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 <[email protected]>
---
Documentation/filesystems/proc.rst | 17 ++++++++++++++
fs/proc/fd.c | 36 ++++++++++++++++++++++++++++++
2 files changed, 53 insertions(+)
diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index e7aafc82be99..394548d26187 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -47,6 +47,7 @@ fixes/update part 1.1 Stefani Seibold <[email protected]> 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
@@ -2145,6 +2146,22 @@ 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..c3c4bf2939a7 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -279,6 +279,26 @@ static int proc_readfd_common(struct file *file, struct dir_context *ctx,
return 0;
}
+static int proc_readfd_count(struct inode *inode)
+{
+ struct task_struct *p = get_proc_task(inode);
+ struct fdtable *fdt;
+ unsigned int i, size, open_fds = 0;
+
+ if (!p)
+ return -ENOENT;
+
+ if (p->files) {
+ fdt = files_fdtable(p->files);
+ size = fdt->max_fds;
+
+ for (i = size / BITS_PER_LONG; i > 0;)
+ open_fds += hweight64(fdt->open_fds[--i]);
+ }
+
+ return open_fds;
+}
+
static int proc_readfd(struct file *file, struct dir_context *ctx)
{
return proc_readfd_common(file, ctx, proc_fd_instantiate);
@@ -319,9 +339,25 @@ 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);
+
+ generic_fillattr(&init_user_ns, inode, stat);
+
+ /* If it's a directory, put the number of open fds there */
+ if (S_ISDIR(inode->i_mode))
+ stat->size = proc_readfd_count(inode);
+
+ return 0;
+}
+
const struct inode_operations proc_fd_inode_operations = {
.lookup = proc_lookupfd,
.permission = proc_fd_permission,
+ .getattr = proc_fd_getattr,
.setattr = proc_setattr,
};
--
2.37.2
From: Ivan Babrou
> Sent: 20 September 2022 20:06
...
>
> +static int proc_readfd_count(struct inode *inode)
> +{
> + struct task_struct *p = get_proc_task(inode);
> + struct fdtable *fdt;
> + unsigned int i, size, open_fds = 0;
> +
> + if (!p)
> + return -ENOENT;
> +
> + if (p->files) {
> + fdt = files_fdtable(p->files);
> + size = fdt->max_fds;
> +
> + for (i = size / BITS_PER_LONG; i > 0;)
> + open_fds += hweight64(fdt->open_fds[--i]);
> + }
> +
> + return open_fds;
> +}
> +
Doesn't that need (at least) rcu protection?
There might also be issues reading p->files twice.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
On Wed, Sep 21, 2022 at 3:21 AM David Laight <[email protected]> wrote:
>
> From: Ivan Babrou
> > Sent: 20 September 2022 20:06
> ...
> >
> > +static int proc_readfd_count(struct inode *inode)
> > +{
> > + struct task_struct *p = get_proc_task(inode);
> > + struct fdtable *fdt;
> > + unsigned int i, size, open_fds = 0;
> > +
> > + if (!p)
> > + return -ENOENT;
> > +
> > + if (p->files) {
> > + fdt = files_fdtable(p->files);
> > + size = fdt->max_fds;
> > +
> > + for (i = size / BITS_PER_LONG; i > 0;)
> > + open_fds += hweight64(fdt->open_fds[--i]);
> > + }
I'm missing put_task_struct(p) here.
> > +
> > + return open_fds;
> > +}
> > +
>
> Doesn't that need (at least) rcu protection?
Should I enclose the "if" in rcu_read_lock() / rcu_read_unlock()?
files_fdtable() is this:
* https://elixir.bootlin.com/linux/v6.0-rc6/source/include/linux/fdtable.h#L77
#define files_fdtable(files) \
rcu_dereference_check_fdtable((files), (files)->fdt)
And rcu_dereference_check_fdtable() is
#define rcu_dereference_check_fdtable(files, fdtfd) \
rcu_dereference_check((fdtfd), lockdep_is_held(&(files)->file_lock))
I definitely need some help with locking here.
> There might also be issues reading p->files twice.
This block for reading twice:
if (p->files) {
fdt = files_fdtable(p->files);
Already exists in fs/proc/array.c in task_state():
* https://elixir.bootlin.com/linux/v6.0-rc6/source/fs/proc/array.c#L173
There's task_lock(p) there, so maybe that's why it's allowed.
Should I run files_fdtable(p->files) unconditionally and then check
the result instead? I'm happy to change it to something else if you
can tell me what it should be.
If there are kernel options I should enable for testing this, I'd be
happy to hear them. So far we've tried running with KASAN with no
issues.