size may
> - * be a huge page for this block.
> - * The saving stops at the boundary of the used_length of the block
> - * if the RAMBlock isn't a multiple of the host page size.
> + * Starting at pss->page send pages up to the end of the current host
> + * page or the boundary of used_length of the block (if the RAMBlock
> + * isn't a multiple of the host page size). The min one is selected.
> + * Only dirty target pages are sent.
> + *
> + * Note that the host page size may be a huge page for this block, it's
> + * valid for the initial offset to point into the middle of a host page
> + * in which case the remainder of the hostpage is sent.

How about:

* Send dirty pages between pss->page and either the end of that page or
* the used_length of the RAMBlock, whichever is smaller.
*
* Note that if the host page is a huge page, pss->page may be in the
* middle of that page.

>   *
>   * Returns the number of pages written or negative on error
>   *
> -- 
> 2.23.0

dme.
-- 
I'm in with the in crowd, I know every latest dance.

Re: [PATCH 1/3] migration/ram: Modify the code comment of ram_save_host_page()

[Kunkun Jiang]

On 2021/2/25 6:53, David Edmondson wrote:
> On Tuesday, 2021-02-23 at 10:16:43 +08, Kunkun Jiang wrote:
>
>> The ram_save_host_page() has been modified several times
>> since its birth. But the comment hasn't been modified as it should
>> be. It'd better to modify the comment to explain ram_save_host_page()
>> more clearly.
>>
>> Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
>> Signed-off-by: Kunkun Jiang <jiangkunkun@huawei.com>
>> ---
>>   migration/ram.c | 17 +++++++++--------
>>   1 file changed, 9 insertions(+), 8 deletions(-)
>>
>> diff --git a/migration/ram.c b/migration/ram.c
>> index 72143da0ac..fc49c3f898 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -1970,15 +1970,16 @@ static int ram_save_target_page(RAMState *rs, PageSearchStatus *pss,
>>   }
>>   
>>   /**
>> - * ram_save_host_page: save a whole host page
>> + * ram_save_host_page: save a whole host page or the rest of a block
>>    *
>> - * Starting at *offset send pages up to the end of the current host
>> - * page. It's valid for the initial offset to point into the middle of
>> - * a host page in which case the remainder of the hostpage is sent.
>> - * Only dirty target pages are sent. Note that the host page size may
>> - * be a huge page for this block.
>> - * The saving stops at the boundary of the used_length of the block
>> - * if the RAMBlock isn't a multiple of the host page size.
>> + * Starting at pss->page send pages up to the end of the current host
>> + * page or the boundary of used_length of the block (if the RAMBlock
>> + * isn't a multiple of the host page size). The min one is selected.
>> + * Only dirty target pages are sent.
>> + *
>> + * Note that the host page size may be a huge page for this block, it's
>> + * valid for the initial offset to point into the middle of a host page
>> + * in which case the remainder of the hostpage is sent.
> How about:
>
> * Send dirty pages between pss->page and either the end of that page or
> * the used_length of the RAMBlock, whichever is smaller.
> *
> * Note that if the host page is a huge page, pss->page may be in the
> * middle of that page.

Thank you. It looks concise and comprehensive.

Best Regards

Kunkun Jiang

>>    *
>>    * Returns the number of pages written or negative on error
>>    *
>> -- 
>> 2.23.0
> dme.

[PATCH 2/3] migration/ram: Modify ram_save_host_page() to match the comment

[Kunkun Jiang]

According to the comment, when the host page is a huge page, the
migration_rate_limit() should be executed. If not, this function
can be omitted to save time.

Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
Signed-off-by: Kunkun Jiang <jiangkunkun@huawei.com>
---
 migration/ram.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/migration/ram.c b/migration/ram.c
index fc49c3f898..c7e18dc2fc 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -2017,7 +2017,9 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
         pages += tmppages;
         pss->page++;
         /* Allow rate limiting to happen in the middle of huge pages */
-        migration_rate_limit();
+        if (pagesize_bits > 1) {
+            migration_rate_limit();
+        }
     } while ((pss->page & (pagesize_bits - 1)) &&
              offset_in_ramblock(pss->block,
                                 ((ram_addr_t)pss->page) << TARGET_PAGE_BITS));
-- 
2.23.0

[PATCH 3/3] migration/ram: Optimize ram_save_host_page()

[Kunkun Jiang]

Starting from pss->page, ram_save_host_page() will check every page
and send the dirty pages up to the end of the current host page or
the boundary of used_length of the block. If the host page size is
a huge page, the step "check" will take a lot of time.

This will improve performance to use migration_bitmap_find_dirty().

Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
Signed-off-by: Kunkun Jiang <jiangkunkun@huawei.com>
---
 migration/ram.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index c7e18dc2fc..c7a2350198 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1994,6 +1994,8 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
     int tmppages, pages = 0;
     size_t pagesize_bits =
         qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS;
+    unsigned long hostpage_boundary =
+        QEMU_ALIGN_UP(pss->page + 1, pagesize_bits);
     unsigned long start_page = pss->page;
     int res;
 
@@ -2005,8 +2007,7 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
     do {
         /* Check the pages is dirty and if it is send it */
         if (!migration_bitmap_clear_dirty(rs, pss->block, pss->page)) {
-            pss->page++;
-            continue;
+            goto find_next;
         }
 
         tmppages = ram_save_target_page(rs, pss, last_stage);
@@ -2015,16 +2016,17 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
         }
 
         pages += tmppages;
-        pss->page++;
         /* Allow rate limiting to happen in the middle of huge pages */
         if (pagesize_bits > 1) {
             migration_rate_limit();
         }
-    } while ((pss->page & (pagesize_bits - 1)) &&
+find_next:
+        pss->page = migration_bitmap_find_dirty(rs, pss->block, pss->page);
+    } while ((pss->page < hostpage_boundary) &&
              offset_in_ramblock(pss->block,
                                 ((ram_addr_t)pss->page) << TARGET_PAGE_BITS));
-    /* The offset we leave with is the last one we looked at */
-    pss->page--;
+    /* The offset we leave with is the min boundary of host page and block */
+    pss->page = MIN(pss->page, hostpage_boundary) - 1;
 
     res = ram_save_release_protection(rs, pss, start_page);
     return (res < 0 ? res : pages);
-- 
2.23.0

Re: [PATCH 3/3] migration/ram: Optimize ram_save_host_page()

[David Edmondson]

On Tuesday, 2021-02-23 at 10:16:45 +08, Kunkun Jiang wrote:

> Starting from pss->page, ram_save_host_page() will check every page
> and send the dirty pages up to the end of the current host page or
> the boundary of used_length of the block. If the host page size is
> a huge page, the step "check" will take a lot of time.
>
> This will improve performance to use migration_bitmap_find_dirty().
>
> Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
> Signed-off-by: Kunkun Jiang <jiangkunkun@huawei.com>
> ---
>  migration/ram.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/migration/ram.c b/migration/ram.c
> index c7e18dc2fc..c7a2350198 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1994,6 +1994,8 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
>      int tmppages, pages = 0;
>      size_t pagesize_bits =
>          qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS;
> +    unsigned long hostpage_boundary =
> +        QEMU_ALIGN_UP(pss->page + 1, pagesize_bits);
>      unsigned long start_page = pss->page;
>      int res;
>  
> @@ -2005,8 +2007,7 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
>      do {
>          /* Check the pages is dirty and if it is send it */
>          if (!migration_bitmap_clear_dirty(rs, pss->block, pss->page)) {
> -            pss->page++;
> -            continue;
> +            goto find_next;
>          }
>  
>          tmppages = ram_save_target_page(rs, pss, last_stage);
> @@ -2015,16 +2016,17 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
>          }
>  
>          pages += tmppages;
> -        pss->page++;
>          /* Allow rate limiting to happen in the middle of huge pages */
>          if (pagesize_bits > 1) {
>              migration_rate_limit();
>          }
> -    } while ((pss->page & (pagesize_bits - 1)) &&
> +find_next:
> +        pss->page = migration_bitmap_find_dirty(rs, pss->block, pss->page);
> +    } while ((pss->page < hostpage_boundary) &&
>               offset_in_ramblock(pss->block,
>                                  ((ram_addr_t)pss->page) << TARGET_PAGE_BITS));

This ends up looking very messy, with a goto inside the loop.

Wouldn't it be cleaner to invert the sense of the
migration_bitmap_clear_dirty() test, such that
migration_bitmap_find_dirty() is called after the body of the test?

> -    /* The offset we leave with is the last one we looked at */
> -    pss->page--;
> +    /* The offset we leave with is the min boundary of host page and block */
> +    pss->page = MIN(pss->page, hostpage_boundary) - 1;
>  
>      res = ram_save_release_protection(rs, pss, start_page);
>      return (res < 0 ? res : pages);
> -- 
> 2.23.0

dme.
-- 
Oliver darling, call Mister Haney, I think our speakers are blown.

Re: [PATCH 3/3] migration/ram: Optimize ram_save_host_page()

[Kunkun Jiang]

On 2021/2/25 20:48, David Edmondson wrote:
> On Tuesday, 2021-02-23 at 10:16:45 +08, Kunkun Jiang wrote:
>
>> Starting from pss->page, ram_save_host_page() will check every page
>> and send the dirty pages up to the end of the current host page or
>> the boundary of used_length of the block. If the host page size is
>> a huge page, the step "check" will take a lot of time.
>>
>> This will improve performance to use migration_bitmap_find_dirty().
>>
>> Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
>> Signed-off-by: Kunkun Jiang <jiangkunkun@huawei.com>
>> ---
>>   migration/ram.c | 14 ++++++++------
>>   1 file changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/migration/ram.c b/migration/ram.c
>> index c7e18dc2fc..c7a2350198 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -1994,6 +1994,8 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
>>       int tmppages, pages = 0;
>>       size_t pagesize_bits =
>>           qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS;
>> +    unsigned long hostpage_boundary =
>> +        QEMU_ALIGN_UP(pss->page + 1, pagesize_bits);
>>       unsigned long start_page = pss->page;
>>       int res;
>>   
>> @@ -2005,8 +2007,7 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
>>       do {
>>           /* Check the pages is dirty and if it is send it */
>>           if (!migration_bitmap_clear_dirty(rs, pss->block, pss->page)) {
>> -            pss->page++;
>> -            continue;
>> +            goto find_next;
>>           }
>>   
>>           tmppages = ram_save_target_page(rs, pss, last_stage);
>> @@ -2015,16 +2016,17 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
>>           }
>>   
>>           pages += tmppages;
>> -        pss->page++;
>>           /* Allow rate limiting to happen in the middle of huge pages */
>>           if (pagesize_bits > 1) {
>>               migration_rate_limit();
>>           }
>> -    } while ((pss->page & (pagesize_bits - 1)) &&
>> +find_next:
>> +        pss->page = migration_bitmap_find_dirty(rs, pss->block, pss->page);
>> +    } while ((pss->page < hostpage_boundary) &&
>>                offset_in_ramblock(pss->block,
>>                                   ((ram_addr_t)pss->page) << TARGET_PAGE_BITS));
> This ends up looking very messy, with a goto inside the loop.
>
> Wouldn't it be cleaner to invert the sense of the
> migration_bitmap_clear_dirty() test, such that
> migration_bitmap_find_dirty() is called after the body of the test?
Sorry for the late reply.
Thanks for your advice.  I will post a v2 as soon as possible.

Best Regards.

Kunkun Jiang

>> -    /* The offset we leave with is the last one we looked at */
>> -    pss->page--;
>> +    /* The offset we leave with is the min boundary of host page and block */
>> +    pss->page = MIN(pss->page, hostpage_boundary) - 1;
>>   
>>       res = ram_save_release_protection(rs, pss, start_page);
>>       return (res < 0 ? res : pages);
>> -- 
>> 2.23.0
> dme.