Hi Alice,
Alice Ryhl <aliceryhl@xxxxxxxxxx> writes:
> 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.
I was going to write that this is not going to work because the compiler
is going to reorder the fields of `Lock` and put the `data` field first,
followed by the `state` field. But I checked the layout, and it seems
that I actually get the `state` field first (with an alignment of 4), 60
bytes of padding, and then the `data` field (with alignment 64).
I am wondering why the compiler is not reordering these fields? Am I
guaranteed that the fields will not be reordered? Looking at the
definition of `Lock` there does not seem to be anything that prevents
rustc from swapping `state` and `data`.
>
> 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.
Alright.
>
> 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)) }
> }
> ```
Got it 👍
BR Andreas
