From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751783AbeBTX41 (ORCPT ); Tue, 20 Feb 2018 18:56:27 -0500 Received: from mail-wr0-f178.google.com ([209.85.128.178]:37032 "EHLO mail-wr0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751683AbeBTX4Z (ORCPT ); Tue, 20 Feb 2018 18:56:25 -0500 X-Google-Smtp-Source: AH8x226hvvHh3gvLAwKcVZWCidbl+y4Vr8czaBMveHXcBtfQbNAihv8l4QPSwzmFf2rrzOXPTN+xdA== Date: Wed, 21 Feb 2018 00:56:21 +0100 From: Daniel Vetter To: Peter Zijlstra Cc: christian.koenig@amd.com, dev@mblankhorst.nl, amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/4] locking/ww_mutex: add ww_mutex_is_owned_by function v3 Message-ID: <20180220235621.GD22199@phenom.ffwll.local> Mail-Followup-To: Peter Zijlstra , christian.koenig@amd.com, dev@mblankhorst.nl, amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org References: <20180220125829.27060-1-christian.koenig@amd.com> <20180220131253.GF25314@hirez.programming.kicks-ass.net> <8fd80334-4d0e-8ed0-8a09-02a7e36a0eae@gmail.com> <20180220135709.GD25201@hirez.programming.kicks-ass.net> <20180220145413.GF25201@hirez.programming.kicks-ass.net> <2dac8ec9-c54a-7dd1-af4e-62f2bc1a959c@gmail.com> <20180220152158.GH25201@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20180220152158.GH25201@hirez.programming.kicks-ass.net> X-Operating-System: Linux phenom 4.14.0-3-amd64 User-Agent: Mutt/1.9.3 (2018-01-21) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 20, 2018 at 04:21:58PM +0100, Peter Zijlstra wrote: > On Tue, Feb 20, 2018 at 04:05:49PM +0100, Christian König wrote: > > Am 20.02.2018 um 15:54 schrieb Peter Zijlstra: > > > On Tue, Feb 20, 2018 at 03:34:07PM +0100, Christian König wrote: > > > > > OK, but neither case would in fact need the !ctx case right? That's just > > > > > there for completeness sake? > > > > Unfortunately not. TTM uses trylock to lock BOs which are about to be > > > > evicted to make room for all the BOs locked with a ctx. > > > > > > > > I need to be able to distinct between the BOs which are trylocked and those > > > > which are locked with a ctx. > > > > > > > > Writing this I actually noticed the current version is buggy, cause even > > > > when we check the mutex owner we still need to make sure that the ctx in the > > > > lock is NULL. > > > Hurm... I can't remember why trylocks behave like that, and it seems > > > rather unfortunate / inconsistent. > > > > Actually for me that is rather fortunate, cause I need to distinct between > > the locks acquired through trylock and lock. > > I suppose that would always be possible using: > ww_mutex_trylock(.ctx=NULL), and it could be that there simply weren't > any immediate uses for a !NULL trylock and it was thus not implemented. > > But that is all very long ago.. I think we simple never had a use-case for interleaving ww_mutex_lock(ctx) and ww_mutex_trylock(ctx). Nesting multiple trylocks in ctx-locks happens plenty, but not further: The common use-case for that is locking a bunch of buffers you need (for command submission or whatever), and then trylocking other buffers to make space for the buffers you need to move into VRAM. I guess if only trylocking buffers doesn't succeed in freeing up enough VRAM then we could go into blocking ww_mutex_locks which need the ctx (and which would need all the trylock-acquired buffers to be annotated with the ctx too). TTM currently tries to be far enough away from that corner case (using a defensive "never use more than 50% of all memory for gfx" approach) that it doesn't seem to need that. Once we get there it should indeed be simply to add a ctx parameter to ww_mutex_trylock to fix this case. The TTM side rework is definitely going to be the much bigger issue here ... -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch