[PATCH 1/1] mm/percpu.c: simplify grouping cpu logic in pcpu_build_alloc_info()

[PATCH 1/1] mm/percpu.c: simplify grouping cpu logic in pcpu_build_alloc_info()

[zijun_hu]

From: zijun_hu <zijun_hu@htc.com>

simplify grouping cpu logic in pcpu_build_alloc_info() to improve
readability and performance, it discards the goto statement too

for every possible cpu, decide whether it can share group id of any
lower index CPU, use the group id if so, otherwise a new group id
is allocated to it

Signed-off-by: zijun_hu <zijun_hu@htc.com>
---
 mm/percpu.c | 28 +++++++++++++++-------------
 1 file changed, 15 insertions(+), 13 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 9903830aaebb..fcaaac977954 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1824,23 +1824,25 @@ static struct pcpu_alloc_info * __init pcpu_build_alloc_info(
 	max_upa = upa;
 
 	/* group cpus according to their proximity */
-	for_each_possible_cpu(cpu) {
-		group = 0;
-	next_group:
+	group = 0;
+	for_each_possible_cpu(cpu)
 		for_each_possible_cpu(tcpu) {
-			if (cpu == tcpu)
-				break;
-			if (group_map[tcpu] == group && cpu_distance_fn &&
-			    (cpu_distance_fn(cpu, tcpu) > LOCAL_DISTANCE ||
-			     cpu_distance_fn(tcpu, cpu) > LOCAL_DISTANCE)) {
+			if (tcpu == cpu) {
+				group_map[cpu] = group;
+				group_cnt[group] = 1;
 				group++;
-				nr_groups = max(nr_groups, group + 1);
-				goto next_group;
+				break;
+			}
+
+			if (!cpu_distance_fn ||
+			    (cpu_distance_fn(cpu, tcpu) == LOCAL_DISTANCE &&
+			     cpu_distance_fn(tcpu, cpu) == LOCAL_DISTANCE)) {
+				group_map[cpu] = group_map[tcpu];
+				group_cnt[group_map[cpu]]++;
+				break;
 			}
 		}
-		group_map[cpu] = group;
-		group_cnt[group]++;
-	}
+	nr_groups = group;
 
 	/*
 	 * Expand unit size until address space usage goes over 75%
-- 
1.9.1

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Re: [PATCH 1/1] mm/percpu.c: simplify grouping cpu logic in pcpu_build_alloc_info()

[Tejun Heo]

On Sat, Sep 24, 2016 at 02:15:09AM +0800, zijun_hu wrote:
> From: zijun_hu <zijun_hu@htc.com>
> 
> simplify grouping cpu logic in pcpu_build_alloc_info() to improve
> readability and performance, it discards the goto statement too
> 
> for every possible cpu, decide whether it can share group id of any
> lower index CPU, use the group id if so, otherwise a new group id
> is allocated to it
> 
> Signed-off-by: zijun_hu <zijun_hu@htc.com>

I'm not gonna change that code unless there are clear upsides.  It's a
complicated code path which is run once during boot.  It's not worth
optimizing, the author doesn't explain how the change has been tested
or verified and doesn't respond to people pointing out that these
drive-by patches aren't helpful.

I won't engage with his patches until he changes his approach and
think that it's advisable for others to do so too.

Nacked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun

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