This RFC is an attempt to share a POC for our LSFMM'25[0] proposal. We initially prepared it based on the percpu writeback context design. But based on feedback received initially, we moved towards a more FS-geometry aware design. The patches are still clumsy, and there is softlockup issue while running fstests as well(mentioned below). Yet, we would like to gather feedback which will help in ironing out the infra. Kernel tree: https://github.com/SamsungDS/linux/tree/priv/pw/rfc Design ========================== - A new "wb_ctx" structure is introduced that represents a writeback context/thread. A new "wb_ctx_list" field is introduced in bdi_writeback that describes a wb_ctx array, which gets initialized during wb_init. - Writeback handling is modified to use "wb_ctx_list[0]" writeback context, so it continues to remain single threaded. - In xfs, during AG initialization, we try to assign a different writeback context to each AG from the wb_ctx_list. - During writeback, a "get_wb_ctx" handler is introduced in super_operations to fetch the writeback context from the inode, which gets used for writeback handling. Currently this handler is implemented only by xfs, so writeback handling is parallelized (per AG) for xfs only. It can be easily extended to other filesystems. Discussion ========================== - Design for API that would be used for deciding the number of writeback contexts by FS (suggested by Dave). TBDs ========================== - Need to explore cgroup related writeback handling. - Add a list_lock field in struct wb_ctx for protecting pctx_b_* lists. Currently there is a "bdi_writeback->list_lock" for protecting per writeback context list. - Ability to create writeback contexts dynamically. Currently, NR_WB_CTX number of writeback contexts are present in bdi_writeback and get initialized during wb_init. Testing ========================== - Encountered a softlockup issue while running fstests (test generic/051). Investigating the same. [0] https://lore.kernel.org/all/20250129102627.161448-1-kundan.kumar@xxxxxxxxxxx/ Kundan Kumar (3): writeback: add parallel writeback infrastructure fs: modify writeback infra to work with wb_ctx xfs: use the parallel writeback infra per AG fs/fs-writeback.c | 507 ++++++++++++++++++++----------- fs/xfs/libxfs/xfs_ag.c | 16 + fs/xfs/libxfs/xfs_ag.h | 1 + fs/xfs/xfs_super.c | 20 ++ include/linux/backing-dev-defs.h | 67 +++- include/linux/backing-dev.h | 34 ++- include/linux/fs.h | 1 + mm/backing-dev.c | 68 +++-- mm/page-writeback.c | 82 ++--- 9 files changed, 551 insertions(+), 245 deletions(-) base-commit: 57d357a7663c4025c68ae07974b679d84e2d5d1a -- 2.25.1
