[PATCH v3 03/32] dcache: convert dentry_stat.nr_unused to per-cpu counters

[Glauber Costa <glommer@parallels.com>]

From: Dave Chinner <dchinner@redhat.com>

Before we split up the dcache_lru_lock, the unused dentry counter
needs to be made independent of the global dcache_lru_lock. Convert
it to per-cpu counters to do this.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
 fs/dcache.c | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index fbfae008..ffdd461 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -118,6 +118,14 @@ struct dentry_stat_t dentry_stat = {
 };
 
 static DEFINE_PER_CPU(unsigned int, nr_dentry);
+/*
+ * The total counts for nr_dentry_unused are hotplug-safe, since we always loop
+ * through all possible cpus. It is quite possible, though, that the counters
+ * go negative.  That could easily happen for a dentry that is marked unused in
+ * one CPU but decrements that count after being preempted to another CPU.
+ * Therefore, we must use a signed quantity in here.
+ */
+static DEFINE_PER_CPU(long , nr_dentry_unused);
 
 #if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS)
 static int get_nr_dentry(void)
@@ -129,10 +137,20 @@ static int get_nr_dentry(void)
 	return sum < 0 ? 0 : sum;
 }
 
+static int get_nr_dentry_unused(void)
+{
+	int i;
+	int sum = 0;
+	for_each_possible_cpu(i)
+		sum += per_cpu(nr_dentry_unused, i);
+	return sum < 0 ? 0 : sum;
+}
+
 int proc_nr_dentry(ctl_table *table, int write, void __user *buffer,
 		   size_t *lenp, loff_t *ppos)
 {
 	dentry_stat.nr_dentry = get_nr_dentry();
+	dentry_stat.nr_unused = get_nr_dentry_unused();
 	return proc_dointvec(table, write, buffer, lenp, ppos);
 }
 #endif
@@ -312,7 +330,7 @@ static void dentry_lru_add(struct dentry *dentry)
 		spin_lock(&dcache_lru_lock);
 		list_add(&dentry->d_lru, &dentry->d_sb->s_dentry_lru);
 		dentry->d_sb->s_nr_dentry_unused++;
-		dentry_stat.nr_unused++;
+		this_cpu_inc(nr_dentry_unused);
 		spin_unlock(&dcache_lru_lock);
 	}
 }
@@ -322,7 +340,7 @@ static void __dentry_lru_del(struct dentry *dentry)
 	list_del_init(&dentry->d_lru);
 	dentry->d_flags &= ~DCACHE_SHRINK_LIST;
 	dentry->d_sb->s_nr_dentry_unused--;
-	dentry_stat.nr_unused--;
+	this_cpu_dec(nr_dentry_unused);
 }
 
 /*
@@ -360,7 +378,7 @@ static void dentry_lru_move_list(struct dentry *dentry, struct list_head *list)
 	if (list_empty(&dentry->d_lru)) {
 		list_add_tail(&dentry->d_lru, list);
 		dentry->d_sb->s_nr_dentry_unused++;
-		dentry_stat.nr_unused++;
+		this_cpu_inc(nr_dentry_unused);
 	} else {
 		list_move_tail(&dentry->d_lru, list);
 	}
-- 
1.8.1.4

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