Re: [patch 2/2] fix file counting

From: Eric Dumazet
Date: Thu Jan 26 2006 - 15:13:50 EST

Dipankar Sarma a écrit :
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 atomic_t, 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@xxxxxxxxxx>
---

Well...

I am using a patch that seems sligthly better : It removes the filp_count_lock as yours but introduces a percpu variable, and a lazy nr_files . (Its value can be off with a delta of +/- 16*num_possible_cpus()

Pros :
- No more delay caused by the slab dtor hack (like your patch)
- No more ping pongs in SMP/NUMA machines because of the nr_files being constantly modified.
- litle memory footprint of NR_CPUS*4 bytes
- No more cli/sti games (because of filp ctor/dtor spinlock_bh)
- Moves files_stat definition in a new file (include/linux/files_stat.h)

Cons : The lazy nr_files value in SMP, but who cares ? Do we want the exact value displayed in /proc/sys/fs/file-nr ?

Thank you

Signed-off-by: Eric Dumazet <dada1@xxxxxxxxxxxxx>


--- linux-2.6.16-rc1-mm3/include/linux/fs.h 2006-01-26 21:42:54.000000000 +0100
+++ linux-2.6.16-rc1-mm3-ed/include/linux/fs.h 2006-01-26 21:46:44.000000000 +0100
@@ -29,12 +29,6 @@
#define BLOCK_SIZE (1<<BLOCK_SIZE_BITS)

/* And dynamically-tunable limits and defaults: */
-struct files_stat_struct {
- int nr_files; /* read only */
- int nr_free_files; /* read only */
- int max_files; /* tunable */
-};
-extern struct files_stat_struct files_stat;

struct inodes_stat_t {
int nr_inodes;
--- linux-2.6.16-rc1-mm3/include/linux/files_stat.h 1970-01-01 01:00:00.000000000 +0100
+++ linux-2.6.16-rc1-mm3-ed/include/linux/files_stat.h 2006-01-26 21:26:08.000000000 +0100
@@ -0,0 +1,34 @@
+#ifndef _LINUX_FILES_STAT_H
+#define _LINUX_FILES_STAT_H
+/*
+ * nr_files uses a percpu_counter to reduce atomic ops and/or cli/sti
+ * Using a percpu_counter is lazy, but we dont need exact values,
+ * and can consider max_files as a lazy limit, at least for SMP machines.
+ */
+#include <linux/percpu_counter.h>
+
+struct files_stat_struct {
+ /*
+ * WARNING : The 3 first fields of this structure
+ * must be of int type :_nr_files, nr_free_files, max_files
+ * in this exact order.
+ * (see kernel/sysctl.c )
+ */
+ int _nr_files; /* rd only, updated when reading /proc/sys/fs/file-nr */
+ int nr_free_files; /* rd only , obsolete */
+ int max_files; /* rw (/proc/sys/fs/file-max) */
+ atomic_t nr_files;
+};
+extern struct files_stat_struct files_stat;
+
+static inline int get_nr_files(void)
+{
+ return atomic_read(&files_stat.nr_files);
+}
+
+static inline int get_max_files(void)
+{
+ return files_stat.max_files;
+}
+
+#endif /* _LINUX_FILES_STAT_H */
--- linux-2.6.16-rc1-mm3/fs/file_table.c 2006-01-26 21:42:53.000000000 +0100
+++ linux-2.6.16-rc1-mm3-ed/fs/file_table.c 2006-01-26 21:48:27.000000000 +0100
@@ -19,10 +19,12 @@
#include <linux/capability.h>
#include <linux/cdev.h>
#include <linux/fsnotify.h>
+#include <linux/files_stat.h>

/* sysctl tunables... */
-struct files_stat_struct files_stat = {
- .max_files = NR_FILE
+__cacheline_aligned_in_smp struct files_stat_struct files_stat = {
+ .max_files = NR_FILE,
+ .nr_files = ATOMIC_INIT(0)
};

EXPORT_SYMBOL(files_stat); /* Needed by unix.o */
@@ -30,35 +32,33 @@
/* public. Not pretty! */
__cacheline_aligned_in_smp DEFINE_SPINLOCK(files_lock);

-static DEFINE_SPINLOCK(filp_count_lock);
-
-/* slab constructors and destructors are called from arbitrary
- * context and must be fully threaded - use a local spinlock
- * to protect files_stat.nr_files
- */
-void filp_ctor(void *objp, struct kmem_cache *cachep, unsigned long cflags)
-{
- 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);
+#ifdef CONFIG_SMP
+#define NR_FILES_THRESHOLD 16
+static DEFINE_PER_CPU(int, nr_files);
+static void add_nr_files(int delta)
+{
+ int *local;
+ preempt_disable();
+ local = &__get_cpu_var(nr_files);
+ *local += delta;
+ if (*local > NR_FILES_THRESHOLD || *local < -NR_FILES_THRESHOLD) {
+ atomic_add(*local, &files_stat.nr_files);
+ *local = 0;
}
+ preempt_enable();
}
-
-void filp_dtor(void *objp, struct kmem_cache *cachep, unsigned long dflags)
-{
- unsigned long flags;
- spin_lock_irqsave(&filp_count_lock, flags);
- files_stat.nr_files--;
- spin_unlock_irqrestore(&filp_count_lock, flags);
-}
+#define dec_nr_files() add_nr_files(-1)
+#define inc_nr_files() add_nr_files(1)
+#else
+# define dec_nr_files() atomic_dec(&files_stat.nr_files)
+# define inc_nr_files() atomic_inc(&files_stat.nr_files)
+#endif

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);
+ dec_nr_files();
}

static inline void file_free(struct file *f)
@@ -75,11 +75,13 @@
struct task_struct *tsk;
static int old_max;
struct file * f;
+ int nr_files;

/*
* Privileged users can go above max_files
*/
- if (files_stat.nr_files >= files_stat.max_files &&
+ nr_files = get_nr_files();
+ if (nr_files >= files_stat.max_files &&
!capable(CAP_SYS_ADMIN))
goto over;

@@ -91,6 +93,7 @@
if (security_file_alloc(f))
goto fail_sec;

+ inc_nr_files();
tsk = current;
INIT_LIST_HEAD(&f->f_u.fu_list);
atomic_set(&f->f_count, 1);
@@ -103,10 +106,10 @@

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

--- linux-2.6.16-rc1-mm3/kernel/sysctl.c 2006-01-26 21:42:54.000000000 +0100
+++ linux-2.6.16-rc1-mm3-ed/kernel/sysctl.c 2006-01-26 19:40:04.000000000 +0100
@@ -46,6 +46,7 @@
#include <linux/syscalls.h>
#include <linux/nfs_fs.h>
#include <linux/acpi.h>
+#include <linux/files_stat.h>

#include <asm/uaccess.h>
#include <asm/processor.h>
@@ -131,6 +132,9 @@
static int proc_doutsstring(ctl_table *table, int write, struct file *filp,
void __user *buffer, size_t *lenp, loff_t *ppos);

+static int proc_dointvec_files_stat(ctl_table *table, int write, struct file *filp,
+ void __user *buffer, size_t *lenp, loff_t *ppos);
+
static ctl_table root_table[];
static struct ctl_table_header root_table_header =
{ root_table, LIST_HEAD_INIT(root_table_header.ctl_entry) };
@@ -920,7 +924,7 @@
.data = &files_stat,
.maxlen = 3*sizeof(int),
.mode = 0444,
- .proc_handler = &proc_dointvec,
+ .proc_handler = &proc_dointvec_files_stat,
},
{
.ctl_name = FS_MAXFILE,
@@ -1754,6 +1758,12 @@
return do_proc_dointvec(table,write,filp,buffer,lenp,ppos,
NULL,NULL);
}
+static int proc_dointvec_files_stat(ctl_table *table, int write, struct file *filp,
+ void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+ files_stat._nr_files = get_nr_files();
+ return proc_dointvec(table,write,filp,buffer,lenp,ppos);
+}

#define OP_SET 0
#define OP_AND 1
--- linux-2.6.16-rc1-mm3/net/unix/af_unix.c 2006-01-26 21:42:54.000000000 +0100
+++ linux-2.6.16-rc1-mm3-ed/net/unix/af_unix.c 2006-01-26 19:19:07.000000000 +0100
@@ -100,6 +100,7 @@
#include <linux/net.h>
#include <linux/in.h>
#include <linux/fs.h>
+#include <linux/files_stat.h>
#include <linux/slab.h>
#include <asm/uaccess.h>
#include <linux/skbuff.h>
@@ -547,7 +548,7 @@
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);
--- linux-2.6.16-rc1-mm3/fs/dcache.c 2006-01-26 21:42:53.000000000 +0100
+++ linux-2.6.16-rc1-mm3-ed/fs/dcache.c 2006-01-26 21:15:11.000000000 +0100
@@ -33,6 +33,7 @@
#include <linux/seqlock.h>
#include <linux/swap.h>
#include <linux/bootmem.h>
+#include <linux/files_stat.h>

/* #define DCACHE_DEBUG 1 */

@@ -1738,7 +1739,7 @@
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);
--- linux-2.6.16-rc1-mm3/include/linux/file.h 2006-01-26 21:42:54.000000000 +0100
+++ linux-2.6.16-rc1-mm3-ed/include/linux/file.h 2006-01-26 21:45:23.000000000 +0100
@@ -79,9 +79,6 @@
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);