On 09/22/2016 08:35 AM, David Rientjes wrote:

On Thu, 22 Sep 2016, zijun_hu wrote:

quoted

On 2016/9/22 5:21, David Rientjes wrote:

quoted

On Wed, 21 Sep 2016, zijun_hu wrote:

quoted

From: zijun_hu <redacted>

correct lazy_max_pages() return value if the number of online
CPUs is power of 2

Signed-off-by: zijun_hu <redacted>
---
 mm/vmalloc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index a125ae8..2804224 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c

@@ -594,7 +594,9 @@ static unsigned long lazy_max_pages(void)
 {
 	unsigned int log;
 
-	log = fls(num_online_cpus());
+	log = num_online_cpus();
+	if (log > 1)
+		log = (unsigned int)get_count_order(log);
 
 	return log * (32UL * 1024 * 1024 / PAGE_SIZE);
 }

The implementation of lazy_max_pages() is somewhat arbitrarily defined, 
the existing approximation has been around for eight years and 
num_online_cpus() isn't intended to be rounded up to the next power of 2.  
I'd be inclined to just leave it as it is.

do i understand the intent in current code logic as below ?
[8, 15) roundup to 16?
[32, 63) roundup to 64?

The intent is as it is implemented; with your change, lazy_max_pages() is 
potentially increased depending on the number of online cpus.  This is 
only a heuristic, changing it would need justification on why the new 
value is better.  It is opposite to what the comment says: "to be 
conservative and not introduce a big latency on huge systems, so go with
a less aggressive log scale."  NACK to the patch.

my change potentially make lazy_max_pages() decreased not increased, i seems
conform with the comment

if the number of online CPUs is not power of 2, both have no any difference
otherwise, my change remain power of 2 value, and the original code rounds up
to next power of 2 value, for instance

my change : (32, 64] -> 64
	     32 -> 32, 64 -> 64
the original code: [32, 63) -> 64
                   32 -> 64, 64 -> 128


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