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Date: Mon, 24 Oct 2022 10:31:40 -0700
From: Ivan Babrou <ivan@...udflare.com>
To: linux-fsdevel@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, kernel-team@...udflare.com,
Alexey Dobriyan <adobriyan@...il.com>,
Al Viro <viro@...iv.linux.org.uk>,
Theodore Ts'o <tytso@....edu>,
David Laight <David.Laight@...lab.com>,
Jonathan Corbet <corbet@....net>,
Andrew Morton <akpm@...ux-foundation.org>,
David Hildenbrand <david@...hat.com>,
Johannes Weiner <hannes@...xchg.org>,
Christoph Anton Mitterer <mail@...istoph.anton.mitterer.name>,
Mike Rapoport <rppt@...nel.org>,
Paul Gortmaker <paul.gortmaker@...driver.com>,
Kalesh Singh <kaleshsingh@...gle.com>,
Brian Foster <bfoster@...hat.com>,
Ivan Babrou <ivan@...udflare.com>
Subject: [PATCH v4] proc: report open files as size in stat() for /proc/pid/fd
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@...udflare.com>
---
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@...bold.net> 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);
+
+ /* 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;
+ }
+
+ 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
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