[PATCH] Drivers: hv: hv_balloon: eliminate jumps in piecewiese linear floor function

[PATCH] Drivers: hv: hv_balloon: eliminate jumps in piecewiese linear floor function

[Vitaly Kuznetsov]

Commit 79208c57da53 ("Drivers: hv: hv_balloon: Make adjustments in computing
the floor") was inacurate as it introduced a jump in our piecewiese linear
'floor' function:

At 2048MB we have:
Left limit:
104 + 2048/8 = 360
Right limit:
256 + 2048/16 = 384 (so the right value is 232)

We now have to make an adjustment at 8192 boundary:
232 + 8192/16 = 744
512 + 8192/32 = 768 (so the right value is 488)

Suggested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 drivers/hv/hv_balloon.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
index 014256a..16d52da 100644
--- a/drivers/hv/hv_balloon.c
+++ b/drivers/hv/hv_balloon.c
@@ -966,8 +966,8 @@ static unsigned long compute_balloon_floor(void)
 	 *     128        72    (1/2)
 	 *     512       168    (1/4)
 	 *    2048       360    (1/8)
-	 *    8192       768    (1/16)
-	 *   32768      1536	(1/32)
+	 *    8192       744    (1/16)
+	 *   32768      1512	(1/32)
 	 */
 	if (totalram_pages < MB2PAGES(128))
 		min_pages = MB2PAGES(8) + (totalram_pages >> 1);
@@ -976,9 +976,9 @@ static unsigned long compute_balloon_floor(void)
 	else if (totalram_pages < MB2PAGES(2048))
 		min_pages = MB2PAGES(104) + (totalram_pages >> 3);
 	else if (totalram_pages < MB2PAGES(8192))
-		min_pages = MB2PAGES(256) + (totalram_pages >> 4);
+		min_pages = MB2PAGES(232) + (totalram_pages >> 4);
 	else
-		min_pages = MB2PAGES(512) + (totalram_pages >> 5);
+		min_pages = MB2PAGES(488) + (totalram_pages >> 5);
 #undef MB2PAGES
 	return min_pages;
 }
-- 
1.9.3

Re: [PATCH] Drivers: hv: hv_balloon: eliminate jumps in piecewiese linear floor function

[Laszlo Ersek]

On 03/26/15 11:26, Vitaly Kuznetsov wrote:
> Commit 79208c57da53 ("Drivers: hv: hv_balloon: Make adjustments in computing
> the floor") was inacurate as it introduced a jump in our piecewiese linear
> 'floor' function:
> 
> At 2048MB we have:
> Left limit:
> 104 + 2048/8 = 360
> Right limit:
> 256 + 2048/16 = 384 (so the right value is 232)
> 
> We now have to make an adjustment at 8192 boundary:
> 232 + 8192/16 = 744
> 512 + 8192/32 = 768 (so the right value is 488)
> 
> Suggested-by: Laszlo Ersek <lersek@redhat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  drivers/hv/hv_balloon.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> index 014256a..16d52da 100644
> --- a/drivers/hv/hv_balloon.c
> +++ b/drivers/hv/hv_balloon.c
> @@ -966,8 +966,8 @@ static unsigned long compute_balloon_floor(void)
>  	 *     128        72    (1/2)
>  	 *     512       168    (1/4)
>  	 *    2048       360    (1/8)
> -	 *    8192       768    (1/16)
> -	 *   32768      1536	(1/32)
> +	 *    8192       744    (1/16)
> +	 *   32768      1512	(1/32)
>  	 */
>  	if (totalram_pages < MB2PAGES(128))
>  		min_pages = MB2PAGES(8) + (totalram_pages >> 1);
> @@ -976,9 +976,9 @@ static unsigned long compute_balloon_floor(void)
>  	else if (totalram_pages < MB2PAGES(2048))
>  		min_pages = MB2PAGES(104) + (totalram_pages >> 3);
>  	else if (totalram_pages < MB2PAGES(8192))
> -		min_pages = MB2PAGES(256) + (totalram_pages >> 4);
> +		min_pages = MB2PAGES(232) + (totalram_pages >> 4);
>  	else
> -		min_pages = MB2PAGES(512) + (totalram_pages >> 5);
> +		min_pages = MB2PAGES(488) + (totalram_pages >> 5);
>  #undef MB2PAGES
>  	return min_pages;
>  }
> 

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Thanks!
Laszlo