Hi, It was noticed that some workloads suffer from contention on increasing/decrementing the ->usage counter in their credentials, those refcount operations are associated with overriding/reverting the current task credentials. (the linked thread adds more context) In some specialized cases, overlayfs is one of them, the credentials in question have a longer lifetime than the override/revert "critical section". In the overlayfs case, the credentials are created when the fs is mounted and destroyed when it's unmounted. In this case of long lived credentials, the usage counter doesn't need to be incremented/decremented. Add a lighter version of credentials override/revert to be used in these specialized cases. To make sure that the override/revert calls are paired, add a cleanup guard macro. This was suggested here: https://lore.kernel.org/all/20231219-marken-pochen-26d888fb9bb9@brauner/ With a small number of tweaks: - Used inline functions instead of macros; - A small change to store the credentials into the passed argument, the guard is now defined as (note the added '_T ='): DEFINE_GUARD(cred, const struct cred *, _T = override_creds_light(_T), revert_creds_light(_T)); - Allow "const" arguments to be used with these kind of guards; Some comments: - If patch 1/4 is not a good idea (adding the cast), the alternative I can see is using some kind of container for the credentials; - The only user for the backing file ops is overlayfs, so these changes make sense, but may not make sense in the most general case; For the numbers, some from 'perf c2c', before this series: (edited to fit) # # ----- HITM ----- Shared # Num RmtHitm LclHitm Symbol Object Source:Line Node # ..... ....... ....... .......................... ................ .................. .... # ------------------------- 0 412 1028 ------------------------- 41.50% 42.22% [k] revert_creds [kernel.vmlinux] atomic64_64.h:39 0 1 15.05% 10.60% [k] override_creds [kernel.vmlinux] atomic64_64.h:25 0 1 0.73% 0.58% [k] init_file [kernel.vmlinux] atomic64_64.h:25 0 1 0.24% 0.10% [k] revert_creds [kernel.vmlinux] cred.h:266 0 1 32.28% 37.16% [k] generic_permission [kernel.vmlinux] mnt_idmapping.h:81 0 1 9.47% 8.75% [k] generic_permission [kernel.vmlinux] mnt_idmapping.h:81 0 1 0.49% 0.58% [k] inode_owner_or_capable [kernel.vmlinux] mnt_idmapping.h:81 0 1 0.24% 0.00% [k] generic_permission [kernel.vmlinux] namei.c:354 0 ------------------------- 1 50 103 ------------------------- 100.00% 100.00% [k] update_cfs_group [kernel.vmlinux] atomic64_64.h:15 0 1 ------------------------- 2 50 98 ------------------------- 96.00% 96.94% [k] update_cfs_group [kernel.vmlinux] atomic64_64.h:15 0 1 2.00% 1.02% [k] update_load_avg [kernel.vmlinux] atomic64_64.h:25 0 1 0.00% 2.04% [k] update_load_avg [kernel.vmlinux] fair.c:4118 0 2.00% 0.00% [k] update_cfs_group [kernel.vmlinux] fair.c:3932 0 1 after this series: # # ----- HITM ----- Shared # Num RmtHitm LclHitm Symbol Object Source:Line Node # ..... ....... ....... .................... ................ ................ .... # ------------------------- 0 54 88 ------------------------- 100.00% 100.00% [k] update_cfs_group [kernel.vmlinux] atomic64_64.h:15 0 1 ------------------------- 1 48 83 ------------------------- 97.92% 97.59% [k] update_cfs_group [kernel.vmlinux] atomic64_64.h:15 0 1 2.08% 1.20% [k] update_load_avg [kernel.vmlinux] atomic64_64.h:25 0 1 0.00% 1.20% [k] update_load_avg [kernel.vmlinux] fair.c:4118 0 1 ------------------------- 2 28 44 ------------------------- 85.71% 79.55% [k] generic_permission [kernel.vmlinux] mnt_idmapping.h:81 0 1 14.29% 20.45% [k] generic_permission [kernel.vmlinux] mnt_idmapping.h:81 0 1 The contention is practically gone. Link: https://lore.kernel.org/all/20231018074553.41333-1-hu1.chen@xxxxxxxxx/ Vinicius Costa Gomes (4): cleanup: Fix discarded const warning when defining guards cred: Add a light version of override/revert_creds() overlayfs: Optimize credentials usage fs: Optimize credentials reference count for backing file ops fs/backing-file.c | 124 +++++++++++++++++++--------------------- fs/overlayfs/copy_up.c | 4 +- fs/overlayfs/dir.c | 22 +++---- fs/overlayfs/file.c | 70 ++++++++++++----------- fs/overlayfs/inode.c | 60 ++++++++++--------- fs/overlayfs/namei.c | 21 ++++--- fs/overlayfs/readdir.c | 18 +++--- fs/overlayfs/util.c | 23 ++++---- fs/overlayfs/xattrs.c | 34 +++++------ include/linux/cleanup.h | 2 +- include/linux/cred.h | 21 +++++++ kernel/cred.c | 6 +- 12 files changed, 215 insertions(+), 190 deletions(-) -- 2.43.0
