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 663B8C433F5 for ; Sat, 2 Apr 2022 14:09:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1354857AbiDBOLc (ORCPT ); Sat, 2 Apr 2022 10:11:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59766 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1346612AbiDBOL2 (ORCPT ); Sat, 2 Apr 2022 10:11:28 -0400 Received: from sipsolutions.net (s3.sipsolutions.net [IPv6:2a01:4f8:191:4433::2]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 84ADB13D45 for ; Sat, 2 Apr 2022 07:09:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=sipsolutions.net; s=mail; h=Content-Transfer-Encoding:MIME-Version: Content-Type:References:In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender :Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From:Resent-To: Resent-Cc:Resent-Message-ID; bh=zWvGZKI9SeXG33QH0VHAxJwyqLjTHGSTR8wnBkMrRiA=; t=1648908576; x=1650118176; b=lvTXhxeYBpyqUGZ33w4DsAOnnl+bzRrCjQoOolk9Aw5caSb Yuk7xhqEV9ZWh0Uuh168F5xENm/2wk+yQ7NCsQcEkcumieqFdI6Xz5FZFNrNjfgWDnRlGIeIEiivx tDn5A/ddOQ6edT6x/CmxuiOUBZPvDNYpUQ8hnfM4rU9PFIkouZxfKk5zWAra9LgrOfGm2PCjCHA1V AM/LL1+rDd3jE92GRJ7sBfkmSzKqH4J4VIXKtSZ2Nd6zJijXWXYOpY9C+iLjTampT7WKPG9YuNceH /XCcAVDv8oPXCiuQIRe+mt+AnG+d3G71w6v9YbsXRxQIn/7biw4Xw1H2XNs5FUGQ==; Received: by sipsolutions.net with esmtpsa (TLS1.3:ECDHE_SECP256R1__RSA_PSS_RSAE_SHA256__AES_256_GCM:256) (Exim 4.95) (envelope-from ) id 1naeRW-003sRf-PW; Sat, 02 Apr 2022 16:09:30 +0200 Message-ID: <84f9d627092660c38400b607198c3b83f795be7f.camel@sipsolutions.net> Subject: Re: UML time-travel warning from __run_timers From: Johannes Berg To: Vincent Whitchurch Cc: linux-um@lists.infradead.org, linux-kernel@vger.kernel.org, Anna-Maria Gleixner , Thomas Gleixner , Frederic Weisbecker Date: Sat, 02 Apr 2022 16:09:29 +0200 In-Reply-To: <20220330110156.GA9250@axis.com> References: <20220330110156.GA9250@axis.com> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.42.4 (3.42.4-1.fc35) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-malware-bazaar: not-scanned Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Vincent, > [10737482.720000][ C0] ------------[ cut here ]------------ > [10737482.720000][ C0] WARNING: CPU: 0 PID: 0 at kernel/time/timer.c:1729 __run_timers+0x36d/0x380 > [for those new on the thread, full message and config here: https://lore.kernel.org/r/20220330110156.GA9250@axis.com] I think maybe you found a bug in the timers code? Your config has CONFIG_NO_HZ_COMMON, so we have both BASE_STD and BASE_DEF. Evidently, in your config, we *never* have any timer with TIMER_DEFERRABLE, which would put it into BASE_DEF. (I put a WARN_ON into get_timer_cpu_base() and get_timer_this_cpu_base() in the if, and it never triggered; I guess my config has something that creates a deferrable timer, so it didn't trigger, but I didn't check that now.) Therefore, base->next_expiry never changes or something? At init, we get init_timer_cpu(0) base 0 clk=0xffff8ad0, next_expiry=0x13fff8acf init_timer_cpu(0) base 1 clk=0xffff8ad0, next_expiry=0x13fff8acf which makes sense, jiffies is set up to wrap very quickly after boot. The warning triggers when we have jiffies=0x13fff9600, so it's just after the "next_expiry", so in this code: static inline void __run_timers(struct timer_base *base) { struct hlist_head heads[LVL_DEPTH]; int levels; if (time_before(jiffies, base->next_expiry)) return; we no longer return. Previously, we've *never* executed past that if for BASE_DEF. But we never touched this timer base nor did we ever want to recalc it I guess, so WARN_ON_ONCE(!levels && !base->next_expiry_recalc); triggers. I thought about changing that condition to if (time_before(...) || !base->timers_pending) return; and that *does* make the splat go away, but I fear that might make it not recalculate when needed, so perhaps in the condition we should have if (time_before(...) || (!base->timers_pending && !base->next_expiry_recalc)) return; or something? (which also avoids hitting the warning) But I really don't know anything about this code, so adding a few CCs. Can you help? Thanks, johannes