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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 08560C433F5 for ; Fri, 7 Jan 2022 18:53:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230450AbiAGSxh (ORCPT ); Fri, 7 Jan 2022 13:53:37 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48030 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229763AbiAGSxg (ORCPT ); Fri, 7 Jan 2022 13:53:36 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1E581C061574 for ; Fri, 7 Jan 2022 10:53:36 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 9B4B461AFE for ; Fri, 7 Jan 2022 18:53:35 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 10FAFC36AED; Fri, 7 Jan 2022 18:53:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1641581615; bh=YQYKQfrCVjazS8G99udYRgnaScR6PMEJUv2tsSUVEWk=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=R2B37sPUzR6udDBPamAxRTXt7R3JiTHf65Wj7EkLf3XTnmwq9fGv9vERWdyYFLVJQ WhLiAZnTkeQBsKBpJltAjOuFrPWOArkmp8gfvf3v4SHk2OoSh031wHnYRb3lCEJlrn KhKoo8A+K3gtHnF3qcaHoUljKXRYgbuqxdKrvowCtZOmE9ktyKQZQcfUFadvlvTkS7 MpTuish3CC4TIE1JhwnjgZ1BwGUtZZHsUSoc81u2LukEU/Dtz/NY2xG1lOl/Jtb3n8 EeXl+etXZvO1FYP4d19Vf2w6RycDx+4UI/tlm2Ona/VRAZA6LHWfGj3XGqjLlSquVr UPJlRvgfuK4/g== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id C1F3B5C08AA; Fri, 7 Jan 2022 10:53:34 -0800 (PST) Date: Fri, 7 Jan 2022 10:53:34 -0800 From: "Paul E. McKenney" To: Wander Costa Cc: Sergey Senozhatsky , Wander Lairson Costa , Petr Mladek , Steven Rostedt , John Ogness , open list Subject: Re: [PATCH v2 1/1] printk: suppress rcu stall warnings caused by slow console devices Message-ID: <20220107185334.GA3360783@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20211111195904.618253-1-wander@redhat.com> <20211111195904.618253-2-wander@redhat.com> <20211112145755.GX641268@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211112145755.GX641268@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 12, 2021 at 06:57:55AM -0800, Paul E. McKenney wrote: > On Fri, Nov 12, 2021 at 11:42:39AM -0300, Wander Costa wrote: > > On Thu, Nov 11, 2021 at 10:42 PM Sergey Senozhatsky > > wrote: > > > > > > On (21/11/11 16:59), Wander Lairson Costa wrote: > > > > > > > > If we have a reasonable large dataset to flush in the printk ring > > > > buffer in the presence of a slow console device (like a serial port > > > > with a low baud rate configured), the RCU stall detector may report > > > > warnings. > > > > > > > > This patch suppresses RCU stall warnings while flushing the ring buffer > > > > to the console. > > > > > > > [..] > > > > +extern int rcu_cpu_stall_suppress; > > > > + > > > > +static void rcu_console_stall_suppress(void) > > > > +{ > > > > + if (!rcu_cpu_stall_suppress) > > > > + rcu_cpu_stall_suppress = 4; > > > > +} > > > > + > > > > +static void rcu_console_stall_unsuppress(void) > > > > +{ > > > > + if (rcu_cpu_stall_suppress == 4) > > > > + rcu_cpu_stall_suppress = 0; > > > > +} > > > > + > > > > /** > > > > * console_unlock - unlock the console system > > > > * > > > > @@ -2634,6 +2648,9 @@ void console_unlock(void) > > > > * and cleared after the "again" goto label. > > > > */ > > > > do_cond_resched = console_may_schedule; > > > > + > > > > + rcu_console_stall_suppress(); > > > > + > > > > again: > > > > console_may_schedule = 0; > > > > > > > > @@ -2645,6 +2662,7 @@ void console_unlock(void) > > > > if (!can_use_console()) { > > > > console_locked = 0; > > > > up_console_sem(); > > > > + rcu_console_stall_unsuppress(); > > > > return; > > > > } > > > > > > > > @@ -2716,8 +2734,10 @@ void console_unlock(void) > > > > > > > > handover = console_lock_spinning_disable_and_check(); > > > > printk_safe_exit_irqrestore(flags); > > > > - if (handover) > > > > + if (handover) { > > > > + rcu_console_stall_unsuppress(); > > > > return; > > > > + } > > > > > > > > if (do_cond_resched) > > > > cond_resched(); > > > > @@ -2738,6 +2758,8 @@ void console_unlock(void) > > > > retry = prb_read_valid(prb, next_seq, NULL); > > > > if (retry && console_trylock()) > > > > goto again; > > > > + > > > > + rcu_console_stall_unsuppress(); > > > > } > > > > > > May be we can just start touching watchdogs from printing routine? > > > > > Hrm, console_unlock is called from vprintk_emit [0] with preemption > > disabled. and it already has the logic implemented to call > > cond_resched when possible [1]. > > > > [0] https://elixir.bootlin.com/linux/latest/source/kernel/printk/printk.c#L2244 > > [1] https://elixir.bootlin.com/linux/latest/source/kernel/printk/printk.c#L2719 > > So when we are having problems is when console_may_schedule == 0? Just following up... Any progress on this? The ability to suppress RCU CPU stall warnings due to console slowness would likely be valuable to quite a few people. Thanx, Paul