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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. ------------------------------------------------------------------ Changes since v7: - Addressed review comments from Al Viro - Remove reference counting for kernfs_open_node - Remove kernfs_open_node_lock and make ->attr.open RCU protected - Change kernfs_rename_lock to a read-write lock - Lock involved nodes before invoking kernfs_find_ns from kernfs_walk_ns - Change kernfs_notify_list to llist - Addressed review comments from Eric - Move interface to access kernfs_open_file_mutex to file.c - Update document about hashed locks - Rebase on tag next-20220407 Changes since v6: - Addressed review comments from Tejun - Make locking helpers compact and remove unlock_parent from node. - Addressed review comments from Al Viro - Make multi node lock helpers non-inline - Account for change of parent while waiting on semaphores during rename operation - Add a document to describe hashed locks introduced in this patch and to provide proof of correctness - Fix for some issues that I observed while preparing lock correctness document - Lock both parent and target while looking for symlink - Introduce a per-fs mutex to synchronize lookup and removal of a node - Avoid locking parent in remove_self, because only the first instance does actual removal so other invocations of remove_self don't need to lock the parent - Remove refcounting from lock helpers - Refcounting was present in lock helpers for cases where an execution path acquires node's rwsem and then deletes the node. For such cases release helpers should not try to acquire semaphore for this already freed node. Refcounting was ensuring that release helpers could get an still existing node. I have modified locking helpers such that helpers that acquire rwsem returns its address which can later be used by release helpers. Changes since v5: - Addressed review comments from Greg - Clean up duplicate code. - Addressed review comments from Tejun - Explain how current value of NR_KERNFS_LOCKS were obtained for different values of NR_CPUS. - Use single hash table for locks in place of per-fs hash table for locks. - Move introduction of supers_rwsem to a separate patch. - Separate interface and usage part of hashed rwsem implementation into 2 patches. - Use address of rwsems to determine locking order in case of nested locking. This approach avoids ABBA deadlock possibility. - Change some #define macros into enum, with proper prefix. - Taking a cue from Tejun's feedback about separating hashed rwsem interface and usage into 2 patches, I have also divided the patch that introduced hashed mutex and spinlock, into separate patches. - Rebase on linux-next tag: next-20220211 Changes since v4: - Removed compilation warnings reported by the 0-day bot. Changes since v3: - Make open_file_mutex and open_node_lock per-fs. - Replace per-fs rwsem with per-fs hashed rwsem. (Suggested by Tejun Heo ) Imran Khan (10): kernfs: Remove reference counting for kernfs_open_node. kernfs: make ->attr.open RCU protected. kernfs: Change kernfs_notify_list to llist. kernfs: Introduce interface to access global kernfs_open_file_mutex. kernfs: Replace global kernfs_open_file_mutex with hashed mutexes. 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 | 272 ++++++++++++------ fs/kernfs/file.c | 228 ++++++++------- fs/kernfs/inode.c | 48 +++- fs/kernfs/kernfs-internal.c | 259 +++++++++++++++++ fs/kernfs/kernfs-internal.h | 127 +++++++- fs/kernfs/mount.c | 30 +- fs/kernfs/symlink.c | 11 +- include/linux/kernfs.h | 62 +++- 10 files changed, 1041 insertions(+), 212 deletions(-) create mode 100644 Documentation/filesystems/kernfs-hashed-locks.rst create mode 100644 fs/kernfs/kernfs-internal.c base-commit: 2e9a9857569ec27e64d2ddd01294bbe3c736acb1 -- 2.30.2