[PATCH v2 0/4] fs/dcache: Track # of negative dentries

[PATCH v2 0/4] fs/dcache: Track # of negative dentries

[Waiman Long]

 v1->v2:
  - Clarify what the new nr_dentry_negative per-cpu counter is tracking
    and open-code the increment and decrement as suggested by Dave Chinner.
  - Append the new nr_dentry_negative count as the 7th element of dentry-state
    instead of replacing one of the dummy entries.
  - Remove patch "fs/dcache: Make negative dentries easier to be
    reclaimed" for now as I need more time to think about what
    to do with it.
  - Add 2 more patches to address issues found while reviewing the
    dentry code.
  - Add another patch to change the conditional branch of
    nr_dentry_negative accounting to conditional move so as to reduce
    the performance impact of the accounting code.

This patchset addresses 2 issues found in the dentry code and adds a
new nr_dentry_negative per-cpu counter to track the total number of
negative dentries in all the LRU lists.

Patch 1 fixes a bug in the accounting of nr_dentry_unused in
shrink_dcache_sb().

Patch 2 removes the ____cacheline_aligned_in_smp tag from super_block
LRU lists.

Patch 3 adds the new nr_dentry_negative per-cpu counter.

Patch 4 removes conditional branches in nr_dentry_negative accounting
code.

Various filesystem related tests were run and no statistically
significant changes in performance was observed.

Waiman Long (4):
  fs/dcache: Fix incorrect nr_dentry_unused accounting in
    shrink_dcache_sb()
  fs: Don't need to put list_lru into its own cacheline
  fs/dcache: Track & report number of negative dentries
  fs/dcache: Eliminate branches in nr_dentry_negative accounting

 Documentation/sysctl/fs.txt | 22 +++++++++++-------
 fs/dcache.c                 | 54 ++++++++++++++++++++++++++++++++++++++++-----
 include/linux/dcache.h      |  5 +++--
 include/linux/fs.h          |  9 ++++----
 kernel/sysctl.c             |  2 +-
 5 files changed, 72 insertions(+), 20 deletions(-)

-- 
1.8.3.1

[PATCH v2 1/4] fs/dcache: Fix incorrect nr_dentry_unused accounting in shrink_dcache_sb()

[Waiman Long]

The nr_dentry_unused per-cpu counter tracks dentries in both the
LRU lists and the shrink lists where the DCACHE_LRU_LIST bit is set.
The shrink_dcache_sb() function moves dentries from the LRU list to a
shrink list and subtracts the dentry count from nr_dentry_unused. This
is incorrect as the nr_dentry_unused count Will also be decremented in
shrink_dentry_list() via d_shrink_del(). To fix this double decrement,
the decrement in the shrink_dcache_sb() function is taken out.

Fixes: 4e717f5c1083 ("list_lru: remove special case function list_lru_dispose_all."

Signed-off-by: Waiman Long <longman@redhat.com>
---
 fs/dcache.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 2e7e8d8..cb515f1 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1202,15 +1202,11 @@ static enum lru_status dentry_lru_isolate_shrink(struct list_head *item,
  */
 void shrink_dcache_sb(struct super_block *sb)
 {
-	long freed;
-
 	do {
 		LIST_HEAD(dispose);
 
-		freed = list_lru_walk(&sb->s_dentry_lru,
+		list_lru_walk(&sb->s_dentry_lru,
 			dentry_lru_isolate_shrink, &dispose, 1024);
-
-		this_cpu_sub(nr_dentry_unused, freed);
 		shrink_dentry_list(&dispose);
 	} while (list_lru_count(&sb->s_dentry_lru) > 0);
 }
-- 
1.8.3.1

[PATCH v2 2/4] fs: Don't need to put list_lru into its own cacheline

[Waiman Long]

The list_lru structure is essentially just a pointer to a table of
per-node LRU lists. Even if CONFIG_MEMCG_KMEM is defined, the list
field is just used for LRU list registration and shrinker_id is set
at initialization. Those fields won't need to be touched that often.

So there is no point to make the list_lru structures to sit in their
own cachelines.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/fs.h | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 3332270..fd4cd8a 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1443,11 +1443,12 @@ struct super_block {
 	struct user_namespace *s_user_ns;
 
 	/*
-	 * Keep the lru lists last in the structure so they always sit on their
-	 * own individual cachelines.
+	 * The list_lru structure is essentially just a pointer to a table
+	 * of per-node lru lists, each of which has its own spinlock.
+	 * There is no need to put them into separate cachelines.
 	 */
-	struct list_lru		s_dentry_lru ____cacheline_aligned_in_smp;
-	struct list_lru		s_inode_lru ____cacheline_aligned_in_smp;
+	struct list_lru		s_dentry_lru;
+	struct list_lru		s_inode_lru;
 	struct rcu_head		rcu;
 	struct work_struct	destroy_work;
 
-- 
1.8.3.1

[PATCH v2 3/4] fs/dcache: Track & report number of negative dentries

[Waiman Long]

The current dentry number tracking code doesn't distinguish between
positive & negative dentries. It just reports the total number of
dentries in the LRU lists.

As excessive number of negative dentries can have an impact on system
performance, it will be wise to track the number of positive and
negative dentries separately.

This patch adds tracking for the total number of negative dentries
in the system LRU lists and reports it in the 7th field in the
/proc/sys/fs/dentry-state file. The number, however, does not include
negative dentries that are in flight but not in the LRU yet as well
as those in the shrinker lists.

The number of positive dentries in the LRU lists can be roughly found
by subtracting the number of negative dentries from the unused count.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 Documentation/sysctl/fs.txt | 22 ++++++++++++++--------
 fs/dcache.c                 | 31 +++++++++++++++++++++++++++++++
 include/linux/dcache.h      |  5 +++--
 kernel/sysctl.c             |  2 +-
 4 files changed, 49 insertions(+), 11 deletions(-)

diff --git a/Documentation/sysctl/fs.txt b/Documentation/sysctl/fs.txt
index 819caf8..21679d0 100644
--- a/Documentation/sysctl/fs.txt
+++ b/Documentation/sysctl/fs.txt
@@ -56,26 +56,32 @@ of any kernel data structures.
 
 dentry-state:
 
-From linux/fs/dentry.c:
+From linux/include/linux/dcache.h:
 --------------------------------------------------------------
-struct {
+struct dentry_stat_t dentry_stat {
         int nr_dentry;
         int nr_unused;
         int age_limit;         /* age in seconds */
         int want_pages;        /* pages requested by system */
         int dummy[2];
-} dentry_stat = {0, 0, 45, 0,};
--------------------------------------------------------------- 
+        int nr_negative;       /* # of unused negative dentries */
+};
+--------------------------------------------------------------
+
+Dentries are dynamically allocated and deallocated.
+
+nr_dentry shows the total number of dentries allocated (active
++ unused). nr_unused shows the number of dentries that are not
+actively used, but are saved in the LRU list for future reuse.
 
-Dentries are dynamically allocated and deallocated, and
-nr_dentry seems to be 0 all the time. Hence it's safe to
-assume that only nr_unused, age_limit and want_pages are
-used. Nr_unused seems to be exactly what its name says.
 Age_limit is the age in seconds after which dcache entries
 can be reclaimed when memory is short and want_pages is
 nonzero when shrink_dcache_pages() has been called and the
 dcache isn't pruned yet.
 
+nr_negative shows the number of unused dentries that are also
+negative dentries which do not mapped to actual files.
+
 ==============================================================
 
 dquot-max & dquot-nr:
diff --git a/fs/dcache.c b/fs/dcache.c
index cb515f1..c1cc956 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -119,6 +119,7 @@ struct dentry_stat_t dentry_stat = {
 
 static DEFINE_PER_CPU(long, nr_dentry);
 static DEFINE_PER_CPU(long, nr_dentry_unused);
+static DEFINE_PER_CPU(long, nr_dentry_negative);
 
 #if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS)
 
@@ -152,11 +153,22 @@ static long get_nr_dentry_unused(void)
 	return sum < 0 ? 0 : sum;
 }
 
+static long get_nr_dentry_negative(void)
+{
+	int i;
+	long sum = 0;
+
+	for_each_possible_cpu(i)
+		sum += per_cpu(nr_dentry_negative, i);
+	return sum < 0 ? 0 : sum;
+}
+
 int proc_nr_dentry(struct 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();
+	dentry_stat.nr_negative = get_nr_dentry_negative();
 	return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
 }
 #endif
@@ -331,6 +343,8 @@ static inline void __d_clear_type_and_inode(struct dentry *dentry)
 	flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
 	WRITE_ONCE(dentry->d_flags, flags);
 	dentry->d_inode = NULL;
+	if (dentry->d_flags & DCACHE_LRU_LIST)
+		this_cpu_inc(nr_dentry_negative);
 }
 
 static void dentry_free(struct dentry *dentry)
@@ -385,6 +399,10 @@ static void dentry_unlink_inode(struct dentry * dentry)
  * The per-cpu "nr_dentry_unused" counters are updated with
  * the DCACHE_LRU_LIST bit.
  *
+ * The per-cpu "nr_dentry_negative" counters are only updated
+ * when deleted or added to the per-superblock LRU list, not
+ * on the shrink list.
+ *
  * These helper functions make sure we always follow the
  * rules. d_lock must be held by the caller.
  */
@@ -394,6 +412,8 @@ static void d_lru_add(struct dentry *dentry)
 	D_FLAG_VERIFY(dentry, 0);
 	dentry->d_flags |= DCACHE_LRU_LIST;
 	this_cpu_inc(nr_dentry_unused);
+	if (d_is_negative(dentry))
+		this_cpu_inc(nr_dentry_negative);
 	WARN_ON_ONCE(!list_lru_add(&dentry->d_sb->s_dentry_lru, &dentry->d_lru));
 }
 
@@ -402,6 +422,8 @@ static void d_lru_del(struct dentry *dentry)
 	D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
 	dentry->d_flags &= ~DCACHE_LRU_LIST;
 	this_cpu_dec(nr_dentry_unused);
+	if (d_is_negative(dentry))
+		this_cpu_dec(nr_dentry_negative);
 	WARN_ON_ONCE(!list_lru_del(&dentry->d_sb->s_dentry_lru, &dentry->d_lru));
 }
 
@@ -432,6 +454,8 @@ static void d_lru_isolate(struct list_lru_one *lru, struct dentry *dentry)
 	D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
 	dentry->d_flags &= ~DCACHE_LRU_LIST;
 	this_cpu_dec(nr_dentry_unused);
+	if (d_is_negative(dentry))
+		this_cpu_dec(nr_dentry_negative);
 	list_lru_isolate(lru, &dentry->d_lru);
 }
 
@@ -440,6 +464,8 @@ static void d_lru_shrink_move(struct list_lru_one *lru, struct dentry *dentry,
 {
 	D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
 	dentry->d_flags |= DCACHE_SHRINK_LIST;
+	if (d_is_negative(dentry))
+		this_cpu_dec(nr_dentry_negative);
 	list_lru_isolate_move(lru, &dentry->d_lru, list);
 }
 
@@ -1836,6 +1862,11 @@ static void __d_instantiate(struct dentry *dentry, struct inode *inode)
 	WARN_ON(d_in_lookup(dentry));
 
 	spin_lock(&dentry->d_lock);
+	/*
+	 * Decrement negative dentry count if it was in the LRU list.
+	 */
+	if (dentry->d_flags & DCACHE_LRU_LIST)
+		this_cpu_dec(nr_dentry_negative);
 	hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
 	raw_write_seqcount_begin(&dentry->d_seq);
 	__d_set_inode_and_type(dentry, inode, add_flags);
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index ef4b70f..619a249 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -62,9 +62,10 @@ struct qstr {
 struct dentry_stat_t {
 	long nr_dentry;
 	long nr_unused;
-	long age_limit;          /* age in seconds */
-	long want_pages;         /* pages requested by system */
+	long age_limit;		/* age in seconds */
+	long want_pages;	/* pages requested by system */
 	long dummy[2];
+	long nr_negative;	/* # of unused negative dentries */
 };
 extern struct dentry_stat_t dentry_stat;
 
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index cc02050..77ff61c 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1708,7 +1708,7 @@ static int sysrq_sysctl_handler(struct ctl_table *table, int write,
 	{
 		.procname	= "dentry-state",
 		.data		= &dentry_stat,
-		.maxlen		= 6*sizeof(long),
+		.maxlen		= 7*sizeof(long),
 		.mode		= 0444,
 		.proc_handler	= proc_nr_dentry,
 	},
-- 
1.8.3.1

[PATCH v2 4/4] fs/dcache: Eliminate branches in nr_dentry_negative accounting

[Waiman Long]

Because the accounting of nr_dentry_negative depends on whether a dentry
is a negative one or not, branch instructions are introduced to handle
the accounting conditionally. That may potentially slow down the task
by a noticeable amount if that introduces sizeable amount of additional
branch mispredictions.

To avoid that, the accounting code is now modified to use conditional
move instructions instead, if supported by the architecture.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 fs/dcache.c | 41 +++++++++++++++++++++++++++++------------
 1 file changed, 29 insertions(+), 12 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index c1cc956..dfd5628 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -171,6 +171,29 @@ int proc_nr_dentry(struct ctl_table *table, int write, void __user *buffer,
 	dentry_stat.nr_negative = get_nr_dentry_negative();
 	return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
 }
+
+/*
+ * Increment/Decrement nr_dentry_negative if the condition is true.
+ * For architectures that support some kind of conditional move, compiler
+ * should be able generate code to inc/dec negative dentry counter
+ * without any branch instruction.
+ */
+static inline void cond_negative_dentry_inc(bool cond)
+{
+	int val = !!cond;
+
+	this_cpu_add(nr_dentry_negative, val);
+}
+
+static inline void cond_negative_dentry_dec(bool cond)
+{
+	int val = !!cond;
+
+	this_cpu_sub(nr_dentry_negative, val);
+}
+#else
+static inline void cond_negative_dentry_inc(bool cond) { }
+static inline void cond_negative_dentry_dec(bool cond) { }
 #endif
 
 /*
@@ -343,8 +366,7 @@ static inline void __d_clear_type_and_inode(struct dentry *dentry)
 	flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
 	WRITE_ONCE(dentry->d_flags, flags);
 	dentry->d_inode = NULL;
-	if (dentry->d_flags & DCACHE_LRU_LIST)
-		this_cpu_inc(nr_dentry_negative);
+	cond_negative_dentry_inc(dentry->d_flags & DCACHE_LRU_LIST);
 }
 
 static void dentry_free(struct dentry *dentry)
@@ -412,8 +434,7 @@ static void d_lru_add(struct dentry *dentry)
 	D_FLAG_VERIFY(dentry, 0);
 	dentry->d_flags |= DCACHE_LRU_LIST;
 	this_cpu_inc(nr_dentry_unused);
-	if (d_is_negative(dentry))
-		this_cpu_inc(nr_dentry_negative);
+	cond_negative_dentry_inc(d_is_negative(dentry));
 	WARN_ON_ONCE(!list_lru_add(&dentry->d_sb->s_dentry_lru, &dentry->d_lru));
 }
 
@@ -422,8 +443,7 @@ static void d_lru_del(struct dentry *dentry)
 	D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
 	dentry->d_flags &= ~DCACHE_LRU_LIST;
 	this_cpu_dec(nr_dentry_unused);
-	if (d_is_negative(dentry))
-		this_cpu_dec(nr_dentry_negative);
+	cond_negative_dentry_dec(d_is_negative(dentry));
 	WARN_ON_ONCE(!list_lru_del(&dentry->d_sb->s_dentry_lru, &dentry->d_lru));
 }
 
@@ -454,8 +474,7 @@ static void d_lru_isolate(struct list_lru_one *lru, struct dentry *dentry)
 	D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
 	dentry->d_flags &= ~DCACHE_LRU_LIST;
 	this_cpu_dec(nr_dentry_unused);
-	if (d_is_negative(dentry))
-		this_cpu_dec(nr_dentry_negative);
+	cond_negative_dentry_dec(d_is_negative(dentry));
 	list_lru_isolate(lru, &dentry->d_lru);
 }
 
@@ -464,8 +483,7 @@ static void d_lru_shrink_move(struct list_lru_one *lru, struct dentry *dentry,
 {
 	D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
 	dentry->d_flags |= DCACHE_SHRINK_LIST;
-	if (d_is_negative(dentry))
-		this_cpu_dec(nr_dentry_negative);
+	cond_negative_dentry_dec(d_is_negative(dentry));
 	list_lru_isolate_move(lru, &dentry->d_lru, list);
 }
 
@@ -1865,8 +1883,7 @@ static void __d_instantiate(struct dentry *dentry, struct inode *inode)
 	/*
 	 * Decrement negative dentry count if it was in the LRU list.
 	 */
-	if (dentry->d_flags & DCACHE_LRU_LIST)
-		this_cpu_dec(nr_dentry_negative);
+	cond_negative_dentry_dec(dentry->d_flags & DCACHE_LRU_LIST);
 	hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
 	raw_write_seqcount_begin(&dentry->d_seq);
 	__d_set_inode_and_type(dentry, inode, add_flags);
-- 
1.8.3.1