[RFC PATCH v1 0/6] Introducing `wq_cpu_set` mount option for btrfs

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

Hi,

This is an RFC patchset that introduces the `wq_cpu_set` mount option.
This option lets the user specify a CPU set that the Btrfs workqueues
will use.

Btrfs workqueues can slow sensitive user tasks down because they can use
any online CPU to perform heavy workloads on an SMP system. Add a mount
option to isolate the Btrfs workqueues to a set of CPUs. It is helpful
to avoid sensitive user tasks being preempted by Btrfs heavy workqueues.

This option is similar to the taskset bitmask except that the comma
separator is replaced with a dot. The reason for this is that the mount
option parser uses commas to separate mount options.

Figure (the CPU usage when `wq_cpu_set` is used VS when it is not):
https://gist.githubusercontent.com/ammarfaizi2/a10f8073e58d1712c1ed49af83ae4ad1/raw/a4f7cbc4eb163db792a669d570ff542495e8c704/wq_cpu_set.png

A simple stress testing:

1. Open htop.
2. Open a new terminal.
3. Mount and perform a heavy workload on the mounted Btrfs filesystem.

## Test without wq_cpu_set
sudo mount -t btrfs -o rw,compress-force=zstd:15,commit=1500 /dev/sda2 hdd/a;
cp -rf /path/folder_with_many_large_files/ hdd/a/test;
sync; # See the CPU usage in htop.
sudo umount hdd/a;

## Test wq_cpu_set
sudo mount -t btrfs -o rw,compress-force=zstd:15,commit=1500,wq_cpu_set=0.4.1.5 /dev/sda2 hdd/a;
cp -rf /path/folder_with_many_large_files/ hdd/a/test;
sync; # See the CPU usage in htop.
sudo umount hdd/a;

Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---

Ammar Faizi (6):
  workqueue: Add set_workqueue_cpumask() helper function
  btrfs: Change `mount_opt` type in `struct btrfs_fs_info` to `u64`
  btrfs: Create btrfs CPU set struct and helpers
  btrfs: Add wq_cpu_set=%s mount option
  btrfs: Adjust the default thread pool size when `wq_cpu_set` option is used
  btrfs: Add `BTRFS_DEFAULT_MAX_THREAD_POOL_SIZE` macro

 fs/btrfs/async-thread.c   | 51 ++++++++++++++++++++
 fs/btrfs/async-thread.h   |  3 ++
 fs/btrfs/disk-io.c        |  6 ++-
 fs/btrfs/fs.c             | 97 +++++++++++++++++++++++++++++++++++++++
 fs/btrfs/fs.h             | 12 ++++-
 fs/btrfs/super.c          | 83 +++++++++++++++++++++++++++++++++
 include/linux/workqueue.h |  3 ++
 kernel/workqueue.c        | 19 ++++++++
 8 files changed, 271 insertions(+), 3 deletions(-)


base-commit: 2fcd07b7ccd5fd10b2120d298363e4e6c53ccf9c
-- 
Ammar Faizi