From patchwork Thu Jul 9 12:33:38 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Daniel Vetter X-Patchwork-Id: 11654355 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 7A52792A for ; Thu, 9 Jul 2020 12:33:57 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 5910E20708 for ; Thu, 9 Jul 2020 12:33:57 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=ffwll.ch header.i=@ffwll.ch header.b="FT1zeqK0" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5910E20708 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ffwll.ch Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=dri-devel-bounces@lists.freedesktop.org Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 9F97A6EA60; Thu, 9 Jul 2020 12:33:53 +0000 (UTC) X-Original-To: dri-devel@lists.freedesktop.org Delivered-To: dri-devel@lists.freedesktop.org Received: from mail-wm1-x341.google.com (mail-wm1-x341.google.com [IPv6:2a00:1450:4864:20::341]) by gabe.freedesktop.org (Postfix) with ESMTPS id 894506E42F for ; Thu, 9 Jul 2020 12:33:52 +0000 (UTC) Received: by mail-wm1-x341.google.com with SMTP id g10so6175510wmc.1 for ; Thu, 09 Jul 2020 05:33:52 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ffwll.ch; s=google; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=9ml7yu61orPGiyeLHj7KPJdBGRuwIj4VGf7RIdg6+6E=; b=FT1zeqK0F6re7x3hxU6WuwKnuhLdZTTGsE6uj0ZmBrbf9TVkVqTK3LOGBYbBEjYddX UQxsmufvcNCR+EiwLcPUmPYWFaC5pZ5CGfGNvzGthBXT2lli8Ur+GfzSgeLxXbdC//jm w7Dqku33+bwbEzRQvB6e/WItr0k784kTvA0/0= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=9ml7yu61orPGiyeLHj7KPJdBGRuwIj4VGf7RIdg6+6E=; b=nnKxmk/M/FZbPu8jFGT+g8s7H0PchXXX9Oywzx8neybZ2eZudKFOjqlPqMdQs0TPId gWYAaI/BooNqJH6ipbCpeS5+7/htn3Zv27gwBYonWgR+yP8Z3Y3xFFoy8Bu3lcOEHiZl di42efE/DJHkikMxV2cCTsY9camhXebFtGRK/bA8BUGwThHivtnRzp6Dk23lUpkuNl0Q MikgwKsTeDEiggjMIQ5FiPsZtRbMKXBWuimbf6mTovt+Dn292PJYtEpy59Zqq+GLhUE5 FTJiIsfkTbLLqQbiw0RMq7383hfbQu/oS0vNpQTPAXFUG3uOjjlRIQu5k2NX8GWOjBXS RcvQ== X-Gm-Message-State: AOAM533CcBhOJb9JshYPeBwDgW4yilQAgATH64YB85BFMzy5Ja+3NZfn WCh7dJzXSp0EirmFwbpq9QQb5gwTbj0= X-Google-Smtp-Source: ABdhPJyW1X39siqvN1tXyO+a5xJDhtYxZB7qgF8t/3W1eiXrhQsMLgXCOAKI1Lx9mgsJn2P0+ZV+SQ== X-Received: by 2002:a7b:c313:: with SMTP id k19mr14181981wmj.67.1594298030902; Thu, 09 Jul 2020 05:33:50 -0700 (PDT) Received: from phenom.ffwll.local ([2a02:168:57f4:0:efd0:b9e5:5ae6:c2fa]) by smtp.gmail.com with ESMTPSA id o205sm4925347wme.24.2020.07.09.05.33.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 09 Jul 2020 05:33:50 -0700 (PDT) From: Daniel Vetter To: DRI Development Subject: [PATCH 1/2] dma-buf.rst: Document why indefinite fences are a bad idea Date: Thu, 9 Jul 2020 14:33:38 +0200 Message-Id: <20200709123339.547390-1-daniel.vetter@ffwll.ch> X-Mailer: git-send-email 2.27.0 In-Reply-To: <20200707201229.472834-4-daniel.vetter@ffwll.ch> References: <20200707201229.472834-4-daniel.vetter@ffwll.ch> MIME-Version: 1.0 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: , Cc: Daniel Stone , linux-rdma@vger.kernel.org, Daniel Vetter , Intel Graphics Development , amd-gfx@lists.freedesktop.org, Chris Wilson , linaro-mm-sig@lists.linaro.org, Steve Pronovost , Daniel Vetter , Jason Ekstrand , Jesse Natalie , Felix Kuehling , Thomas Hellstrom , linux-media@vger.kernel.org, =?utf-8?q?Christian_K=C3=B6nig?= , Mika Kuoppala Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" Comes up every few years, gets somewhat tedious to discuss, let's write this down once and for all. What I'm not sure about is whether the text should be more explicit in flat out mandating the amdkfd eviction fences for long running compute workloads or workloads where userspace fencing is allowed. v2: Now with dot graph! v3: Typo (Dave Airlie) Acked-by: Christian König Acked-by: Daniel Stone Cc: Jesse Natalie Cc: Steve Pronovost Cc: Jason Ekstrand Cc: Felix Kuehling Cc: Mika Kuoppala Cc: Thomas Hellstrom Cc: linux-media@vger.kernel.org Cc: linaro-mm-sig@lists.linaro.org Cc: linux-rdma@vger.kernel.org Cc: amd-gfx@lists.freedesktop.org Cc: intel-gfx@lists.freedesktop.org Cc: Chris Wilson Cc: Maarten Lankhorst Cc: Christian König Signed-off-by: Daniel Vetter Reviewed-by: Maarten Lankhorst Acked-by: Jason Ekstrand Reviewed-by: Thomas Hellstrom --- Documentation/driver-api/dma-buf.rst | 70 ++++++++++++++++++++++++++++ 1 file changed, 70 insertions(+) diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst index f8f6decde359..100bfd227265 100644 --- a/Documentation/driver-api/dma-buf.rst +++ b/Documentation/driver-api/dma-buf.rst @@ -178,3 +178,73 @@ DMA Fence uABI/Sync File .. kernel-doc:: include/linux/sync_file.h :internal: +Indefinite DMA Fences +~~~~~~~~~~~~~~~~~~~~ + +At various times &dma_fence with an indefinite time until dma_fence_wait() +finishes have been proposed. Examples include: + +* Future fences, used in HWC1 to signal when a buffer isn't used by the display + any longer, and created with the screen update that makes the buffer visible. + The time this fence completes is entirely under userspace's control. + +* Proxy fences, proposed to handle &drm_syncobj for which the fence has not yet + been set. Used to asynchronously delay command submission. + +* Userspace fences or gpu futexes, fine-grained locking within a command buffer + that userspace uses for synchronization across engines or with the CPU, which + are then imported as a DMA fence for integration into existing winsys + protocols. + +* Long-running compute command buffers, while still using traditional end of + batch DMA fences for memory management instead of context preemption DMA + fences which get reattached when the compute job is rescheduled. + +Common to all these schemes is that userspace controls the dependencies of these +fences and controls when they fire. Mixing indefinite fences with normal +in-kernel DMA fences does not work, even when a fallback timeout is included to +protect against malicious userspace: + +* Only the kernel knows about all DMA fence dependencies, userspace is not aware + of dependencies injected due to memory management or scheduler decisions. + +* Only userspace knows about all dependencies in indefinite fences and when + exactly they will complete, the kernel has no visibility. + +Furthermore the kernel has to be able to hold up userspace command submission +for memory management needs, which means we must support indefinite fences being +dependent upon DMA fences. If the kernel also support indefinite fences in the +kernel like a DMA fence, like any of the above proposal would, there is the +potential for deadlocks. + +.. kernel-render:: DOT + :alt: Indefinite Fencing Dependency Cycle + :caption: Indefinite Fencing Dependency Cycle + + digraph "Fencing Cycle" { + node [shape=box bgcolor=grey style=filled] + kernel [label="Kernel DMA Fences"] + userspace [label="userspace controlled fences"] + kernel -> userspace [label="memory management"] + userspace -> kernel [label="Future fence, fence proxy, ..."] + + { rank=same; kernel userspace } + } + +This means that the kernel might accidentally create deadlocks +through memory management dependencies which userspace is unaware of, which +randomly hangs workloads until the timeout kicks in. Workloads, which from +userspace's perspective, do not contain a deadlock. In such a mixed fencing +architecture there is no single entity with knowledge of all dependencies. +Thefore preventing such deadlocks from within the kernel is not possible. + +The only solution to avoid dependencies loops is by not allowing indefinite +fences in the kernel. This means: + +* No future fences, proxy fences or userspace fences imported as DMA fences, + with or without a timeout. + +* No DMA fences that signal end of batchbuffer for command submission where + userspace is allowed to use userspace fencing or long running compute + workloads. This also means no implicit fencing for shared buffers in these + cases.