[RFC PATCH v1 0/2] Documentation: Introducing `wq_cpu_set` mount option for btrfs

[RFC PATCH v1 0/2] Documentation: Introducing `wq_cpu_set` mount option for btrfs

[Ammar Faizi]

This is a follow up RFC of this series:
https://lore.kernel.org/linux-btrfs/20230226160259.18354-1-ammarfaizi2@gnuweeb.org

It contains the documentation for the `wq_cpu_set` mount option.

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

Ammar Faizi (2):
  Documentation: btrfs: Document wq_cpu_set mount option
  Documentation: btrfs: Document the influence of wq_cpu_set to thread_pool option

 Documentation/ch-mount-options.rst | 32 ++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)


base-commit: 908b4b4450320e30fdef693f09b42f4eb93702c3
-- 
Ammar Faizi

[RFC PATCH v1 1/2] Documentation: btrfs: Document wq_cpu_set mount option

[Ammar Faizi]

Document a new Btrfs mount option, wq_cpu_set.

Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---
 Documentation/ch-mount-options.rst | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/Documentation/ch-mount-options.rst b/Documentation/ch-mount-options.rst
index f0f205dc20fa15ff..48fe63ee5e95c297 100644
--- a/Documentation/ch-mount-options.rst
+++ b/Documentation/ch-mount-options.rst
@@ -451,6 +451,35 @@ user_subvol_rm_allowed
                 ordinary directory. Whether this is possible can be detected at runtime, see
                 *rmdir_subvol* feature in *FILESYSTEM FEATURES*.
 
+wq_cpu_set=<cpu_set>
+        (since: 6.5, default: all online CPUs)
+
+        Btrfs workqueues can slow sensitive user tasks down because they can use any
+        online CPU to perform heavy workloads on an SMP system. This option is used to
+        isolate the Btrfs workqueues to a set of CPUs. It is helpful to avoid
+        sensitive user tasks being preempted by Btrfs heavy workqueues.
+
+        The *cpu_set* is a dot-separated list of decimal numbers and ranges. The
+        numbers are CPU numbers, the ranges are inclusive. For example:
+
+                - *wq_cpu_set=0.3-7* will use CPUs 0, 3, 4, 5, 6 and 7.
+
+                - *wq_cpu_set=0.4.1.5* will use CPUs 0, 1, 4 and 5.
+
+        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.
+
+        If *wq_cpu_set* option is specificed and the *thread_pool* option is not, the
+        number of default max thread pool size will be set to the number of online
+        CPUs in the specified CPU set plus 2, if and only if the resulting number is
+        less than 8.
+
+        If *wq_cpu_set* option is specificed and the *thread_pool* option is also
+        specified, the thread pool size will be set to the value of *thread_pool*
+        option.
+
+
 DEPRECATED MOUNT OPTIONS
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
-- 
Ammar Faizi

Re: [RFC PATCH v1 1/2] Documentation: btrfs: Document wq_cpu_set mount option

[Bagas Sanjaya]

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On Sun, Feb 26, 2023 at 11:26:38PM +0700, Ammar Faizi wrote:
> +wq_cpu_set=<cpu_set>
> +        (since: 6.5, default: all online CPUs)

Why will the knob be introduced in 6.5 instead?

> +
> +        Btrfs workqueues can slow sensitive user tasks down because they can use any
> +        online CPU to perform heavy workloads on an SMP system. This option is used to
> +        isolate the Btrfs workqueues to a set of CPUs. It is helpful to avoid
> +        sensitive user tasks being preempted by Btrfs heavy workqueues.
> +
> +        The *cpu_set* is a dot-separated list of decimal numbers and ranges. The
> +        numbers are CPU numbers, the ranges are inclusive. For example:

"*cpu_set* is a dot-separated list of CPU numbers. Both individual
number and range (inclusive) can be listed".

> +
> +                - *wq_cpu_set=0.3-7* will use CPUs 0, 3, 4, 5, 6 and 7.
> +
> +                - *wq_cpu_set=0.4.1.5* will use CPUs 0, 1, 4 and 5.
> +
> +        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.

"... replaced by dots, since commas has already been used as mount
options separator".

> +        If *wq_cpu_set* option is specificed and the *thread_pool* option is also
> +        specified, the thread pool size will be set to the value of *thread_pool*
> +        option.

"If both options are set ..."

Thanks.

-- 
An old man doll... just what I always wanted! - Clara

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Re: [RFC PATCH v1 1/2] Documentation: btrfs: Document wq_cpu_set mount option

[Ammar Faizi]

On Mon, Feb 27, 2023 at 09:24:16AM +0700, Bagas Sanjaya wrote:
> "*cpu_set* is a dot-separated list of CPU numbers. Both individual
> number and range (inclusive) can be listed".

Folded in.

> "... replaced by dots, since commas has already been used as mount
> options separator".

Folded in with s/commas/comma/.

> "If both options are set ..."

Folded in.

Thanks,

-- 
Ammar Faizi

[RFC PATCH v1 2/2] Documentation: btrfs: Document the influence of wq_cpu_set to thread_pool option

[Ammar Faizi]

If wq_cpu_set option is set, the default thread_pool value will be
adjusted accordingly.

Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---
 Documentation/ch-mount-options.rst | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Documentation/ch-mount-options.rst b/Documentation/ch-mount-options.rst
index 48fe63ee5e95c297..c38caf5e5fd0b719 100644
--- a/Documentation/ch-mount-options.rst
+++ b/Documentation/ch-mount-options.rst
@@ -411,6 +411,9 @@ thread_pool=<number>
         due to increased locking contention, process scheduling, cache-line bouncing or
         costly data transfers between local CPU memories.
 
+        Since 6.5, if *wq_cpu_set* is set, the default value will be the number of
+        online CPUs in the CPU wq_cpu_set plus 2.
+
 treelog, notreelog
         (default: on)
 
-- 
Ammar Faizi

Re: [RFC PATCH v1 2/2] Documentation: btrfs: Document the influence of wq_cpu_set to thread_pool option

[Bagas Sanjaya]

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On Sun, Feb 26, 2023 at 11:26:39PM +0700, Ammar Faizi wrote:
> +        Since 6.5, if *wq_cpu_set* is set, the default value will be the number of
> +        online CPUs in the CPU wq_cpu_set plus 2.
> +
  
Why will the behavior be introduced in such future version (6.5)?

-- 
An old man doll... just what I always wanted! - Clara

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Re: [RFC PATCH v1 2/2] Documentation: btrfs: Document the influence of wq_cpu_set to thread_pool option

[Ammar Faizi]

On Mon, Feb 27, 2023 at 09:15:58AM +0700, Bagas Sanjaya wrote:
> Why will the behavior be introduced in such future version (6.5)?

It's not like it has been staged for the next merge window. It's still
in an RFC state. The changes are not trivial and need further review.

I don't know if it can hit the next merge window. As such, I picked a
long distance for this proposal. If it ends up going upstream sooner, we
can change this document.

-- 
Ammar Faizi

Re: [RFC PATCH v1 2/2] Documentation: btrfs: Document the influence of wq_cpu_set to thread_pool option

[Bagas Sanjaya]

On 2/27/23 17:02, Ammar Faizi wrote:
> On Mon, Feb 27, 2023 at 09:15:58AM +0700, Bagas Sanjaya wrote:
>> Why will the behavior be introduced in such future version (6.5)?
> 
> It's not like it has been staged for the next merge window. It's still
> in an RFC state. The changes are not trivial and need further review.
> 
> I don't know if it can hit the next merge window. As such, I picked a
> long distance for this proposal. If it ends up going upstream sooner, we
> can change this document.
> 

OK, thanks!

-- 
An old man doll... just what I always wanted! - Clara