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 3BD72C433F5 for ; Thu, 17 Feb 2022 10:11:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239026AbiBQKL3 (ORCPT ); Thu, 17 Feb 2022 05:11:29 -0500 Received: from mxb-00190b01.gslb.pphosted.com ([23.128.96.19]:48660 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239013AbiBQKL2 (ORCPT ); Thu, 17 Feb 2022 05:11:28 -0500 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6507F2AAB3D for ; Thu, 17 Feb 2022 02:11:14 -0800 (PST) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id 23C581F383; Thu, 17 Feb 2022 10:11:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1645092673; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=cyUregw8hAXko0ZRmaRWn7Astnni7nXWdxjsvYQJs/4=; b=e8i0Bm1Ax5TGlX4yqI0sNNIIXDYo1UGJuNGmPaHp9NVtUgjkBypLOsEtKUiGy5yP+ycgmS yZRoaPG3syvfeyYSgDDI0/GNPj+ofGHW/MEKdjXYlTARN7xfE+m5xqgj4wFbR4agmxeLJa gvT1HwNvuV09xG5RpqHUC0sET0wLGcU= Received: from suse.cz (unknown [10.100.224.162]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id D75D9A3B87; Thu, 17 Feb 2022 10:11:12 +0000 (UTC) Date: Thu, 17 Feb 2022 11:11:12 +0100 From: Petr Mladek To: John Ogness Cc: Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: Re: [PATCH printk v1 08/13] printk: add pr_flush() Message-ID: References: <20220207194323.273637-1-john.ogness@linutronix.de> <20220207194323.273637-9-john.ogness@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220207194323.273637-9-john.ogness@linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 2022-02-07 20:49:18, John Ogness wrote: > Provide a might-sleep function to allow waiting for threaded console > printers to catch up to the latest logged message. > > Use pr_flush() whenever it is desirable to get buffered messages > printed before continuing: suspend_console(), resume_console(), > console_stop(), console_start(), console_unblank(). > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 02bde45c1149..1e80fd052bd5 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -2449,6 +2449,7 @@ void suspend_console(void) > if (!console_suspend_enabled) > return; > pr_info("Suspending console(s) (use no_console_suspend to debug)\n"); > + pr_flush(1000, true); > console_lock(); > console_suspended = 1; > up_console_sem(); > @@ -2461,6 +2462,7 @@ void resume_console(void) > down_console_sem(); > console_suspended = 0; > console_unlock(); > + pr_flush(1000, true); > } > > /** > @@ -2802,8 +2804,10 @@ void console_unblank(void) > if (oops_in_progress) { > if (down_trylock_console_sem() != 0) > return; > - } else > + } else { > + pr_flush(1000, true); It would make more sense to flush the consoles after they are unblanked. I mean to move this to the end of the function. Also it is not obvious why this is not called when oops_in_progress is set. I guess that it is because trylock is needed in this case. It should be handled inside pr_flush(). I mean that pr_flush() should internally use trylock when @oops_in_progress is set. It will make it safe even in this mode. > console_lock(); > + } > > console_locked = 1; > console_may_schedule = 0; > @@ -2869,6 +2873,7 @@ struct tty_driver *console_device(int *index) > */ > void console_stop(struct console *console) > { > + pr_flush(1000, true); It would be enough to flush just the given @console. It might be possible to take over the job from the related kthread and flush it in this context. Well, I am not sure if it is a good idea. > console_lock(); > console->flags &= ~CON_ENABLED; > console_unlock(); > @@ -2880,6 +2885,7 @@ void console_start(struct console *console) > console_lock(); > console->flags |= CON_ENABLED; > console_unlock(); > + pr_flush(1000, true); Same here. > } > EXPORT_SYMBOL(console_start); > > @@ -3249,6 +3255,71 @@ static int __init printk_late_init(void) > late_initcall(printk_late_init); > > #if defined CONFIG_PRINTK > +/** > + * pr_flush() - Wait for printing threads to catch up. > + * Alternative solution would be to take over the job from the kthreads and flush the consoles in this context. Well, I am not sure if it is a good idea or not. > + * @timeout_ms: The maximum time (in ms) to wait. > + * @reset_on_progress: Reset the timeout if forward progress is seen. > + * > + * A value of 0 for @timeout_ms means no waiting will occur. A value of -1 > + * represents infinite waiting. > + * > + * If @reset_on_progress is true, the timeout will be reset whenever any > + * printer has been seen to make some forward progress. > + * > + * Context: Process context. May sleep while acquiring console lock. > + * Return: true if all enabled printers are caught up. > + */ > +bool pr_flush(int timeout_ms, bool reset_on_progress) > +{ > + int remaining = timeout_ms; > + struct console *con; > + u64 last_diff = 0; > + u64 printk_seq; > + u64 diff; > + u64 seq; > + > + might_sleep(); > + > + seq = prb_next_seq(prb); > + > + for (;;) { > + diff = 0; > + > + console_lock(); > + for_each_console(con) { > + if (!console_is_usable(con)) > + continue; > + printk_seq = con->seq; > + if (printk_seq < seq) > + diff += seq - printk_seq; > + } > + console_unlock(); > + > + if (diff != last_diff && reset_on_progress) > + remaining = timeout_ms; > + > + if (diff == 0 || remaining == 0) > + break; > + > + if (remaining < 0) { > + /* no timeout limit */ > + msleep(100); > + } else if (remaining < 100) { > + msleep(remaining); > + remaining = 0; > + } else { > + msleep(100); > + remaining -= 100; > + } > + > + last_diff = diff; > + } > + > + return (diff == 0); > +} > +EXPORT_SYMBOL(pr_flush); Summary: The pr_flush() API and the optional timeout look reasonable to me. Please, handle oops_in_progress in pr_flush() and make it safe in this mode. It will allow to move it at the end of console_unblank() where it makes more sense. I do not resist on flushing only the given consoles in console_stop() and console_start(). It is nice to have and can be done later. Also I do not resist on doing the flush in the context of the caller. I am not even sure if it is a good idea. We could play with it later when there are some problems with the current approach in practice. Best Regards, Petr