[PATCH v2 0/3] fs: replace wq users and add WQ_PERCPU to alloc_workqueue() users

[Marco Crivellari <marco.crivellari@suse.com>]

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