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As per [2], the patches of [1] were divided into 2 parts so that they can be reviewed feature by feature. Patches of [2] are now in linux-next. So this patchset is for resuming review (after addressing latest review comments) of rest of the patches of [1]. The patches in this change set are as follows: PATCH-1: Use a per-fs rwsem to protect per-fs list of kernfs_super_info. PATCH-2: Change kernfs_rename_lock into a read-write lock. PATCH-3: Introduce interface to access per-fs rwsem. PATCH-4: Replace per-fs rwsem with hashed rwsems. PATCH-5: Add a document to describe hashed locks used in kernfs. Original cover letter --------------------------------------------------------------- Reduce contention around global locks used in kernfs. The current kernfs design makes use of 3 global locks to synchronize various operations. There are a few other global locks as well but they are used for specific cases and hence don't cause severe contention. The above mentioned 3 main global locks used in kernfs are: 1. A global mutex, kernfs_open_file_mutex, to protect the list of kernfs_open_file instances correspondng to a sysfs attribute. 2. A global spinlock, kernfs_open_node_lock, to protect kernfs_node->attr.open which points to kernfs_open_node instance corresponding to a kernfs_node. 3. A global per-fs rw semaphore, root->kernfs_rwsem, to synchronize most of the other operations like adding, removing, renaming etc. of a file or directory. Since these locks are global, they can cause contention when multiple (for example few hundred) applications try to access sysfs (or other kernfs based file system) files in parallel, even if the applications are accessing different and unrelated files. For example on a system with 384 cores, if I run 200 instances of an application which is mostly executing the following loop: for (int loop = 0; loop <100 ; loop++) { for (int port_num = 1; port_num < 2; port_num++) { for (int gid_index = 0; gid_index < 254; gid_index++ ) { char ret_buf[64], ret_buf_lo[64]; char gid_file_path[1024]; int ret_len; int ret_fd; ssize_t ret_rd; ub4 i, saved_errno; memset(ret_buf, 0, sizeof(ret_buf)); memset(gid_file_path, 0, sizeof(gid_file_path)); ret_len = snprintf(gid_file_path, sizeof(gid_file_path), "/sys/class/infiniband/%s/ports/%d/gids/%d", dev_name, port_num, gid_index); ret_fd = open(gid_file_path, O_RDONLY | O_CLOEXEC); if (ret_fd < 0) { printf("Failed to open %s\n", gid_file_path); continue; } /* Read the GID */ ret_rd = read(ret_fd, ret_buf, 40); if (ret_rd == -1) { printf("Failed to read from file %s, errno: %u\n", gid_file_path, saved_errno); continue; } close(ret_fd); } } I can see contention around above mentioned locks as follows: - 54.07% 53.60% showgids [kernel.kallsyms] [k] osq_lock - 53.60% __libc_start_main - 32.29% __GI___libc_open entry_SYSCALL_64_after_hwframe do_syscall_64 sys_open do_sys_open do_filp_open path_openat vfs_open do_dentry_open kernfs_fop_open mutex_lock - __mutex_lock_slowpath - 32.23% __mutex_lock.isra.5 osq_lock - 21.31% __GI___libc_close entry_SYSCALL_64_after_hwframe do_syscall_64 exit_to_usermode_loop task_work_run ____fput __fput kernfs_fop_release kernfs_put_open_node.isra.8 mutex_lock - __mutex_lock_slowpath - 21.28% __mutex_lock.isra.5 osq_lock - 10.49% 10.39% showgids [kernel.kallsyms] [k] down_read 10.39% __libc_start_main __GI___libc_open entry_SYSCALL_64_after_hwframe do_syscall_64 sys_open do_sys_open do_filp_open - path_openat - 9.72% link_path_walk - 5.21% inode_permission - __inode_permission - 5.21% kernfs_iop_permission down_read - 4.08% walk_component lookup_fast - d_revalidate.part.24 - 4.08% kernfs_dop_revalidate - 7.48% 7.41% showgids [kernel.kallsyms] [k] up_read 7.41% __libc_start_main __GI___libc_open entry_SYSCALL_64_after_hwframe do_syscall_64 sys_open do_sys_open do_filp_open - path_openat - 7.01% link_path_walk - 4.12% inode_permission - __inode_permission - 4.12% kernfs_iop_permission up_read - 2.61% walk_component lookup_fast - d_revalidate.part.24 - 2.61% kernfs_dop_revalidate Moreover this run of 200 application isntances takes 32-34 secs. to complete. This patch set is reducing the above mentioned contention by replacing these global locks with hashed locks. For example with the patched kernel and on the same test setup, we no longer see contention around osq_lock (i.e kernfs_open_file_mutex) and also contention around per-fs kernfs_rwsem has reduced significantly as well. This can be seen in the following perf snippet: - 1.66% 1.65% showgids [kernel.kallsyms] [k] down_read 1.65% __libc_start_main __GI___libc_open entry_SYSCALL_64_after_hwframe do_syscall_64 sys_open do_sys_open do_filp_open - path_openat - 1.62% link_path_walk - 0.98% inode_permission - __inode_permission + 0.98% kernfs_iop_permission - 0.52% walk_component lookup_fast - d_revalidate.part.24 - 0.52% kernfs_dop_revalidate - 1.12% 1.11% showgids [kernel.kallsyms] [k] up_read 1.11% __libc_start_main __GI___libc_open entry_SYSCALL_64_after_hwframe do_syscall_64 sys_open do_sys_open do_filp_open - path_openat - 1.11% link_path_walk - 0.69% inode_permission - __inode_permission - 0.69% kernfs_iop_permission up_read Moreover the test execution time has reduced from 32-34 secs to 15-16 secs. The patches of this patchset introduce following changes: PATCH-1: Remove reference counting from kernfs_open_node. PATCH-2: Make kernfs_open_node->attr.open RCU protected. PATCH-3: Change kernfs_notify_list to llist. PATCH-4: Introduce interface to access kernfs_open_file_mutex. PATCH-5: Replace global kernfs_open_file_mutex with hashed mutexes. PATCH-6: Use a per-fs rwsem to protect per-fs list of kernfs_super_info. PATCH-7: Change kernfs_rename_lock into a read-write lock. PATCH-8: Introduce interface to access per-fs rwsem. PATCH-9: Replace per-fs rwsem with hashed rwsems. PATCH-10: Add a document to describe hashed locks used in kernfs. ------------------------------------------------------------------ I did not receive any feedback earlier so resending after rebasing on tag next-20220810 of linux-next. Imran Khan (5): kernfs: Use a per-fs rwsem to protect per-fs list of kernfs_super_info. kernfs: Change kernfs_rename_lock into a read-write lock. kernfs: Introduce interface to access per-fs rwsem. kernfs: Replace per-fs rwsem with hashed rwsems. kernfs: Add a document to describe hashed locks used in kernfs. .../filesystems/kernfs-hashed-locks.rst | 214 ++++++++++++++ fs/kernfs/Makefile | 2 +- fs/kernfs/dir.c | 269 ++++++++++++------ fs/kernfs/file.c | 7 +- fs/kernfs/inode.c | 48 +++- fs/kernfs/kernfs-internal.c | 259 +++++++++++++++++ fs/kernfs/kernfs-internal.h | 122 +++++++- fs/kernfs/mount.c | 23 +- fs/kernfs/symlink.c | 13 +- include/linux/kernfs.h | 1 + 10 files changed, 845 insertions(+), 113 deletions(-) create mode 100644 Documentation/filesystems/kernfs-hashed-locks.rst create mode 100644 fs/kernfs/kernfs-internal.c base-commit: bc6c6584ffb27b62e19ea89553b22b4cad1abaca