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f1f1f25699
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> |
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| .. | ||
| caching | ||
| cifs | ||
| ext4 | ||
| nfs | ||
| spufs | ||
| 9p.rst | ||
| adfs.rst | ||
| affs.rst | ||
| afs.rst | ||
| api-summary.rst | ||
| autofs-mount-control.rst | ||
| autofs.rst | ||
| automount-support.rst | ||
| befs.rst | ||
| bfs.rst | ||
| btrfs.rst | ||
| ceph.rst | ||
| coda.rst | ||
| configfs.rst | ||
| cramfs.rst | ||
| dax.rst | ||
| debugfs.rst | ||
| devpts.rst | ||
| directory-locking.rst | ||
| dlmfs.rst | ||
| dnotify.rst | ||
| ecryptfs.rst | ||
| efivarfs.rst | ||
| erofs.rst | ||
| ext2.rst | ||
| ext3.rst | ||
| f2fs.rst | ||
| fiemap.rst | ||
| files.rst | ||
| fscrypt.rst | ||
| fsverity.rst | ||
| fuse-io.rst | ||
| fuse.rst | ||
| gfs2-glocks.rst | ||
| gfs2-uevents.rst | ||
| gfs2.rst | ||
| hfs.rst | ||
| hfsplus.rst | ||
| hpfs.rst | ||
| idmappings.rst | ||
| index.rst | ||
| inotify.rst | ||
| isofs.rst | ||
| journalling.rst | ||
| locking.rst | ||
| locks.rst | ||
| mount_api.rst | ||
| netfs_library.rst | ||
| nilfs2.rst | ||
| ntfs3.rst | ||
| ntfs.rst | ||
| ocfs2-online-filecheck.rst | ||
| ocfs2.rst | ||
| omfs.rst | ||
| orangefs.rst | ||
| overlayfs.rst | ||
| path-lookup.rst | ||
| path-lookup.txt | ||
| porting.rst | ||
| proc.rst | ||
| qnx6.rst | ||
| quota.rst | ||
| ramfs-rootfs-initramfs.rst | ||
| relay.rst | ||
| romfs.rst | ||
| seq_file.rst | ||
| sharedsubtree.rst | ||
| splice.rst | ||
| squashfs.rst | ||
| sysfs.rst | ||
| sysv-fs.rst | ||
| tmpfs.rst | ||
| ubifs-authentication.rst | ||
| ubifs.rst | ||
| udf.rst | ||
| vfat.rst | ||
| vfs.rst | ||
| virtiofs.rst | ||
| xfs-delayed-logging-design.rst | ||
| xfs-self-describing-metadata.rst | ||
| zonefs.rst | ||