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authorIvan Babrou <ivan@cloudflare.com>2022-09-23 01:40:26 +0300
committerAndrew Morton <akpm@linux-foundation.org>2022-11-19 00:55:07 +0300
commitf1f1f2569901ec5b9d425f2e91c09a0e320768f3 (patch)
tree006a670ef38a9dcd190e9471165ba342521e049c /Documentation/filesystems
parent2122e2a4efc2cd139474079e11939b6e07adfacd (diff)
downloadlinux-f1f1f2569901ec5b9d425f2e91c09a0e320768f3.tar.xz
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. [ivan@cloudflare.com: use bitmap_weight() to count the bits] Link: https://lkml.kernel.org/r/20221018045844.37697-1-ivan@cloudflare.com [akpm@linux-foundation.org: include linux/bitmap.h for bitmap_weight()] [ivan@cloudflare.com: return errno from proc_fd_getattr() instead of setting negative size] Link: https://lkml.kernel.org/r/20221024173140.30673-1-ivan@cloudflare.com Link: https://lkml.kernel.org/r/20220922224027.59266-1-ivan@cloudflare.com Signed-off-by: Ivan Babrou <ivan@cloudflare.com> Cc: Alexey Dobriyan <adobriyan@gmail.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name> Cc: David Hildenbrand <david@redhat.com> Cc: David Laight <David.Laight@ACULAB.COM> Cc: Ivan Babrou <ivan@cloudflare.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Kalesh Singh <kaleshsingh@google.com> Cc: Mike Rapoport <rppt@kernel.org> Cc: Paul Gortmaker <paul.gortmaker@windriver.com> Cc: Theodore Ts'o <tytso@mit.edu> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'Documentation/filesystems')
-rw-r--r--Documentation/filesystems/proc.rst17
1 files changed, 17 insertions, 0 deletions
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@seibold.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
=============================