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From:   "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
To:     linux-mm@kvack.org, akpm@linux-foundation.org
Cc:     Wei Xu <weixugc@google.com>, Huang Ying <ying.huang@intel.com>,
        Yang Shi <shy828301@gmail.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Tim C Chen <tim.c.chen@intel.com>,
        Michal Hocko <mhocko@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Hesham Almatary <hesham.almatary@huawei.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Alistair Popple <apopple@nvidia.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Johannes Weiner <hannes@cmpxchg.org>, jvgediya.oss@gmail.com,
        Bharata B Rao <bharata@amd.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Subject: [PATCH v14 10/10] lib/nodemask: Optimize node_random for nodemask with single NUMA node
Date:   Fri, 12 Aug 2022 11:27:09 +0530
Message-Id: <20220812055710.357820-11-aneesh.kumar@linux.ibm.com>
In-Reply-To: <20220812055710.357820-1-aneesh.kumar@linux.ibm.com>
References: <20220812055710.357820-1-aneesh.kumar@linux.ibm.com>
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The most common case for certain node_random usage (demotion nodemask) is with
nodemask weight 1. We can avoid calling get_random_init() in that case and
always return the only node set in the nodemask.

A simple test as below
  before = rdtsc_ordered();
  for (i= 0; i < 100; i++) {
      rand = node_random(&nmask);
  }
  after = rdtsc_ordered();

Without fix after - before : 16438
With fix after - before : 816

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
---
 include/linux/nodemask.h | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/linux/nodemask.h b/include/linux/nodemask.h
index 4b71a96190a8..ac5b6a371be5 100644
--- a/include/linux/nodemask.h
+++ b/include/linux/nodemask.h
@@ -504,12 +504,21 @@ static inline int num_node_state(enum node_states state)
 static inline int node_random(const nodemask_t *maskp)
 {
 #if defined(CONFIG_NUMA) && (MAX_NUMNODES > 1)
-	int w, bit = NUMA_NO_NODE;
+	int w, bit;
 
 	w = nodes_weight(*maskp);
-	if (w)
+	switch (w) {
+	case 0:
+		bit = NUMA_NO_NODE;
+		break;
+	case 1:
+		bit = first_node(*maskp);
+		break;
+	default:
 		bit = bitmap_ord_to_pos(maskp->bits,
-			get_random_int() % w, MAX_NUMNODES);
+					get_random_int() % w, MAX_NUMNODES);
+		break;
+	}
 	return bit;
 #else
 	return 0;
-- 
2.37.1