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Date:   Wed, 06 Dec 2023 10:09:23 +0000
In-Reply-To: <20231206-rb-new-condvar-methods-v1-0-33a4cab7fdaa@google.com>
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Subject: [PATCH 1/2] rust: sync: add `CondVar::notify_sync`
From:   Alice Ryhl <aliceryhl@google.com>
To:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        "=?utf-8?q?Bj=C3=B6rn_Roy_Baron?=" <bjorn3_gh@protonmail.com>,
        Benno Lossin <benno.lossin@proton.me>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>
Cc:     rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        Alice Ryhl <aliceryhl@google.com>
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Wake up another thread synchronously.

This method behaves like `notify_one`, except that it hints to the
scheduler that the current thread is about to go to sleep, so it should
schedule the target thread on the same CPU.

This is used by Rust Binder as a performance optimization. When sending
a transaction to a different process, we usually know which thread will
handle it, so we can schedule that thread for execution next on this
CPU for better cache locality.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/sync/condvar.rs | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/rust/kernel/sync/condvar.rs b/rust/kernel/sync/condvar.rs
index b679b6f6dbeb..9861c6749ad0 100644
--- a/rust/kernel/sync/condvar.rs
+++ b/rust/kernel/sync/condvar.rs
@@ -155,6 +155,12 @@ fn notify(&self, count: i32, flags: u32) {
         };
     }
 
+    /// Calls the kernel function to notify one thread synchronously.
+    pub fn notify_sync(&self) {
+        // SAFETY: `wait_list` points to valid memory.
+        unsafe { bindings::__wake_up_sync(self.wait_list.get(), bindings::TASK_NORMAL) };
+    }
+
     /// Wakes a single waiter up, if any.
     ///
     /// This is not 'sticky' in the sense that if no thread is waiting, the notification is lost

-- 
2.43.0.rc2.451.g8631bc7472-goog