Hi!
Below is a summary of a discussion about the Workqueue API and cpu isolation
considerations. Details and more information are available here:
"workqueue: Always use wq_select_unbound_cpu() for WORK_CPU_UNBOUND."
https://lore.kernel.org/all/20250221112003.1dSuoGyc@linutronix.de/
=== Current situation: problems ===
Let's consider a nohz_full system with isolated CPUs: wq_unbound_cpumask is
set to the housekeeping CPUs, for !WQ_UNBOUND the local CPU is selected.
This leads to different scenarios if a work item is scheduled on an isolated
CPU where "delay" value is 0 or greater then 0:
schedule_delayed_work(, 0);
This will be handled by __queue_work() that will queue the work item on the
current local (isolated) CPU, while:
schedule_delayed_work(, 1);
Will move the timer on an housekeeping CPU, and schedule the work there.
Currently if a user enqueue a work item using schedule_delayed_work() the
used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use
WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to
schedule_work() that is using system_wq and queue_work(), that makes use
again of WORK_CPU_UNBOUND.
This lack of consistentcy cannot be addressed without refactoring the API.
=== Plan and future plans ===
This patchset is the first stone on a refactoring needed in order to
address the points aforementioned; it will have a positive impact also
on the cpu isolation, in the long term, moving away percpu workqueue in
favor to an unbound model.
These are the main steps:
1) API refactoring (that this patch is introducing)
- Make more clear and uniform the system wq names, both per-cpu and
unbound. This to avoid any possible confusion on what should be
used.
- Introduction of WQ_PERCPU: this flag is the complement of WQ_UNBOUND,
introduced in this patchset and used on all the callers that are not
currently using WQ_UNBOUND.
WQ_UNBOUND will be removed in a future release cycle.
Most users don't need to be per-cpu, because they don't have
locality requirements, because of that, a next future step will be
make "unbound" the default behavior.
2) Check who really needs to be per-cpu
- Remove the WQ_PERCPU flag when is not strictly required.
3) Add a new API (prefer local cpu)
- There are users that don't require a local execution, like mentioned
above; despite that, local execution yeld to performance gain.
This new API will prefer the local execution, without requiring it.
=== Introduced Changes by this series ===
1) [P 1-2] Replace use of system_wq and system_unbound_wq
system_wq is a per-CPU workqueue, but his name is not clear.
system_unbound_wq is to be used when locality is not required.
Because of that, system_wq has been renamed in system_percpu_wq, and
system_unbound_wq has been renamed in system_dfl_wq.
2) [P 3] add WQ_PERCPU to remaining alloc_workqueue() users
Every alloc_workqueue() caller should use one among WQ_PERCPU or
WQ_UNBOUND. This is actually enforced warning if both or none of them
are present at the same time.
WQ_UNBOUND will be removed in a next release cycle.
Thanks!
---
Changes in v2:
- rebased on 6.17-rc5
- removed the "For maintainers" section from the cover letter: patches are
already merged, there are no prerequisites.
Marco Crivellari (3):
fs: replace use of system_unbound_wq with system_dfl_wq
fs: replace use of system_wq with system_percpu_wq
fs: WQ_PERCPU added to alloc_workqueue users
fs/afs/callback.c | 4 ++--
fs/afs/main.c | 4 ++--
fs/afs/write.c | 2 +-
fs/aio.c | 2 +-
fs/bcachefs/btree_write_buffer.c | 2 +-
fs/bcachefs/io_read.c | 8 ++++----
fs/bcachefs/journal_io.c | 2 +-
fs/bcachefs/super.c | 10 +++++-----
fs/btrfs/block-group.c | 2 +-
fs/btrfs/disk-io.c | 2 +-
fs/btrfs/extent_map.c | 2 +-
fs/btrfs/space-info.c | 4 ++--
fs/btrfs/zoned.c | 2 +-
fs/ceph/super.c | 2 +-
fs/coredump.c | 2 +-
fs/dlm/lowcomms.c | 2 +-
fs/dlm/main.c | 2 +-
fs/ext4/mballoc.c | 2 +-
fs/fs-writeback.c | 4 ++--
fs/fuse/dev.c | 2 +-
fs/fuse/inode.c | 2 +-
fs/gfs2/main.c | 5 +++--
fs/gfs2/ops_fstype.c | 6 ++++--
fs/netfs/misc.c | 2 +-
fs/netfs/objects.c | 2 +-
fs/nfs/namespace.c | 2 +-
fs/nfs/nfs4renewd.c | 2 +-
fs/nfsd/filecache.c | 2 +-
fs/notify/mark.c | 4 ++--
fs/ocfs2/dlm/dlmdomain.c | 3 ++-
fs/ocfs2/dlmfs/dlmfs.c | 3 ++-
fs/quota/dquot.c | 2 +-
fs/smb/client/cifsfs.c | 16 +++++++++++-----
fs/smb/server/ksmbd_work.c | 2 +-
fs/smb/server/transport_rdma.c | 3 ++-
fs/super.c | 3 ++-
fs/verity/verify.c | 2 +-
fs/xfs/xfs_log.c | 3 +--
fs/xfs/xfs_mru_cache.c | 3 ++-
fs/xfs/xfs_super.c | 15 ++++++++-------
40 files changed, 79 insertions(+), 65 deletions(-)
--
2.51.0