Re: [PATCH] fs/file.c: don't acquire files->file_lock in fd_install()

[Mateusz Guzik]

On Tue, Apr 21, 2015 at 09:59:28PM -0700, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@xxxxxxxxxx>
> 
> Mateusz Guzik reported :
> 
>  Currently obtaining a new file descriptor results in locking fdtable
>  twice - once in order to reserve a slot and second time to fill it.
> 
> Holding the spinlock in __fd_install() is needed in case a resize is
> done, or to prevent a resize.
> 
> Mateusz provided an RFC patch and a micro benchmark :
>   http://people.redhat.com/~mguzik/pipebench.c
> 
> A resize is an unlikely operation in a process lifetime,
> as table size is at least doubled at every resize.
> 
> We can use RCU instead of the spinlock :
> 
> __fd_install() must wait if a resize is in progress.
> 
> The resize must block new __fd_install() callers from starting,
> and wait that ongoing install are finished (synchronize_sched())
> 
> resize should be attempted by a single thread to not waste resources.
> 
> rcu_sched variant is used, as __fd_install() and expand_fdtable() run
> from process context.
> 
> It gives us a ~30% speedup using pipebench with 16 threads, and a ~10%
> speedup with another program using TCP sockets.
> 
> Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> Reported-by: Mateusz Guzik <mguzik@xxxxxxxxxx>

Reviewed-by: Mateusz Guzik <mguzik@xxxxxxxxxx>
Tested-by: Mateusz Guzik <mguzik@xxxxxxxxxx>

Thanks,

> ---
>  fs/file.c               |   66 +++++++++++++++++++++++++++-----------
>  include/linux/fdtable.h |    3 +
>  2 files changed, 50 insertions(+), 19 deletions(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index 93c5f89c248b..17cef5e52f16 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -147,6 +147,13 @@ static int expand_fdtable(struct files_struct *files, int nr)
>  
>  	spin_unlock(&files->file_lock);
>  	new_fdt = alloc_fdtable(nr);
> +
> +	/* make sure all __fd_install() have seen resize_in_progress
> +	 * or have finished their rcu_read_lock_sched() section.
> +	 */
> +	if (atomic_read(&files->count) > 1)
> +		synchronize_sched();
> +
>  	spin_lock(&files->file_lock);
>  	if (!new_fdt)
>  		return -ENOMEM;
> @@ -158,21 +165,14 @@ static int expand_fdtable(struct files_struct *files, int nr)
>  		__free_fdtable(new_fdt);
>  		return -EMFILE;
>  	}
> -	/*
> -	 * Check again since another task may have expanded the fd table while
> -	 * we dropped the lock
> -	 */
>  	cur_fdt = files_fdtable(files);
> -	if (nr >= cur_fdt->max_fds) {
> -		/* Continue as planned */
> -		copy_fdtable(new_fdt, cur_fdt);
> -		rcu_assign_pointer(files->fdt, new_fdt);
> -		if (cur_fdt != &files->fdtab)
> -			call_rcu(&cur_fdt->rcu, free_fdtable_rcu);
> -	} else {
> -		/* Somebody else expanded, so undo our attempt */
> -		__free_fdtable(new_fdt);
> -	}
> +	BUG_ON(nr < cur_fdt->max_fds);
> +	copy_fdtable(new_fdt, cur_fdt);
> +	rcu_assign_pointer(files->fdt, new_fdt);
> +	if (cur_fdt != &files->fdtab)
> +		call_rcu(&cur_fdt->rcu, free_fdtable_rcu);
> +	/* coupled with smp_rmb() in __fd_install() */
> +	smp_wmb();
>  	return 1;
>  }
>  
> @@ -185,21 +185,38 @@ static int expand_fdtable(struct files_struct *files, int nr)
>   * The files->file_lock should be held on entry, and will be held on exit.
>   */
>  static int expand_files(struct files_struct *files, int nr)
> +	__releases(files->file_lock)
> +	__acquires(files->file_lock)
>  {
>  	struct fdtable *fdt;
> +	int expanded = 0;
>  
> +repeat:
>  	fdt = files_fdtable(files);
>  
>  	/* Do we need to expand? */
>  	if (nr < fdt->max_fds)
> -		return 0;
> +		return expanded;
>  
>  	/* Can we expand? */
>  	if (nr >= sysctl_nr_open)
>  		return -EMFILE;
>  
> +	if (unlikely(files->resize_in_progress)) {
> +		spin_unlock(&files->file_lock);
> +		expanded = 1;
> +		wait_event(files->resize_wait, !files->resize_in_progress);
> +		spin_lock(&files->file_lock);
> +		goto repeat;
> +	}
> +
>  	/* All good, so we try */
> -	return expand_fdtable(files, nr);
> +	files->resize_in_progress = true;
> +	expanded = expand_fdtable(files, nr);
> +	files->resize_in_progress = false;
> +
> +	wake_up_all(&files->resize_wait);
> +	return expanded;
>  }
>  
>  static inline void __set_close_on_exec(int fd, struct fdtable *fdt)
> @@ -256,6 +273,8 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
>  	atomic_set(&newf->count, 1);
>  
>  	spin_lock_init(&newf->file_lock);
> +	newf->resize_in_progress = false;
> +	init_waitqueue_head(&newf->resize_wait);
>  	newf->next_fd = 0;
>  	new_fdt = &newf->fdtab;
>  	new_fdt->max_fds = NR_OPEN_DEFAULT;
> @@ -553,11 +572,20 @@ void __fd_install(struct files_struct *files, unsigned int fd,
>  		struct file *file)
>  {
>  	struct fdtable *fdt;
> -	spin_lock(&files->file_lock);
> -	fdt = files_fdtable(files);
> +
> +	rcu_read_lock_sched();
> +
> +	while (unlikely(files->resize_in_progress)) {
> +		rcu_read_unlock_sched();
> +		wait_event(files->resize_wait, !files->resize_in_progress);
> +		rcu_read_lock_sched();
> +	}
> +	/* coupled with smp_wmb() in expand_fdtable() */
> +	smp_rmb();
> +	fdt = READ_ONCE(files->fdt);
>  	BUG_ON(fdt->fd[fd] != NULL);
>  	rcu_assign_pointer(fdt->fd[fd], file);
> -	spin_unlock(&files->file_lock);
> +	rcu_read_unlock_sched();
>  }
>  
>  void fd_install(unsigned int fd, struct file *file)
> diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
> index 230f87bdf5ad..fbb88740634a 100644
> --- a/include/linux/fdtable.h
> +++ b/include/linux/fdtable.h
> @@ -47,6 +47,9 @@ struct files_struct {
>     * read mostly part
>     */
>  	atomic_t count;
> +	bool resize_in_progress;
> +	wait_queue_head_t resize_wait;
> +
>  	struct fdtable __rcu *fdt;
>  	struct fdtable fdtab;
>    /*
> 
> 

-- 
Mateusz Guzik
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