linux-kernel - [PATCH 2/2] clk: Use hashtable for global clk lookups

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Message-ID: <20250812085328.3306705-2-wenst@chromium.org>
Date: Tue, 12 Aug 2025 16:53:27 +0800
From: Chen-Yu Tsai <wenst@...omium.org>
To: Stephen Boyd <sboyd@...nel.org>
Cc: Chen-Yu Tsai <wenst@...omium.org>,
	linux-clk@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] clk: Use hashtable for global clk lookups

A clk lookup using clk_core_lookup() is currently somewhat expensive
since it has to walk the whole clk tree to find a match. This is
extremely bad in the clk_core_init() function where it is used to look
for clk name conflicts, which is always the worst case of walking the
whole tree. Moreover, the number of clks checked increases as more
clks are registered, causing each subsequent clk registration becoming
slower.

Add a hashtable for doing clk lookups to replace the tree walk method.
On arm64 this increases kernel memory usage by 4 KB for the hashtable,
and 16 bytes (2 pointers) for |struct hlist_node| in each clk. On a
platform with around 800 clks, this reduces the time spent in
clk_core_lookup() significantly:

          |      PID 0      |     kworker     |
          | before |  after | before |  after |
    -------------------------------------------
    avg   | 203 us | 2.7 us | 123 us | 1.5 us |
    -------------------------------------------
    min   | 4.7 us | 2.3 us | 102 us | 0.9 us |
    -------------------------------------------
    max   | 867 us | 4.8 us | 237 us | 3.5 us |
    -------------------------------------------
    culm  | 109 ms | 1.5 ms |  21 ms | 0.3 ms |

This in turn reduces the time spent in clk_hw_register(), and
ultimately, boot time. On a different system with close to 700 clks,
This reduces boot time by around 110 ms. While this doesn't seem like
a lot, this helps in cases where minimizing boot time is important.

Signed-off-by: Chen-Yu Tsai <wenst@...omium.org>
---
 drivers/clk/clk.c | 50 +++++++++++++++++------------------------------
 1 file changed, 18 insertions(+), 32 deletions(-)

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index 2eb63d610cbb..2d54224fb3ef 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -12,6 +12,7 @@
 #include <linux/clk-provider.h>
 #include <linux/device.h>
 #include <linux/err.h>
+#include <linux/hashtable.h>
 #include <linux/init.h>
 #include <linux/list.h>
 #include <linux/module.h>
@@ -21,6 +22,8 @@
 #include <linux/sched.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
+#include <linux/string.h>
+#include <linux/stringhash.h>
 
 #include "clk.h"
 
@@ -33,6 +36,9 @@ static struct task_struct *enable_owner;
 static int prepare_refcnt;
 static int enable_refcnt;
 
+#define CLK_HASH_BITS 9
+DEFINE_HASHTABLE(clk_hashtable, CLK_HASH_BITS);
+
 static HLIST_HEAD(clk_root_list);
 static HLIST_HEAD(clk_orphan_list);
 static LIST_HEAD(clk_notifier_list);
@@ -87,6 +93,7 @@ struct clk_core {
 	struct clk_duty		duty;
 	struct hlist_head	children;
 	struct hlist_node	child_node;
+	struct hlist_node	hashtable_node;
 	struct hlist_head	clks;
 	unsigned int		notifier_count;
 #ifdef CONFIG_DEBUG_FS
@@ -395,45 +402,20 @@ struct clk_hw *clk_hw_get_parent(const struct clk_hw *hw)
 }
 EXPORT_SYMBOL_GPL(clk_hw_get_parent);
 
-static struct clk_core *__clk_lookup_subtree(const char *name,
-					     struct clk_core *core)
-{
-	struct clk_core *child;
-	struct clk_core *ret;
-
-	if (!strcmp(core->name, name))
-		return core;
-
-	hlist_for_each_entry(child, &core->children, child_node) {
-		ret = __clk_lookup_subtree(name, child);
-		if (ret)
-			return ret;
-	}
-
-	return NULL;
-}
-
 static struct clk_core *clk_core_lookup(const char *name)
 {
-	struct clk_core *root_clk;
-	struct clk_core *ret;
+	struct clk_core *core;
+	u32 hash;
 
 	if (!name)
 		return NULL;
 
-	/* search the 'proper' clk tree first */
-	hlist_for_each_entry(root_clk, &clk_root_list, child_node) {
-		ret = __clk_lookup_subtree(name, root_clk);
-		if (ret)
-			return ret;
-	}
+	hash = full_name_hash(NULL, name, strlen(name));
 
-	/* if not found, then search the orphan tree */
-	hlist_for_each_entry(root_clk, &clk_orphan_list, child_node) {
-		ret = __clk_lookup_subtree(name, root_clk);
-		if (ret)
-			return ret;
-	}
+	/* search the hashtable */
+	hash_for_each_possible(clk_hashtable, core, hashtable_node, hash)
+		if (!strcmp(core->name, name))
+			return core;
 
 	return NULL;
 }
@@ -4013,6 +3995,8 @@ static int __clk_core_init(struct clk_core *core)
 		hlist_add_head(&core->child_node, &clk_orphan_list);
 		core->orphan = true;
 	}
+	hash_add(clk_hashtable, &core->hashtable_node,
+		 full_name_hash(NULL, core->name, strlen(core->name)));
 
 	/*
 	 * Set clk's accuracy.  The preferred method is to use
@@ -4089,6 +4073,7 @@ static int __clk_core_init(struct clk_core *core)
 	clk_pm_runtime_put(core);
 unlock:
 	if (ret) {
+		hash_del(&core->hashtable_node);
 		hlist_del_init(&core->child_node);
 		core->hw->core = NULL;
 	}
@@ -4610,6 +4595,7 @@ void clk_unregister(struct clk *clk)
 
 	clk_core_evict_parent_cache(clk->core);
 
+	hash_del(&clk->core->hashtable_node);
 	hlist_del_init(&clk->core->child_node);
 
 	if (clk->core->prepare_count)
-- 
2.51.0.rc0.215.g125493bb4a-goog