From patchwork Mon Sep 14 20:42:11 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Thomas Gleixner X-Patchwork-Id: 11775717 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 25E5392C for ; Tue, 15 Sep 2020 07:08:13 +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 EACED20EDD for ; Tue, 15 Sep 2020 07:08:12 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="FKqpL270"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="3ZJjdClz" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EACED20EDD Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linutronix.de 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 6140A6E853; Tue, 15 Sep 2020 07:08:09 +0000 (UTC) X-Original-To: dri-devel@lists.freedesktop.org Delivered-To: dri-devel@lists.freedesktop.org Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by gabe.freedesktop.org (Postfix) with ESMTPS id 4281C6E5A0 for ; Mon, 14 Sep 2020 20:45:09 +0000 (UTC) Message-Id: <20200914204441.115488215@linutronix.de> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1600116306; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: references:references; bh=+wlx0B5bXFGE1ZqmF2GOQz7Vu5f4dOGKH6MLdtZ/des=; b=FKqpL2700K2KI8YRyNg486WotbPXB8OPjTNR7CD6hpf4WOsECuFHsf9Fl7xQ/njOCC4us1 V7/8dgt2Y8Q5bsUgPdX3cx6XhZz7xzGxkW9H08+RkTCs1S3zZRCuJ6BO6NdCa1sGP2ZZpX 3bB1p3j0W8RPnI56xz7EqLgYOttnzUhttUOz6CqCP4LTLNoe43UQ+fBcsmVdaPtzMUWgf2 T1f1tMxghsrPiIICkGbkbj/aNUjRjRxUCjZE6DU4Hd0Nghaw+wzNONpacrfVovmhca4M2C rQnc8p8FaD3ZJfM8rRAgyRFUboeWQsqzuPbUDTsqtxOETbLcX/pUl6ZnCc32nQ== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1600116306; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: references:references; bh=+wlx0B5bXFGE1ZqmF2GOQz7Vu5f4dOGKH6MLdtZ/des=; b=3ZJjdClz5oMHjfFfg9NKeHYyaj0Ejpc30NZ9Tracs4udi2vCXYHDiLEh6EZOsJaUpk6Jwn e+AMA/J0ha4T1+Bg== Date: Mon, 14 Sep 2020 22:42:11 +0200 From: Thomas Gleixner To: LKML Subject: [patch 02/13] preempt: Make preempt count unconditional References: <20200914204209.256266093@linutronix.de> MIME-Version: 1.0 X-Mailman-Approved-At: Tue, 15 Sep 2020 07:07:18 +0000 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: Juri Lelli , Peter Zijlstra , Sebastian Andrzej Siewior , Lai Jiangshan , dri-devel@lists.freedesktop.org, Ben Segall , linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-hexagon@vger.kernel.org, Will Deacon , Ingo Molnar , Anton Ivanov , linux-arch@vger.kernel.org, Vincent Guittot , Brian Cain , Richard Weinberger , Russell King , David Airlie , Ingo Molnar , Geert Uytterhoeven , Mel Gorman , intel-gfx@lists.freedesktop.org, Matt Turner , Valentin Schneider , linux-xtensa@linux-xtensa.org, Shuah Khan , "Paul E. McKenney" , Jeff Dike , linux-um@lists.infradead.org, Josh Triplett , Steven Rostedt , rcu@vger.kernel.org, linux-m68k@lists.linux-m68k.org, Ivan Kokshaysky , Rodrigo Vivi , Dietmar Eggemann , linux-arm-kernel@lists.infradead.org, Richard Henderson , Chris Zankel , Max Filippov , Linus Torvalds , linux-alpha@vger.kernel.org, Mathieu Desnoyers , Andrew Morton , Daniel Bristot de Oliveira Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" The handling of preempt_count() is inconsistent accross kernel configurations. On kernels which have PREEMPT_COUNT=n preempt_disable/enable() and the lock/unlock functions are not affecting the preempt count, only local_bh_disable/enable() and _bh variants of locking, soft interrupt delivery, hard interrupt and NMI context affect it. It's therefore impossible to have a consistent set of checks which provide information about the context in which a function is called. In many cases it makes sense to have seperate functions for seperate contexts, but there are valid reasons to avoid that and handle different calling contexts conditionally. The lack of such indicators which work on all kernel configuratios is a constant source of trouble because developers either do not understand the implications or try to work around this inconsistency in weird ways. Neither seem these issues be catched by reviewers and testing. Recently merged code does: gfp = preemptible() ? GFP_KERNEL : GFP_ATOMIC; Looks obviously correct, except for the fact that preemptible() is unconditionally false for CONFIF_PREEMPT_COUNT=n, i.e. all allocations in that code use GFP_ATOMIC on such kernels. Attempts to make preempt count unconditional and consistent have been rejected in the past with handwaving performance arguments. Freshly conducted benchmarks did not reveal any measurable impact from enabling preempt count unconditionally. On kernels with CONFIG_PREEMPT_NONE or CONFIG_PREEMPT_VOLUNTARY the preempt count is only incremented and decremented but the result of the decrement is not tested. Contrary to that enabling CONFIG_PREEMPT which tests the result has a small but measurable impact due to the conditional branch/call. It's about time to make essential functionality of the kernel consistent accross the various preemption models. Enable CONFIG_PREEMPT_COUNT unconditionally. Follow up changes will remove the #ifdeffery and remove the config option at the end. Signed-off-by: Thomas Gleixner --- kernel/Kconfig.preempt | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) --- a/kernel/Kconfig.preempt +++ b/kernel/Kconfig.preempt @@ -75,8 +75,7 @@ config PREEMPT_RT endchoice config PREEMPT_COUNT - bool + def_bool y config PREEMPTION bool - select PREEMPT_COUNT