Re: [RFC PATCH 06/11] rust: apply cache line padding for `SpinLock`

[Alice Ryhl]

On Wed, 3 May 2023 11:07:03 +0200, Andreas Hindborg <a.hindborg@xxxxxxxxxxx> wrote:
> The kernel `struct spinlock` is 4 bytes on x86 when lockdep is not enabled. The
> structure is not padded to fit a cache line. The effect of this for `SpinLock`
> is that the lock variable and the value protected by the lock will share a cache
> line, depending on the alignment requirements of the protected value. Aligning
> the lock variable and the protected value to a cache line yields a 20%
> performance increase for the Rust null block driver for sequential reads to
> memory backed devices at 6 concurrent readers.
> 
> Signed-off-by: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>

This applies the cacheline padding to all spinlocks unconditionally.
It's not clear to me that we want to do that. Instead, I suggest using
`SpinLock<CachePadded<T>>` in the null block driver to opt-in to the
cache padding there, and let other drivers choose whether or not they
want to cache pad their locks.

On Wed, 3 May 2023 11:07:03 +0200, Andreas Hindborg <a.hindborg@xxxxxxxxxxx> wrote:
> diff --git a/rust/kernel/cache_padded.rs b/rust/kernel/cache_padded.rs
> new file mode 100644
> index 000000000000..758678e71f50
> --- /dev/null
> +++ b/rust/kernel/cache_padded.rs
> 
> +impl<T> CachePadded<T> {
> +    /// Pads and aligns a value to 64 bytes.
> +    #[inline(always)]
> +    pub(crate) const fn new(t: T) -> CachePadded<T> {
> +        CachePadded::<T> { value: t }
> +    }
> +}

Please make this `pub` instead of just `pub(crate)`. Other drivers might
want to use this directly.

On Wed, 3 May 2023 11:07:03 +0200, Andreas Hindborg <a.hindborg@xxxxxxxxxxx> wrote:
> diff --git a/rust/kernel/sync/lock/spinlock.rs b/rust/kernel/sync/lock/spinlock.rs
> index 979b56464a4e..e39142a8148c 100644
> --- a/rust/kernel/sync/lock/spinlock.rs
> +++ b/rust/kernel/sync/lock/spinlock.rs
> @@ -100,18 +103,20 @@ unsafe impl super::Backend for SpinLockBackend {
>      ) {
>          // SAFETY: The safety requirements ensure that `ptr` is valid for writes, and `name` and
>          // `key` are valid for read indefinitely.
> -        unsafe { bindings::__spin_lock_init(ptr, name, key) }
> +        unsafe { bindings::__spin_lock_init((&mut *ptr).deref_mut(), name, key) }
>      }
>  
> +    #[inline(always)]
>      unsafe fn lock(ptr: *mut Self::State) -> Self::GuardState {
>          // SAFETY: The safety requirements of this function ensure that `ptr` points to valid
>          // memory, and that it has been initialised before.
> -        unsafe { bindings::spin_lock(ptr) }
> +        unsafe { bindings::spin_lock((&mut *ptr).deref_mut()) }
>      }
>  
> +    #[inline(always)]
>      unsafe fn unlock(ptr: *mut Self::State, _guard_state: &Self::GuardState) {
>          // SAFETY: The safety requirements of this function ensure that `ptr` is valid and that the
>          // caller is the owner of the mutex.
> -        unsafe { bindings::spin_unlock(ptr) }
> +        unsafe { bindings::spin_unlock((&mut *ptr).deref_mut()) }
>      }
>  }

I would prefer to remain in pointer-land for the above operations. I
think that this leads to core that is more obviously correct.

For example:

```
impl<T> CachePadded<T> {
    pub const fn raw_get(ptr: *mut Self) -> *mut T {
        core::ptr::addr_of_mut!((*ptr).value)
    }
}

#[inline(always)]
unsafe fn unlock(ptr: *mut Self::State, _guard_state: &Self::GuardState) {
    unsafe { bindings::spin_unlock(CachePadded::raw_get(ptr)) }
}
```