From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 52DC0C2E0; Sun, 13 Jul 2025 09:30:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752399037; cv=none; b=PlB/JZ8YxpXNMuxL5i6GPymlExmsPTypcw878q9eIMAMMKUKIVTDV3Pgts7FbGEAslHozhiN7LAJawCwRHHlfE8jQwu0VKQQ//6CY0Iqwvn2s0BuSnfTwdg0yZ/VzuDrjj2oMn+P6iDYF2c1BNZ16lDKkxwUSmRlckrIijzNKxs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752399037; c=relaxed/simple; bh=nKbk6E47aaqEXS0XKE/C976P4mu6SktTQNRnupN5EMs=; h=Mime-Version:Content-Type:Date:Message-Id:Cc:Subject:From:To: References:In-Reply-To; b=l9ElnrWIrbKPWS4A0Nm5cL3q8TqatQt3OIifnwvOjirASJAsk5da5y1xHMoXSgPsjhqcfbPSwpBmGWD4WCX9FWKn+2zG6/zsFLUMRuHCULnSQPpTBqn1U3tGwAuYvRCce9SkjrVemBl/LWdyDM1Pj/nNjQ4pFORK8qHMod6thnw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=SJ1oCnWP; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="SJ1oCnWP" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 97E85C4CEE3; Sun, 13 Jul 2025 09:30:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1752399036; bh=nKbk6E47aaqEXS0XKE/C976P4mu6SktTQNRnupN5EMs=; h=Date:Cc:Subject:From:To:References:In-Reply-To:From; b=SJ1oCnWP7GGo8/oqSsTzDM10/zjCsKRghHncNrZIA3H/wDMnv8HVZPRB5ioeqc8QU x9XPmnpk4ANH8f44LGVmCp69KXLzaLYp2DCDxpieBqJUcjcQPmB1Pb/lxS8oD3RW9e XaSJYkvOTecnQetUNKlAaHJY8dBxuK4ckCZoCmPBeX3CBiazFnOQuFyAvrlsyp/jhS lGIa3GJnJEvs3ewACwYm6ZxSHgC3RpabeRtsxU5GJfmw1ryO/qXnIj074S/oA59xmY C7d+K7CFrFmUPAACLDtykOfLIzl4UXAtLBBB8lo0gwfhUl3lVlBM9PNbO9OSjuHKAk 3l8iNb+AtNJdg== Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Sun, 13 Jul 2025 11:30:31 +0200 Message-Id: Cc: , , Subject: Re: [PATCH v2 3/5] rust: percpu: add a rust per-CPU variable test From: "Benno Lossin" To: "Mitchell Levy" , "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" X-Mailer: aerc 0.20.1 References: <20250712-rust-percpu-v2-0-826f2567521b@gmail.com> <20250712-rust-percpu-v2-3-826f2567521b@gmail.com> In-Reply-To: <20250712-rust-percpu-v2-3-826f2567521b@gmail.com> On Sat Jul 12, 2025 at 11:31 PM CEST, Mitchell Levy wrote: > Add a short exercise for Rust's per-CPU variable API, modelled after > lib/percpu_test.c > > Signed-off-by: Mitchell Levy > --- > lib/Kconfig.debug | 9 ++++ > lib/Makefile | 1 + > lib/percpu_test_rust.rs | 120 ++++++++++++++++++++++++++++++++++++++++++= ++++++ I don't know if this is the correct place, the code looks much more like a sample, so why not place it there instead? > rust/helpers/percpu.c | 11 +++++ > 4 files changed, 141 insertions(+) > diff --git a/lib/percpu_test_rust.rs b/lib/percpu_test_rust.rs > new file mode 100644 > index 000000000000..a9652e6ece08 > --- /dev/null > +++ b/lib/percpu_test_rust.rs > @@ -0,0 +1,120 @@ > +// SPDX-License-Identifier: GPL-2.0 > +//! A simple self test for the rust per-CPU API. > + > +use core::ffi::c_void; > + > +use kernel::{ > + bindings::{on_each_cpu, smp_processor_id}, > + define_per_cpu, > + percpu::{cpu_guard::*, *}, > + pr_info, > + prelude::*, > + unsafe_get_per_cpu, > +}; > + > +module! { > + type: PerCpuTestModule, > + name: "percpu_test_rust", > + author: "Mitchell Levy", > + description: "Test code to exercise the Rust Per CPU variable API", > + license: "GPL v2", > +} > + > +struct PerCpuTestModule; > + > +define_per_cpu!(PERCPU: i64 =3D 0); > +define_per_cpu!(UPERCPU: u64 =3D 0); > + > +impl kernel::Module for PerCpuTestModule { > + fn init(_module: &'static ThisModule) -> Result { > + pr_info!("rust percpu test start\n"); > + > + let mut native: i64 =3D 0; > + // SAFETY: PERCPU is properly defined > + let mut pcpu: StaticPerCpu =3D unsafe { unsafe_get_per_cpu!= (PERCPU) }; I don't understand why we need unsafe here, can't we just create something specially in the `define_per_cpu` macro that is then confirmed by the `get_per_cpu!` macro and thus it can be safe? > + // SAFETY: We only have one PerCpu that points at PERCPU > + unsafe { pcpu.get(CpuGuard::new()) }.with(|val: &mut i64| { Hmm I also don't like the unsafe part here... Can't we use the same API that `thread_local!` in the standard library has: https://doc.rust-lang.org/std/macro.thread_local.html So in this example you would store a `Cell` instead. I'm not familiar with per CPU variables, but if you're usually storing `Copy` types, then this is much better wrt not having unsafe code everywhere. If one also often stores `!Copy` types, then we might be able to get away with `RefCell`, but that's a small runtime overhead -- which is probably bad given that per cpu variables are most likely used for performance reasons? In that case the user might just need to store `UnsafeCell` and use unsafe regardless. (or we invent something specifically for that case, eg tokens that are statically known to be unique etc) --- Cheers, Benno > + pr_info!("The contents of pcpu are {}\n", val); > + > + native +=3D -1; > + *val +=3D -1; > + pr_info!("Native: {}, *pcpu: {}\n", native, val); > + assert!(native =3D=3D *val && native =3D=3D -1); > + > + native +=3D 1; > + *val +=3D 1; > + pr_info!("Native: {}, *pcpu: {}\n", native, val); > + assert!(native =3D=3D *val && native =3D=3D 0); > + });