[RFC PATCH 0/2] avoid the extra atomic on a ref when closing a fd

[RFC PATCH 0/2] avoid the extra atomic on a ref when closing a fd

[Mateusz Guzik]

The stock kernel transitioning the file to no refs held penalizes the
caller with an extra atomic to block any increments.

For cases where the file is highly likely to be going away this is
easily avoidable.

The obvious place is the close() system call, patched in 2/2.

But part from that possible callers (to be evaluated):
- clean up after failed open (common)
- close_range
- close on exec
- process exit

So 1/1 adds some routines and 2/2 uses one of them. The rest can be
tackled on later or I can send a v2 with more places sorted out.

In the open+close case the win is very modest because of the following
problems:
- kmem and memcg having terrible performance
- putname using an atomic (I have a wip to whack that)
- open performing an extra ref/unref on the dentry (there are patches to
  do it, including by Al. I'll be mailing about it separately)
- creds using atomics (I have a wip to whack that)
- apparmor using atomics (ditto, same mechanism)

On top of that I have a WIP patch to dodge some of the work at lookup
itself.

All in all there is several % avoidably lost here.

Mateusz Guzik (2):
  file: add fput and file_ref_put routines optimized for use when
    closing a fd
  fs: use fput_close() in close()

 fs/file.c                | 75 ++++++++++++++++++++++++++++++----------
 fs/file_table.c          | 72 +++++++++++++++++++++++++++-----------
 fs/open.c                |  2 +-
 include/linux/file.h     |  2 ++
 include/linux/file_ref.h |  1 +
 5 files changed, 112 insertions(+), 40 deletions(-)

-- 
2.43.0

[PATCH 1/2] file: add fput and file_ref_put routines optimized for use when closing a fd

[Mateusz Guzik]

Vast majority of the time closing a file descriptor also operates on the
last reference, where a regular fput usage will result in 2 atomics.
This can be changed to only suffer 1.

See commentary above file_ref_put_close() for more information.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
---
 fs/file.c                | 75 ++++++++++++++++++++++++++++++----------
 fs/file_table.c          | 72 +++++++++++++++++++++++++++-----------
 include/linux/file.h     |  2 ++
 include/linux/file_ref.h |  1 +
 4 files changed, 111 insertions(+), 39 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 44efdc8c1e27..ea753f9c8e08 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -26,6 +26,28 @@
 
 #include "internal.h"
 
+static bool __file_ref_put_badval(file_ref_t *ref, unsigned long cnt)
+{
+	/*
+	 * If the reference count was already in the dead zone, then this
+	 * put() operation is imbalanced. Warn, put the reference count back to
+	 * DEAD and tell the caller to not deconstruct the object.
+	 */
+	if (WARN_ONCE(cnt >= FILE_REF_RELEASED, "imbalanced put on file reference count")) {
+		atomic_long_set(&ref->refcnt, FILE_REF_DEAD);
+		return false;
+	}
+
+	/*
+	 * This is a put() operation on a saturated refcount. Restore the
+	 * mean saturation value and tell the caller to not deconstruct the
+	 * object.
+	 */
+	if (cnt > FILE_REF_MAXREF)
+		atomic_long_set(&ref->refcnt, FILE_REF_SATURATED);
+	return false;
+}
+
 /**
  * __file_ref_put - Slowpath of file_ref_put()
  * @ref:	Pointer to the reference count
@@ -67,27 +89,44 @@ bool __file_ref_put(file_ref_t *ref, unsigned long cnt)
 		return true;
 	}
 
-	/*
-	 * If the reference count was already in the dead zone, then this
-	 * put() operation is imbalanced. Warn, put the reference count back to
-	 * DEAD and tell the caller to not deconstruct the object.
-	 */
-	if (WARN_ONCE(cnt >= FILE_REF_RELEASED, "imbalanced put on file reference count")) {
-		atomic_long_set(&ref->refcnt, FILE_REF_DEAD);
-		return false;
-	}
-
-	/*
-	 * This is a put() operation on a saturated refcount. Restore the
-	 * mean saturation value and tell the caller to not deconstruct the
-	 * object.
-	 */
-	if (cnt > FILE_REF_MAXREF)
-		atomic_long_set(&ref->refcnt, FILE_REF_SATURATED);
-	return false;
+	return __file_ref_put_badval(ref, cnt);
 }
 EXPORT_SYMBOL_GPL(__file_ref_put);
 
+/**
+ * file_ref_put_close - drop a reference expecting it would transition to FILE_REF_NOREF
+ * @ref:	Pointer to the reference count
+ *
+ * Semantically it is equivalent to calling file_ref_put(), but it trades lower
+ * performance in face of other CPUs also modifying the refcount for higher
+ * performance when this happens to be the last reference.
+ *
+ * For the last reference file_ref_put() issues 2 atomics. One to drop the
+ * reference and another to transition it to FILE_REF_DEAD. This routine does
+ * the work in one step, but in order to do it has to pre-read the variable which
+ * decreases scalability.
+ *
+ * Use with close() et al, stick to file_ref_put() by default.
+ */
+bool file_ref_put_close(file_ref_t *ref)
+{
+	long old, new;
+
+	old = atomic_long_read(&ref->refcnt);
+	do {
+		if (unlikely(old < 0))
+			return __file_ref_put_badval(ref, old);
+
+		if (old == FILE_REF_ONEREF)
+			new = FILE_REF_DEAD;
+		else
+			new = old - 1;
+	} while (!atomic_long_try_cmpxchg(&ref->refcnt, &old, new));
+
+	return new == FILE_REF_DEAD;
+}
+EXPORT_SYMBOL_GPL(file_ref_put_close);
+
 unsigned int sysctl_nr_open __read_mostly = 1024*1024;
 unsigned int sysctl_nr_open_min = BITS_PER_LONG;
 /* our min() is unusable in constant expressions ;-/ */
diff --git a/fs/file_table.c b/fs/file_table.c
index 5c00dc38558d..82d96ad141e2 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -511,31 +511,37 @@ void flush_delayed_fput(void)
 }
 EXPORT_SYMBOL_GPL(flush_delayed_fput);
 
-void fput(struct file *file)
+static void __fput_defer_free(struct file *file)
 {
-	if (file_ref_put(&file->f_ref)) {
-		struct task_struct *task = current;
+	struct task_struct *task = current;
 
-		if (unlikely(!(file->f_mode & (FMODE_BACKING | FMODE_OPENED)))) {
-			file_free(file);
+	if (unlikely(!(file->f_mode & (FMODE_BACKING | FMODE_OPENED)))) {
+		file_free(file);
+		return;
+	}
+	if (likely(!in_interrupt() && !(task->flags & PF_KTHREAD))) {
+		init_task_work(&file->f_task_work, ____fput);
+		if (!task_work_add(task, &file->f_task_work, TWA_RESUME))
 			return;
-		}
-		if (likely(!in_interrupt() && !(task->flags & PF_KTHREAD))) {
-			init_task_work(&file->f_task_work, ____fput);
-			if (!task_work_add(task, &file->f_task_work, TWA_RESUME))
-				return;
-			/*
-			 * After this task has run exit_task_work(),
-			 * task_work_add() will fail.  Fall through to delayed
-			 * fput to avoid leaking *file.
-			 */
-		}
-
-		if (llist_add(&file->f_llist, &delayed_fput_list))
-			schedule_delayed_work(&delayed_fput_work, 1);
+		/*
+		 * After this task has run exit_task_work(),
+		 * task_work_add() will fail.  Fall through to delayed
+		 * fput to avoid leaking *file.
+		 */
 	}
+
+	if (llist_add(&file->f_llist, &delayed_fput_list))
+		schedule_delayed_work(&delayed_fput_work, 1);
 }
 
+void fput(struct file *file)
+{
+	if (unlikely(file_ref_put(&file->f_ref))) {
+		__fput_defer_free(file);
+	}
+}
+EXPORT_SYMBOL(fput);
+
 /*
  * synchronous analog of fput(); for kernel threads that might be needed
  * in some umount() (and thus can't use flush_delayed_fput() without
@@ -549,10 +555,34 @@ void __fput_sync(struct file *file)
 	if (file_ref_put(&file->f_ref))
 		__fput(file);
 }
-
-EXPORT_SYMBOL(fput);
 EXPORT_SYMBOL(__fput_sync);
 
+/*
+ * Equivalent to __fput_sync(), but optimized for being called with the last
+ * reference.
+ *
+ * See file_ref_put_close() for details.
+ */
+void fput_close_sync(struct file *file)
+{
+	if (file_ref_put_close(&file->f_ref))
+		__fput(file);
+}
+EXPORT_SYMBOL(fput_close_sync);
+
+/*
+ * Equivalent to fput(), but optimized for being called with the last
+ * reference.
+ *
+ * See file_ref_put_close() for details.
+ */
+void fput_close(struct file *file)
+{
+	if (file_ref_put_close(&file->f_ref))
+		__fput_defer_free(file);
+}
+EXPORT_SYMBOL(fput_close);
+
 void __init files_init(void)
 {
 	struct kmem_cache_args args = {
diff --git a/include/linux/file.h b/include/linux/file.h
index 302f11355b10..7b04e87cbde6 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -124,6 +124,8 @@ int receive_fd_replace(int new_fd, struct file *file, unsigned int o_flags);
 
 extern void flush_delayed_fput(void);
 extern void __fput_sync(struct file *);
+void fput_close_sync(struct file *);
+void fput_close(struct file *);
 
 extern unsigned int sysctl_nr_open_min, sysctl_nr_open_max;
 
diff --git a/include/linux/file_ref.h b/include/linux/file_ref.h
index 9b3a8d9b17ab..f269299941aa 100644
--- a/include/linux/file_ref.h
+++ b/include/linux/file_ref.h
@@ -62,6 +62,7 @@ static inline void file_ref_init(file_ref_t *ref, unsigned long cnt)
 }
 
 bool __file_ref_put(file_ref_t *ref, unsigned long cnt);
+bool file_ref_put_close(file_ref_t *ref);
 
 /**
  * file_ref_get - Acquire one reference on a file
-- 
2.43.0

[PATCH 2/2] fs: use fput_close() in close()

[Mateusz Guzik]

This bumps open+close rate by 1% on Sapphire Rapids by eliding one
atomic.

It would be higher if it was not for several other slowdowns of the same
nature.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
---
 fs/open.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/open.c b/fs/open.c
index a5def5611b2f..f2fcfaeb2232 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1577,7 +1577,7 @@ SYSCALL_DEFINE1(close, unsigned int, fd)
 	 * We're returning to user space. Don't bother
 	 * with any delayed fput() cases.
 	 */
-	__fput_sync(file);
+	fput_close_sync(file);
 
 	if (likely(retval == 0))
 		return 0;
-- 
2.43.0