linux-kernel - [PATCH] mm/vmscan: try to optimize branch procedures.

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Date:   Tue, 28 Nov 2017 09:49:45 +0800
From:   Jiang Biao <jiang.biao2@....com.cn>
To:     akpm@...ux-foundation.org, mhocko@...e.com, hannes@...xchg.org,
        hillf.zj@...baba-inc.com, minchan@...nel.org, ying.huang@...el.com,
        mgorman@...hsingularity.net
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        jiang.biao2@....com.cn, zhong.weidong@....com.cn
Subject: [PATCH] mm/vmscan: try to optimize branch procedures.

1. Use unlikely to try to improve branch prediction. The
*total_scan < 0* branch is unlikely to reach, so use unlikely.

2. Optimize *next_deferred >= scanned* condition.
*next_deferred >= scanned* condition could be optimized into
*next_deferred > scanned*, because when *next_deferred == scanned*,
next_deferred shoud be 0, which is covered by the else branch.

3. Merge two branch blocks into one. The *next_deferred > 0* branch
could be merged into *next_deferred > scanned* to simplify the code.

Signed-off-by: Jiang Biao <jiang.biao2@....com.cn>
---
 mm/vmscan.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index eb2f031..5f5d4ab 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -338,7 +338,7 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
 	delta *= freeable;
 	do_div(delta, nr_eligible + 1);
 	total_scan += delta;
-	if (total_scan < 0) {
+	if (unlikely(total_scan < 0)) {
 		pr_err("shrink_slab: %pF negative objects to delete nr=%ld\n",
 		       shrinker->scan_objects, total_scan);
 		total_scan = freeable;
@@ -407,18 +407,16 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
 		cond_resched();
 	}
 
-	if (next_deferred >= scanned)
-		next_deferred -= scanned;
-	else
-		next_deferred = 0;
 	/*
 	 * move the unused scan count back into the shrinker in a
 	 * manner that handles concurrent updates. If we exhausted the
 	 * scan, there is no need to do an update.
 	 */
-	if (next_deferred > 0)
+	if (next_deferred > scanned) {
+		next_deferred -= scanned;
 		new_nr = atomic_long_add_return(next_deferred,
 						&shrinker->nr_deferred[nid]);
+	}
 	else
 		new_nr = atomic_long_read(&shrinker->nr_deferred[nid]);
 
-- 
2.7.4