Most mnt_want/drop_write() pairs are really close in
the code; they aren't held for very long. So, in
practice is hard to get bounced between cpus between
when you mnt_want_write() and mnt_drop_write().
The exception to this is the pair in may_open() and
__fput(). Between those two it is pretty common to
move between cpus. During a kernel compile of around
900 files on a 4-way, I saw it happen ~400 times.
This patch assumes that the cpu doing the allocating of
the 'struct file' is also the one doing the
mnt_want_write(). It is OK that it is wrong sometimes,
it just means that we regress back to the
spinlock-protected search of all of the cpus' counts.
My kernel compile from before went from 400 misses
during a compile to just 20 with this patch.
It might also be helpful to do the writer count per-node
which would _greatly_ decrease the number of migrations
that we see.
---
lxc-dave/fs/file_table.c | 2 +-
lxc-dave/fs/namespace.c | 17 +++++++++--------
lxc-dave/fs/open.c | 4 ++++
lxc-dave/include/linux/fs.h | 1 +
lxc-dave/include/linux/mount.h | 1 +
5 files changed, 16 insertions(+), 9 deletions(-)
diff -puN fs/file_table.c~fput-cpu fs/file_table.c
--- lxc/fs/file_table.c~fput-cpu 2007-02-20 17:59:48.000000000 -0800
+++ lxc-dave/fs/file_table.c 2007-02-20 17:59:49.000000000 -0800
@@ -215,7 +215,7 @@ void fastcall __fput(struct file *file)
if (file->f_mode & FMODE_WRITE) {
put_write_access(inode);
if(!special_file(inode->i_mode))
- mnt_drop_write(mnt);
+ __mnt_drop_write(mnt, file->f_write_cpu);
}
put_pid(file->f_owner.pid);
file_kill(file);
diff -puN fs/namespace.c~fput-cpu fs/namespace.c
--- lxc/fs/namespace.c~fput-cpu 2007-02-20 17:59:48.000000000 -0800
+++ lxc-dave/fs/namespace.c 2007-02-20 18:00:27.000000000 -0800
@@ -89,8 +89,8 @@ struct vfsmount *alloc_vfsmnt(const char
int mnt_want_write(struct vfsmount *mnt)
{
int ret = 0;
- atomic_t *cpu_writecount;
int cpu = get_cpu();
+ atomic_t *cpu_writecount;
retry:
/*
* Not strictly required, but quick and cheap
@@ -122,22 +122,17 @@ out:
put_cpu();
return ret;
}
-EXPORT_SYMBOL_GPL(mnt_want_write);
-void mnt_drop_write(struct vfsmount *mnt)
+void __mnt_drop_write(struct vfsmount *mnt, int cpu)
{
static int miss = 0;
atomic_t *cpu_writecount;
- int cpu;
int borrowed = 0;
int retries = 0;
retry:
- cpu = get_cpu();
cpu_writecount = per_cpu_ptr(mnt->writers, cpu);
- if (atomic_add_unless(cpu_writecount, -1, 0)) {
- put_cpu();
+ if (atomic_add_unless(cpu_writecount, -1, 0))
return;
- }
spin_lock(&vfsmount_lock);
/*
* Holding the spinlock, and only checking cpus that
@@ -167,6 +162,12 @@ retry:
if (!borrowed)
goto retry;
}
+void mnt_drop_write(struct vfsmount *mnt)
+{
+ int cpu = get_cpu();
+ __mnt_drop_write(mnt, cpu);
+ put_cpu();
+}
EXPORT_SYMBOL_GPL(mnt_drop_write);
/*
diff -puN fs/open.c~fput-cpu fs/open.c
--- lxc/fs/open.c~fput-cpu 2007-02-20 17:59:48.000000000 -0800
+++ lxc-dave/fs/open.c 2007-02-20 17:59:49.000000000 -0800
@@ -715,6 +715,10 @@ static struct file *__dentry_open(struct
f->f_path.mnt = mnt;
f->f_pos = 0;
f->f_op = fops_get(inode->i_fop);
+ /*
+ * This is OK to race because it is just a hint
+ */
+ f->f_write_cpu = smp_processor_id();
file_move(f, &inode->i_sb->s_files);
if (!open && f->f_op)
diff -puN include/linux/fs.h~fput-cpu include/linux/fs.h
--- lxc/include/linux/fs.h~fput-cpu 2007-02-20 17:59:48.000000000 -0800
+++ lxc-dave/include/linux/fs.h 2007-02-20 17:59:49.000000000 -0800
@@ -766,6 +766,7 @@ struct file {
struct fown_struct f_owner;
unsigned int f_uid, f_gid;
struct file_ra_state f_ra;
+ int f_write_cpu;
unsigned long f_version;
#ifdef CONFIG_SECURITY
diff -puN include/linux/mount.h~fput-cpu include/linux/mount.h
--- lxc/include/linux/mount.h~fput-cpu 2007-02-20 17:59:49.000000000 -0800
+++ lxc-dave/include/linux/mount.h 2007-02-20 17:59:49.000000000 -0800
@@ -94,6 +94,7 @@ static inline int __mnt_is_readonly(stru
extern int mnt_want_write(struct vfsmount *mnt);
extern void mnt_drop_write(struct vfsmount *mnt);
+extern void __mnt_drop_write(struct vfsmount *mnt, int cpu);
extern void mntput_no_expire(struct vfsmount *mnt);
extern void mnt_pin(struct vfsmount *mnt);
extern void mnt_unpin(struct vfsmount *mnt);
_
-
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