Re: [PATCH v4] proc: report open files as size in stat() for /proc/pid/fd

[Brian Foster]

On Mon, Oct 24, 2022 at 10:31:40AM -0700, Ivan Babrou wrote:
> Many monitoring tools include open file count as a metric. Currently
> the only way to get this number is to enumerate the files in /proc/pid/fd.
> 
> The problem with the current approach is that it does many things people
> generally don't care about when they need one number for a metric.
> In our tests for cadvisor, which reports open file counts per cgroup,
> we observed that reading the number of open files is slow. Out of 35.23%
> of CPU time spent in `proc_readfd_common`, we see 29.43% spent in
> `proc_fill_cache`, which is responsible for filling dentry info.
> Some of this extra time is spinlock contention, but it's a contention
> for the lock we don't want to take to begin with.
> 
> We considered putting the number of open files in /proc/pid/status.
> Unfortunately, counting the number of fds involves iterating the open_files
> bitmap, which has a linear complexity in proportion with the number
> of open files (bitmap slots really, but it's close). We don't want
> to make /proc/pid/status any slower, so instead we put this info
> in /proc/pid/fd as a size member of the stat syscall result.
> Previously the reported number was zero, so there's very little
> risk of breaking anything, while still providing a somewhat logical
> way to count the open files with a fallback if it's zero.
> 
> RFC for this patch included iterating open fds under RCU. Thanks
> to Frank Hofmann for the suggestion to use the bitmap instead.
> 
> Previously:
> 
> ```
> $ sudo stat /proc/1/fd | head -n2
>   File: /proc/1/fd
>   Size: 0         	Blocks: 0          IO Block: 1024   directory
> ```
> 
> With this patch:
> 
> ```
> $ sudo stat /proc/1/fd | head -n2
>   File: /proc/1/fd
>   Size: 65        	Blocks: 0          IO Block: 1024   directory
> ```
> 
> Correctness check:
> 
> ```
> $ sudo ls /proc/1/fd | wc -l
> 65
> ```
> 
> I added the docs for /proc/<pid>/fd while I'm at it.
> 
> Signed-off-by: Ivan Babrou <ivan@xxxxxxxxxxxxxx>
> 
> ---
> v4: Return errno from proc_fd_getattr() instead of setting negative size.
>     Added an explicit include for linux/bitmap.h.
> v3: Made use of bitmap_weight() to count the bits.
> v2: Added missing rcu_read_lock() / rcu_read_unlock(),
>     task_lock() / task_unlock() and put_task_struct().
> ---
>  Documentation/filesystems/proc.rst | 17 +++++++++++
>  fs/proc/fd.c                       | 45 ++++++++++++++++++++++++++++++
>  2 files changed, 62 insertions(+)
> 
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 898c99eae8e4..ec6cfdf1796a 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -47,6 +47,7 @@ fixes/update part 1.1  Stefani Seibold <stefani@xxxxxxxxxxx>    June 9 2009
>    3.10  /proc/<pid>/timerslack_ns - Task timerslack value
>    3.11	/proc/<pid>/patch_state - Livepatch patch operation state
>    3.12	/proc/<pid>/arch_status - Task architecture specific information
> +  3.13  /proc/<pid>/fd - List of symlinks to open files
>  
>    4	Configuring procfs
>    4.1	Mount options
> @@ -2149,6 +2150,22 @@ AVX512_elapsed_ms
>    the task is unlikely an AVX512 user, but depends on the workload and the
>    scheduling scenario, it also could be a false negative mentioned above.
>  
> +3.13 /proc/<pid>/fd - List of symlinks to open files
> +-------------------------------------------------------
> +This directory contains symbolic links which represent open files
> +the process is maintaining.  Example output::
> +
> +  lr-x------ 1 root root 64 Sep 20 17:53 0 -> /dev/null
> +  l-wx------ 1 root root 64 Sep 20 17:53 1 -> /dev/null
> +  lrwx------ 1 root root 64 Sep 20 17:53 10 -> 'socket:[12539]'
> +  lrwx------ 1 root root 64 Sep 20 17:53 11 -> 'socket:[12540]'
> +  lrwx------ 1 root root 64 Sep 20 17:53 12 -> 'socket:[12542]'
> +
> +The number of open files for the process is stored in 'size' member
> +of stat() output for /proc/<pid>/fd for fast access.
> +-------------------------------------------------------
> +
> +
>  Chapter 4: Configuring procfs
>  =============================
>  
> diff --git a/fs/proc/fd.c b/fs/proc/fd.c
> index 913bef0d2a36..fc46d6fe080c 100644
> --- a/fs/proc/fd.c
> +++ b/fs/proc/fd.c
> @@ -7,6 +7,7 @@
>  #include <linux/namei.h>
>  #include <linux/pid.h>
>  #include <linux/ptrace.h>
> +#include <linux/bitmap.h>
>  #include <linux/security.h>
>  #include <linux/file.h>
>  #include <linux/seq_file.h>
> @@ -279,6 +280,30 @@ static int proc_readfd_common(struct file *file, struct dir_context *ctx,
>  	return 0;
>  }
>  
> +static int proc_readfd_count(struct inode *inode, loff_t *count)
> +{
> +	struct task_struct *p = get_proc_task(inode);
> +	struct fdtable *fdt;
> +
> +	if (!p)
> +		return -ENOENT;
> +
> +	task_lock(p);
> +	if (p->files) {
> +		rcu_read_lock();
> +
> +		fdt = files_fdtable(p->files);
> +		*count = bitmap_weight(fdt->open_fds, fdt->max_fds);
> +
> +		rcu_read_unlock();
> +	}
> +	task_unlock(p);
> +
> +	put_task_struct(p);
> +
> +	return 0;
> +}
> +
>  static int proc_readfd(struct file *file, struct dir_context *ctx)
>  {
>  	return proc_readfd_common(file, ctx, proc_fd_instantiate);
> @@ -319,9 +344,29 @@ int proc_fd_permission(struct user_namespace *mnt_userns,
>  	return rv;
>  }
>  
> +static int proc_fd_getattr(struct user_namespace *mnt_userns,
> +			const struct path *path, struct kstat *stat,
> +			u32 request_mask, unsigned int query_flags)
> +{
> +	struct inode *inode = d_inode(path->dentry);
> +	int rv = 0;
> +
> +	generic_fillattr(&init_user_ns, inode, stat);
> +

Sorry I missed this on v3, but shouldn't this pass through the
mnt_userns parameter?

> +	/* If it's a directory, put the number of open fds there */
> +	if (S_ISDIR(inode->i_mode)) {
> +		rv = proc_readfd_count(inode, &stat->size);
> +		if (rv < 0)
> +			return rv;
> +	}

Also I suppose this could just do:

	if (S_ISDIR(inode->i_mode))
		rv = proc_readfd_count(inode, &stat->size);

	return rv;

But that's a nit. Otherwise seems reasonable to me.

Brian

> +
> +	return rv;
> +}
> +
>  const struct inode_operations proc_fd_inode_operations = {
>  	.lookup		= proc_lookupfd,
>  	.permission	= proc_fd_permission,
> +	.getattr	= proc_fd_getattr,
>  	.setattr	= proc_setattr,
>  };
>  
> -- 
> 2.37.3
>