From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CDC30606A319 for ; Mon, 19 Oct 2026 23:40:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=XfSDoPl8 Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 2717810EB5C; Mon, 19 Oct 2026 23:40:47 +0000 (UTC) Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by gabe.freedesktop.org (Postfix) with ESMTPS id EEE7510EB5C for ; Mon, 19 Oct 2026 23:40:39 +0000 (UTC) Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 9307F403FE; Mon, 19 Oct 2026 23:40:39 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4C4C21F000C4; Mon, 19 Oct 2026 23:40:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792453239; bh=cJE4Qd0aWDx6ISxVK9dIk9/aXK/tsToYLAWDqfOlosw=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=XfSDoPl8OrogpTGTm48nflmKwXCVStmIXlEyiwTF4Cpi+fRs6FhmR2i0QguELPaKt kJ8Rm1go/r2UKqjq47J4XuuHfcWd/VVxmU2TWm/2QwKMiP5B6qlXWu1EhPl18IXizL igzJVI2vvKqZhu3JHRky1amXgFRYkTNYxngNZqIOAXxfoyZ10mzYI2shPYanDx7JeA AlORKIY1H/B4Y0FlCFXYfE/0gYWU33/w90tcDR533/q+m3Q6HdK39XME5TZgey4ZlQ 34Kobwh3NZysAD1shOSYTXFOCEKZMbiwb1vwCYvh4YPTWVqv7DloXkd3F4GnqbyUZm kF2XWGcfxN9NA== Date: Mon, 19 Oct 2026 13:40:38 -1000 Message-ID: From: Tejun Heo To: Tvrtko Ursulin Cc: linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, kernel-dev@igalia.com, Boris Brezillon , Bradley Morgan , Chia-I Wu , Liviu Dudau , Matthew Brost , Steven Price , Lai Jiangshan , Breno Leitao Subject: Re: [RFC v6.1 2/3] workqueue: Add support for real-time workers In-Reply-To: <9bb46cf4-7982-463b-8289-06bbc319bba9@igalia.com> References: <20261001160711.59888-3-tvrtko.ursulin@igalia.com> <20261001184848.61776-1-tvrtko.ursulin@igalia.com> <3a139a32135c149f51bac4f2bd58ad56@kernel.org> <9bb46cf4-7982-463b-8289-06bbc319bba9@igalia.com> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "dri-devel" X-Spam-Status: No, score=-6.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 Hello, Tvrtko. On Mon, Oct 19, 2026 at 11:02:08AM +0100, Tvrtko Ursulin wrote: > > apply_workqueue_attrs() is unrestricted, so a caller can hand a WQ_RT wq the > > attrs from alloc_workqueue_attrs() and turn it into a normal non-strict wq > > while the flag stays set. Maybe apply_wqattrs_prepare() should force prio > > and affinity for WQ_RT instead of restricting only the sysfs side? > > I am not entirely sure about this one. It could be done in > apply_wqattrs_prepare(), but it would end up redundant on the sysfs > path. Because currently there are two paths: ... > Am I missing something? The missing one is apply_workqueue_attrs() called directly with attrs from alloc_workqueue_attrs(), which is what padata_setup_cpumasks() does for its own wq. Forcing prio and affinity in apply_wqattrs_prepare() covers all the paths in one place. Being redundant on the sysfs path is fine. > > alloc_ordered_workqueue() with WQ_RT passes this and ends up with a single > > pool spanning all CPUs because ordered wqs use dfl_pwq everywhere, while the > > doc and sysfs say strict per-CPU. Should __WQ_ORDERED be rejected too? The > > nested ifs can also be a single condition. > > Not sure - Tejun what is your guidance here? Is this purely an > implementation challenge because conceptually it should work, no? Yeah, the priority part would work. It's just that an ordered wq has a single pwq for all CPUs, so the per-CPU affinity can't apply. Let's reject __WQ_ORDERED for now and allow it when there's a user. > P.S. The nested ifs could be a single condition but I wanted to mimic > the existing style so it looks nicer: ... > I can change if desired. Your call. A single condition would read fine too. > > The three stores check WQ_RT and return -EINVAL while nice goes read-only > > through is_visible below. Can all four go through > > wq_sysfs_unbound_group_visible() returning 0444 for WQ_RT? That drops the > > three store checks. ... > Could do that if it would be acceptable for root to be still able to > write to them - Tejun? Root can't write to them. sysfs sets KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK, so kernfs_fop_open() fails a write open of a file without a write bit even with CAP_DAC_OVERRIDE. Can you make nice, affinity_scope and affinity_strict read-only through is_visible? cpumask can stay writable as narrowing where the RT workers run doesn't break anything. > > Can we keep printing nice for non-RT pools? prio=120 is the internal > > encoding, sysfs and the pool dumps print nice, and the example output in > > workqueue.rst would go stale. prog['RT_PRIORITY'] would match the rest of > > the file too. > > We probably could if we wanted to. Tejun? > > Complication would be having to hardcode some priority conversion > helpers into the script. Yeah, let's keep nice. It's just prio - DEFAULT_PRIO and hardcoding 120 in the script is fine. > > One more thing which isn't in the diff. The rescuer of a WQ_RT | > > WQ_MEM_RECLAIM wq, which is what panthor-drm-rt is, still runs at nice -20, > > so under memory pressure the RT wq's work items run as CFS. Should > > rescuer_thread() use sched_set_fifo_low() for WQ_RT? > > I did not think we should care much about latency when under severe > memory pressure. It should be enough to guarantee forward progress and > the latency restores back to normal once memory pressure stabilises. Agreed. Thanks. -- tejun