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 X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 12583C63777 for ; Wed, 25 Nov 2020 12:34:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B9507206F7 for ; Wed, 25 Nov 2020 12:34:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729430AbgKYMep (ORCPT ); Wed, 25 Nov 2020 07:34:45 -0500 Received: from mx2.suse.de ([195.135.220.15]:40226 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726162AbgKYMeo (ORCPT ); Wed, 25 Nov 2020 07:34:44 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 576F3AC2F; Wed, 25 Nov 2020 12:34:43 +0000 (UTC) Date: Wed, 25 Nov 2020 13:35:34 +0100 From: Cyril Hrubis To: Thomas Gleixner Cc: Li Wang , ltp@lists.linux.it, Chunyu Hu , LKML , Peter Zijlstra , John Stultz , Arnd Bergmann , Vincenzo Frascino , Andy Lutomirski , "Michael Kerrisk (man-pages)" , Carlos O'Donell Subject: Re: [PATCH 1/2] syscalls: avoid time() using __cvdso_gettimeofday in use-level's VDSO Message-ID: <20201125123534.GA28684@yuki.lan> References: <20201123083137.11575-1-liwang@redhat.com> <20201124153837.GA24470@yuki.lan> <875z5tllih.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <875z5tllih.fsf@nanos.tec.linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi! > This is a general problem and not really just for this particular test > case. > > Due to the internal implementation of ktime_get_real_seconds(), which is > a 2038 safe replacement for the former get_seconds() function, this > accumulation issue can be observed. (time(2) via syscall and newer > versions of VDSO use the same mechanism). > > clock_gettime(CLOCK_REALTIME, &ts); > sec = time(); > assert(sec >= ts.tv_sec); > > That assert can trigger for two reasons: > > 1) Clock was set between the clock_gettime() and time(). > > 2) The clock has advanced far enough that: > > timekeeper.tv_nsec + (clock_now_ns() - last_update_ns) > NSEC_PER_SEC > > #1 is just a property of clock REALTIME. There is nothing we can do > about that. > > #2 is due to the optimized get_seconds()/time() access which avoids to > read the clock. This can happen on bare metal as well, but is far > more likely to be exposed on virt. > > The same problem exists for CLOCK_XXX vs. CLOCK_XXX_COARSE > > clock_gettime(CLOCK_XXX, &ts); > clock_gettime(CLOCK_XXX_COARSE, &tc); > assert(tc.tv_sec >= ts.tv_sec); > > The _COARSE variants return their associated timekeeper.tv_sec,tv_nsec > pair without reading the clock. Same as #2 above just extended to clock > MONOTONIC. Good hint, I guess that easiest fix would be to switch to coarse timers for these tests. > There is no way to fix this except giving up on the fast accessors and > make everything take the slow path and read the clock, which might make > a lot of people unhappy. That's understandable and reasonable. Thanks a lot for the confirmation. > For clock REALTIME #1 is anyway an issue, so I think documenting this > proper is the right thing to do. > > Thoughts? I guess that ideally BUGS section for time(2) and clock_gettime(2) should be updated with this explanation. -- Cyril Hrubis chrubis@suse.cz From mboxrd@z Thu Jan 1 00:00:00 1970 From: Cyril Hrubis Date: Wed, 25 Nov 2020 13:35:34 +0100 Subject: [LTP] [PATCH 1/2] syscalls: avoid time() using __cvdso_gettimeofday in use-level's VDSO In-Reply-To: <875z5tllih.fsf@nanos.tec.linutronix.de> References: <20201123083137.11575-1-liwang@redhat.com> <20201124153837.GA24470@yuki.lan> <875z5tllih.fsf@nanos.tec.linutronix.de> Message-ID: <20201125123534.GA28684@yuki.lan> List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: ltp@lists.linux.it Hi! > This is a general problem and not really just for this particular test > case. > > Due to the internal implementation of ktime_get_real_seconds(), which is > a 2038 safe replacement for the former get_seconds() function, this > accumulation issue can be observed. (time(2) via syscall and newer > versions of VDSO use the same mechanism). > > clock_gettime(CLOCK_REALTIME, &ts); > sec = time(); > assert(sec >= ts.tv_sec); > > That assert can trigger for two reasons: > > 1) Clock was set between the clock_gettime() and time(). > > 2) The clock has advanced far enough that: > > timekeeper.tv_nsec + (clock_now_ns() - last_update_ns) > NSEC_PER_SEC > > #1 is just a property of clock REALTIME. There is nothing we can do > about that. > > #2 is due to the optimized get_seconds()/time() access which avoids to > read the clock. This can happen on bare metal as well, but is far > more likely to be exposed on virt. > > The same problem exists for CLOCK_XXX vs. CLOCK_XXX_COARSE > > clock_gettime(CLOCK_XXX, &ts); > clock_gettime(CLOCK_XXX_COARSE, &tc); > assert(tc.tv_sec >= ts.tv_sec); > > The _COARSE variants return their associated timekeeper.tv_sec,tv_nsec > pair without reading the clock. Same as #2 above just extended to clock > MONOTONIC. Good hint, I guess that easiest fix would be to switch to coarse timers for these tests. > There is no way to fix this except giving up on the fast accessors and > make everything take the slow path and read the clock, which might make > a lot of people unhappy. That's understandable and reasonable. Thanks a lot for the confirmation. > For clock REALTIME #1 is anyway an issue, so I think documenting this > proper is the right thing to do. > > Thoughts? I guess that ideally BUGS section for time(2) and clock_gettime(2) should be updated with this explanation. -- Cyril Hrubis chrubis@suse.cz