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Thu, 28 Aug 2025 12:01:03 -0700 (PDT) From: Mitchell Levy Date: Thu, 28 Aug 2025 12:00:14 -0700 Subject: [PATCH v3 7/7] rust: percpu: cache per-CPU pointers in the dynamic case MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20250828-rust-percpu-v3-7-4dd92e1e7904@gmail.com> References: <20250828-rust-percpu-v3-0-4dd92e1e7904@gmail.com> In-Reply-To: <20250828-rust-percpu-v3-0-4dd92e1e7904@gmail.com> To: Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , Andreas Hindborg , Alice Ryhl , Trevor Gross , Andrew Morton , Dennis Zhou , Tejun Heo , Christoph Lameter , Danilo Krummrich , Benno Lossin , Yury Norov , Viresh Kumar Cc: Tyler Hicks , linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, linux-mm@kvack.org, Mitchell Levy X-Mailer: b4 0.14.0 X-Developer-Signature: v=1; a=ed25519-sha256; t=1756407651; l=3005; i=levymitchell0@gmail.com; s=20240719; h=from:subject:message-id; bh=E3oVGdPxyaJFgaTqf2y8btrQgXMEMCM/zUm8ETp1y5c=; 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While the compiler might be clever enough to consolidate these reads in some cases, the read must happen *somewhere*, which, when we're concerning ourselves with individual instructions, is a very high burden. Instead, cache the `PerCpuPointer` inside the `DynamicPerCpu` structure; then, the `Arc` is used solely to manage the allocation. Signed-off-by: Mitchell Levy --- rust/kernel/percpu/dynamic.rs | 8 ++++++-- rust/kernel/percpu/numeric.rs | 4 ++-- 2 files changed, 8 insertions(+), 4 deletions(-) diff --git a/rust/kernel/percpu/dynamic.rs b/rust/kernel/percpu/dynamic.rs index aad08e4b4251..8683e94bcd9f 100644 --- a/rust/kernel/percpu/dynamic.rs +++ b/rust/kernel/percpu/dynamic.rs @@ -58,7 +58,10 @@ fn drop(&mut self) { pub struct DynamicPerCpu { // INVARIANT: The memory location in each CPU's per-CPU area pointed at by `alloc.0` has been // initialized. + // INVARIANT: `ptr` is the per-CPU pointer managed by `alloc`, which does not change for the + // lifetime of `self`. pub(super) alloc: Arc>, + pub(super) ptr: PerCpuPtr, } impl DynamicPerCpu { @@ -70,9 +73,10 @@ impl DynamicPerCpu { pub fn new_zero(flags: Flags) -> Option { let alloc: PerCpuAllocation = PerCpuAllocation::new_zero()?; + let ptr = alloc.0; let arc = Arc::new(alloc, flags).ok()?; - Some(Self { alloc: arc }) + Some(Self { alloc: arc, ptr }) } } @@ -102,7 +106,7 @@ pub fn new_with(val: T, flags: Flags) -> Option { let arc = Arc::new(alloc, flags).ok()?; - Some(Self { alloc: arc }) + Some(Self { alloc: arc, ptr }) } } diff --git a/rust/kernel/percpu/numeric.rs b/rust/kernel/percpu/numeric.rs index 4de93f653f0e..3412c2de85fd 100644 --- a/rust/kernel/percpu/numeric.rs +++ b/rust/kernel/percpu/numeric.rs @@ -20,7 +20,7 @@ impl DynamicPerCpu<$ty> { pub fn num(&mut self) -> PerCpuNumeric<'_, $ty> { // The invariant is satisfied because `DynamicPerCpu`'s invariant guarantees that // this pointer is valid and initialized on all CPUs. - PerCpuNumeric { ptr: &self.alloc.0 } + PerCpuNumeric { ptr: &self.ptr } } } impl StaticPerCpu<$ty> { @@ -71,7 +71,7 @@ impl DynamicPerCpu<$ty> { pub fn num(&mut self) -> PerCpuNumeric<'_, $ty> { // The invariant is satisfied because `DynamicPerCpu`'s invariant guarantees that // this pointer is valid and initialized on all CPUs. - PerCpuNumeric { ptr: &self.alloc.0 } + PerCpuNumeric { ptr: &self.ptr } } } impl StaticPerCpu<$ty> { -- 2.34.1