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SubjectRe: [PATCH 2/2] fix file counting
I have benchmarked this on an x86_64 NUMA system and see no
significant performance difference on kernbench. Tested on
both x86_64 and powerpc.

The way we do file struct accounting is not very suitable for batched
freeing. For scalability reasons, file accounting was constructor/destructor
based. This meant that nr_files was decremented only when
the object was removed from the slab cache. This is
susceptible to slab fragmentation. With RCU based file structure,
consequent batched freeing and a test program like Serge's,
we just speed this up and end up with a very fragmented slab -

llm22:~ # cat /proc/sys/fs/file-nr
587730 0 758844

At the same time, I see only a 2000+ objects in filp cache.
The following patch I fixes this problem.

This patch changes the file counting by removing the filp_count_lock.
Instead we use a separate percpu counter, nr_files, for now and all
accesses to it are through get_nr_files() api. In the sysctl
handler for nr_files, we populate files_stat.nr_files before returning
to user.

Counting files as an when they are created and destroyed (as opposed
to inside slab) allows us to correctly count open files with RCU.

Signed-off-by: Dipankar Sarma <dipankar@in.ibm.com>
---



fs/dcache.c | 2 -
fs/file_table.c | 80 +++++++++++++++++++++++++++++++--------------------
include/linux/file.h | 2 -
include/linux/fs.h | 2 +
kernel/sysctl.c | 5 ++-
net/unix/af_unix.c | 2 -
6 files changed, 57 insertions(+), 36 deletions(-)
diff -puN fs/dcache.c~fix-file-counting fs/dcache.c
--- linux-2.6.16-rc3-rcu/fs/dcache.c~fix-file-counting 2006-02-15 16:00:02.000000000 +0530
+++ linux-2.6.16-rc3-rcu-dipankar/fs/dcache.c 2006-02-15 16:00:02.000000000 +0530
@@ -1736,7 +1736,7 @@ void __init vfs_caches_init(unsigned lon
SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL, NULL);

filp_cachep = kmem_cache_create("filp", sizeof(struct file), 0,
- SLAB_HWCACHE_ALIGN|SLAB_PANIC, filp_ctor, filp_dtor);
+ SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL, NULL);

dcache_init(mempages);
inode_init(mempages);
diff -puN fs/file_table.c~fix-file-counting fs/file_table.c
--- linux-2.6.16-rc3-rcu/fs/file_table.c~fix-file-counting 2006-02-15 16:00:02.000000000 +0530
+++ linux-2.6.16-rc3-rcu-dipankar/fs/file_table.c 2006-02-16 21:52:35.000000000 +0530
@@ -5,6 +5,7 @@
* Copyright (C) 1997 David S. Miller (davem@caip.rutgers.edu)
*/

+#include <linux/config.h>
#include <linux/string.h>
#include <linux/slab.h>
#include <linux/file.h>
@@ -19,52 +20,67 @@
#include <linux/capability.h>
#include <linux/cdev.h>
#include <linux/fsnotify.h>
-
+#include <linux/sysctl.h>
+#include <linux/percpu_counter.h>
+#include <asm/atomic.h>
+
/* sysctl tunables... */
struct files_stat_struct files_stat = {
.max_files = NR_FILE
};

-EXPORT_SYMBOL(files_stat); /* Needed by unix.o */
-
/* public. Not pretty! */
- __cacheline_aligned_in_smp DEFINE_SPINLOCK(files_lock);
+__cacheline_aligned_in_smp DEFINE_SPINLOCK(files_lock);
+
+static struct percpu_counter nr_files __cacheline_aligned_in_smp;

-static DEFINE_SPINLOCK(filp_count_lock);
+static inline void file_free_rcu(struct rcu_head *head)
+{
+ struct file *f = container_of(head, struct file, f_u.fu_rcuhead);
+ kmem_cache_free(filp_cachep, f);
+}

-/* slab constructors and destructors are called from arbitrary
- * context and must be fully threaded - use a local spinlock
- * to protect files_stat.nr_files
+static inline void file_free(struct file *f)
+{
+ percpu_counter_mod(&nr_files, -1L);
+ call_rcu(&f->f_u.fu_rcuhead, file_free_rcu);
+}
+
+/*
+ * Return the total number of open files in the system
*/
-void filp_ctor(void *objp, struct kmem_cache *cachep, unsigned long cflags)
+int get_nr_files(void)
{
- if ((cflags & (SLAB_CTOR_VERIFY|SLAB_CTOR_CONSTRUCTOR)) ==
- SLAB_CTOR_CONSTRUCTOR) {
- unsigned long flags;
- spin_lock_irqsave(&filp_count_lock, flags);
- files_stat.nr_files++;
- spin_unlock_irqrestore(&filp_count_lock, flags);
- }
+ return percpu_counter_read_positive(&nr_files);
}

-void filp_dtor(void *objp, struct kmem_cache *cachep, unsigned long dflags)
+/*
+ * Return the maximum number of open files in the system
+ */
+int get_max_files(void)
{
- unsigned long flags;
- spin_lock_irqsave(&filp_count_lock, flags);
- files_stat.nr_files--;
- spin_unlock_irqrestore(&filp_count_lock, flags);
+ return files_stat.max_files;
}

-static inline void file_free_rcu(struct rcu_head *head)
+EXPORT_SYMBOL(get_max_files);
+
+/*
+ * Handle nr_files sysctl
+ */
+#if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS)
+int proc_nr_files(ctl_table *table, int write, struct file *filp,
+ void __user *buffer, size_t *lenp, loff_t *ppos)
{
- struct file *f = container_of(head, struct file, f_u.fu_rcuhead);
- kmem_cache_free(filp_cachep, f);
+ files_stat.nr_files = get_nr_files();
+ return proc_dointvec(table, write, filp, buffer, lenp, ppos);
}
-
-static inline void file_free(struct file *f)
+#else
+int proc_nr_files(ctl_table *table, int write, struct file *filp,
+ void __user *buffer, size_t *lenp, loff_t *ppos)
{
- call_rcu(&f->f_u.fu_rcuhead, file_free_rcu);
+ return -ENOSYS;
}
+#endif

/* Find an unused file structure and return a pointer to it.
* Returns NULL, if there are no more free file structures or
@@ -78,7 +94,7 @@ struct file *get_empty_filp(void)
/*
* Privileged users can go above max_files
*/
- if (files_stat.nr_files >= files_stat.max_files &&
+ if (get_nr_files() >= files_stat.max_files &&
!capable(CAP_SYS_ADMIN))
goto over;

@@ -86,6 +102,7 @@ struct file *get_empty_filp(void)
if (f == NULL)
goto fail;

+ percpu_counter_mod(&nr_files, 1L);
memset(f, 0, sizeof(*f));
if (security_file_alloc(f))
goto fail_sec;
@@ -101,10 +118,10 @@ struct file *get_empty_filp(void)

over:
/* Ran out of filps - report that */
- if (files_stat.nr_files > old_max) {
+ if (get_nr_files() > old_max) {
printk(KERN_INFO "VFS: file-max limit %d reached\n",
- files_stat.max_files);
- old_max = files_stat.nr_files;
+ get_max_files());
+ old_max = get_nr_files();
}
goto fail;

@@ -276,4 +293,5 @@ void __init files_init(unsigned long mem
if (files_stat.max_files < NR_FILE)
files_stat.max_files = NR_FILE;
files_defer_init();
+ percpu_counter_init(&nr_files);
}
diff -puN include/linux/file.h~fix-file-counting include/linux/file.h
--- linux-2.6.16-rc3-rcu/include/linux/file.h~fix-file-counting 2006-02-15 16:00:02.000000000 +0530
+++ linux-2.6.16-rc3-rcu-dipankar/include/linux/file.h 2006-02-15 16:00:02.000000000 +0530
@@ -60,8 +60,6 @@ extern void put_filp(struct file *);
extern int get_unused_fd(void);
extern void FASTCALL(put_unused_fd(unsigned int fd));
struct kmem_cache;
-extern void filp_ctor(void * objp, struct kmem_cache *cachep, unsigned long cflags);
-extern void filp_dtor(void * objp, struct kmem_cache *cachep, unsigned long dflags);

extern struct file ** alloc_fd_array(int);
extern void free_fd_array(struct file **, int);
diff -puN include/linux/fs.h~fix-file-counting include/linux/fs.h
--- linux-2.6.16-rc3-rcu/include/linux/fs.h~fix-file-counting 2006-02-15 16:00:02.000000000 +0530
+++ linux-2.6.16-rc3-rcu-dipankar/include/linux/fs.h 2006-02-15 16:00:02.000000000 +0530
@@ -35,6 +35,8 @@ struct files_stat_struct {
int max_files; /* tunable */
};
extern struct files_stat_struct files_stat;
+extern int get_nr_files(void);
+extern int get_max_files(void);

struct inodes_stat_t {
int nr_inodes;
diff -puN kernel/sysctl.c~fix-file-counting kernel/sysctl.c
--- linux-2.6.16-rc3-rcu/kernel/sysctl.c~fix-file-counting 2006-02-15 16:00:02.000000000 +0530
+++ linux-2.6.16-rc3-rcu-dipankar/kernel/sysctl.c 2006-02-15 16:00:02.000000000 +0530
@@ -52,6 +52,9 @@
#include <linux/nfs_fs.h>
#endif

+extern int proc_nr_files(ctl_table *table, int write, struct file *filp,
+ void __user *buffer, size_t *lenp, loff_t *ppos);
+
#if defined(CONFIG_SYSCTL)

/* External variables not in a header file. */
@@ -921,7 +924,7 @@ static ctl_table fs_table[] = {
.data = &files_stat,
.maxlen = 3*sizeof(int),
.mode = 0444,
- .proc_handler = &proc_dointvec,
+ .proc_handler = &proc_nr_files,
},
{
.ctl_name = FS_MAXFILE,
diff -puN net/unix/af_unix.c~fix-file-counting net/unix/af_unix.c
--- linux-2.6.16-rc3-rcu/net/unix/af_unix.c~fix-file-counting 2006-02-15 16:00:02.000000000 +0530
+++ linux-2.6.16-rc3-rcu-dipankar/net/unix/af_unix.c 2006-02-15 16:00:02.000000000 +0530
@@ -547,7 +547,7 @@ static struct sock * unix_create1(struct
struct sock *sk = NULL;
struct unix_sock *u;

- if (atomic_read(&unix_nr_socks) >= 2*files_stat.max_files)
+ if (atomic_read(&unix_nr_socks) >= 2*get_max_files())
goto out;

sk = sk_alloc(PF_UNIX, GFP_KERNEL, &unix_proto, 1);
_
-
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