From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 0FD22E9E31B for ; Wed, 11 Feb 2026 15:33:29 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 70EC910E61E; Wed, 11 Feb 2026 15:33:28 +0000 (UTC) Authentication-Results: gabe.freedesktop.org; dkim=pass (2048-bit key; unprotected) header.d=google.com header.i=@google.com header.b="POC5XQwt"; dkim-atps=neutral Received: from mail-wm1-f74.google.com (mail-wm1-f74.google.com [209.85.128.74]) by gabe.freedesktop.org (Postfix) with ESMTPS id BDB8A10E61E for ; Wed, 11 Feb 2026 15:33:27 +0000 (UTC) Received: by mail-wm1-f74.google.com with SMTP id 5b1f17b1804b1-4806b12ad3fso68935255e9.0 for ; Wed, 11 Feb 2026 07:33:27 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1770824006; x=1771428806; darn=lists.freedesktop.org; h=content-transfer-encoding:cc:to:from:subject:message-id:references :mime-version:in-reply-to:date:from:to:cc:subject:date:message-id :reply-to; bh=69uHbckydWaKM9z8QoKV1Sspu6MeiIeZoVRM+AIu30E=; b=POC5XQwtJMNNOuuPJWD+aZ1/glIyNf+9rIPq3uAT7cZ4tr/0HvhfL+U6l2/6mavd7f M6qXbDj1Xz+wFTFfW5BdvUC/iO69t/E77MI2lOLn3D/D6h/ZMTM2Y7u6+aZc6g3xtt8a w7PHeMSH/ajvyQa+OSc319YBlawittW+/eAZt3zo2GyG/FUqGlYEaB4UDIUzTRAE0vqM MM3NCohdvkiBmI98gQRRWIjTjAZ26XqIFXKftAghxwb+U4o9KJ5SK2X/PdB37aM3hyxX q2Mi9g5euGiBhODkZLnpNsQMiL0Cpj18EmrMFdZbcyRBK8J0vrbVLUxOcyRKraWyuUFO IL8g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1770824006; x=1771428806; h=content-transfer-encoding:cc:to:from:subject:message-id:references :mime-version:in-reply-to:date:x-gm-message-state:from:to:cc:subject :date:message-id:reply-to; bh=69uHbckydWaKM9z8QoKV1Sspu6MeiIeZoVRM+AIu30E=; b=GGECd68hTp9ejc0VkVm7C4ouMGwyRd+W5G85cDRNUL5ErG0w4moZYMD9u9rFWycy7D lkNQ2y9Je/ow8UJpTjXHPTQjXtuN1xM7JLfP6lQ2D2fpIyeVxDt+hl+6ZBMvbg0T5brf IFsJjHPaYSaj8EZ4Who646VybVwwcQAh5MBL+7KJ/YROMX+WIsgJltx/ZwGZAsPhNg02 WjeukvFGddCNIPtdtGJutGoSfbM26dAJFl6Z319WXlwRAo4fiTWfiochSlaS9PVRf+0l yu+KXziwB/UV2x1zMkMaM46oT2vvJsc5W4bB9OVGvP/qwyzJ7inR/MEYCEIBHq6UnEvu hsww== X-Forwarded-Encrypted: i=1; AJvYcCWAg2npQgsPcBtY19lxj3UUpMOm/HlSoC7U5S5sTXVKWLGHbQMwreLodvR6hWE8rNyOsiJy5Yu7lCA=@lists.freedesktop.org X-Gm-Message-State: AOJu0YzynndR3jR4jw+98ZQ2UGrwJupxllaFHmUygmJqdqsw0MOZP900 enKIrcXFQjElQY7GdCG7IDGDlvVeMuk8s2lswTBCtNUzCiXiQvmiduLao0pWycY/xP8toqSlbKj 7fqvHwFpBw6mH7V3K8g== X-Received: from wmpf29.prod.google.com ([2002:a05:600c:491d:b0:480:6a27:9ec2]) (user=aliceryhl job=prod-delivery.src-stubby-dispatcher) by 2002:a05:600c:46cb:b0:483:ea6:8767 with SMTP id 5b1f17b1804b1-4835e2cd408mr35661225e9.36.1770824006380; Wed, 11 Feb 2026 07:33:26 -0800 (PST) Date: Wed, 11 Feb 2026 15:33:25 +0000 In-Reply-To: Mime-Version: 1.0 References: <20260203081403.68733-2-phasta@kernel.org> <20260203081403.68733-5-phasta@kernel.org> <20260210155750.5cdbe6cc@fedora> <8ea48ce49f2c7b6fd715dd54c24e755e8ac3262c.camel@mailbox.org> <20260211120742.0e9e7122@fedora> Message-ID: Subject: Re: [RFC PATCH 3/4] rust/drm: Add DRM Jobqueue From: Alice Ryhl To: Gary Guo Cc: phasta@kernel.org, Boris Brezillon , David Airlie , Simona Vetter , Danilo Krummrich , Benno Lossin , "Christian =?utf-8?B?S8O2bmln?=" , Daniel Almeida , Joel Fernandes , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, rust-for-linux@vger.kernel.org Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Wed, Feb 11, 2026 at 09:45:37PM +0800, Gary Guo wrote: > On Wed Feb 11, 2026 at 8:22 PM CST, Alice Ryhl wrote: > > On Wed, Feb 11, 2026 at 12:19:56PM +0100, Philipp Stanner wrote: > >> On Wed, 2026-02-11 at 12:07 +0100, Boris Brezillon wrote: > >> > On Wed, 11 Feb 2026 11:47:27 +0100 > >> > Philipp Stanner wrote: > >> >=20 > >> > > On Tue, 2026-02-10 at 15:57 +0100, Boris Brezillon wrote: > >> > > > On Tue,=C2=A0 3 Feb 2026 09:14:02 +0100 > >> > > > Philipp Stanner wrote: > >> > > > =C2=A0=20 > >> > > > > +/// A jobqueue Job. > >> > > > > +/// > >> > > > > +/// You can stuff your data in it. The job will be borrowed b= ack to your driver > >> > > > > +/// once the time has come to run it. > >> > > > > +/// > >> > > > > +/// Jobs are consumed by [`Jobqueue::submit_job`] by value (o= wnership transfer). > >> > > > > +/// You can set multiple [`DmaFence`] as dependencies for a j= ob. It will only > >> > > > > +/// get run once all dependency fences have been signaled. > >> > > > > +/// > >> > > > > +/// Jobs cost credits. Jobs will only be run if there are is = enough capacity in > >> > > > > +/// the jobqueue for the job's credits. It is legal to specif= y jobs costing 0 > >> > > > > +/// credits, effectively disabling that mechanism. > >> > > > > +#[pin_data] > >> > > > > +pub struct Job { > >> > > > > +=C2=A0=C2=A0=C2=A0 cost: u32, > >> > > > > +=C2=A0=C2=A0=C2=A0 #[pin] > >> > > > > +=C2=A0=C2=A0=C2=A0 pub data: T, > >> > > > > +=C2=A0=C2=A0=C2=A0 done_fence: Option>>, > >> > > > > +=C2=A0=C2=A0=C2=A0 hardware_fence: Option>= >, > >> > > > > +=C2=A0=C2=A0=C2=A0 nr_of_deps: AtomicU32, > >> > > > > +=C2=A0=C2=A0=C2=A0 dependencies: List,=C2=A0=20 > >> > > >=20 > >> > > > Given how tricky Lists are in rust, I'd recommend going for an X= Array, > >> > > > like we have on the C side. There's a bit of overhead when the j= ob only > >> > > > has a few deps, but I think simplicity beats memory-usage-optimi= zations > >> > > > in that case (especially since the overhead exists and is accept= ed in > >> > > > C).=C2=A0=20 > >> > >=20 > >> > > I mean, the list is now already implemented and works. Considering= the > >> > > XArray would have made sense during the development difficulties. > >> >=20 > >> > I'm sure it does, but that's still more code/tricks to maintain than > >> > what you'd have with the XArray abstraction. > >>=20 > >> The solution than will rather be to make the linked list implementatio= n > >> better. > >>=20 > >> A list is the correct data structure in a huge number of use cases in > >> the kernel. We should not begin here to defer to other structures > >> because of convenience. > > > > Rust vs C aside, linked lists are often used in the kernel despite not > > being the best choice. They are extremely cache unfriendly and > > inefficient; most of the time a vector or xarray is far faster if you > > can accept an ENOMEM failure path when adding elements. I have heard > > several times from C maintainers that overuse of list is making the > > kernel slow in a death from a thousand cuts situation. >=20 > I would rather argue the other way, other than very hot paths where cache > friendliness absolutely matters, if you do not require indexing access th= en the > list is the correct data strucutre more often than not. >=20 > Vector have the issue where resizing requires moving, so it cannot be use= d with > pinned types. XArray doesn't require moving because it requires an indire= ction > and thus an extra allocation, but this means that if you're just iteratin= g over > all elements it also does not benefit from cache locality. Using vectors = also > require careful management of capacity, which is a very common source of = memory > leak for long running programs in user space Rust. XArray does benefit somewhat from cache locality compared to a linked list because you know the address of element i+1 even if you have not yet retrieved element i, which may enable prefetching to happen. Alice > Re: the ENOMEM failure path, I'd argue that even if you *can* accept a EN= OMEM > failure path, it is better to not have a failing path that is unnecessary= . >=20 > Best, > Gary >=20 > > > > This applies to the red/black tree too, by the way. > > > > Alice >=20