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
>
