From patchwork Wed Jun 24 03:38:59 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Steven Rostedt X-Patchwork-Id: 223032 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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH, MAILING_LIST_MULTI, SIGNED_OFF_BY, SPF_HELO_NONE, SPF_PASS autolearn=unavailable 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 D9A2BC433E1 for ; Wed, 24 Jun 2020 03:39:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BF1AC20C09 for ; Wed, 24 Jun 2020 03:39:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388288AbgFXDjp (ORCPT ); Tue, 23 Jun 2020 23:39:45 -0400 Received: from mail.kernel.org ([198.145.29.99]:44334 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388572AbgFXDjf (ORCPT ); Tue, 23 Jun 2020 23:39:35 -0400 Received: from gandalf.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id DA2CF20899; Wed, 24 Jun 2020 03:39:34 +0000 (UTC) Received: from rostedt by gandalf.local.home with local (Exim 4.93) (envelope-from ) id 1jnwG5-003u8Z-Ky; Tue, 23 Jun 2020 23:39:33 -0400 Message-ID: <20200624033933.508480518@goodmis.org> User-Agent: quilt/0.66 Date: Tue, 23 Jun 2020 23:38:59 -0400 From: Steven Rostedt To: linux-kernel@vger.kernel.org Cc: Ingo Molnar , Andrew Morton , Tom Zanussi , stable@vger.kernel.org, Julia Lawall Subject: [for-linus][PATCH 1/4] ring-buffer: Zero out time extend if it is nested and not absolute References: <20200624033858.651927570@goodmis.org> MIME-Version: 1.0 Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: "Steven Rostedt (VMware)" Currently the ring buffer makes events that happen in interrupts that preempt another event have a delta of zero. (Hopefully we can change this soon). But this is to deal with the races of updating a global counter with lockless and nesting functions updating deltas. With the addition of absolute time stamps, the time extend didn't follow this rule. A time extend can happen if two events happen longer than 2^27 nanoseconds appart, as the delta time field in each event is only 27 bits. If that happens, then a time extend is injected with 2^59 bits of nanoseconds to use (18 years). But if the 2^27 nanoseconds happen between two events, and as it is writing the event, an interrupt triggers, it will see the 2^27 difference as well and inject a time extend of its own. But a recent change made the time extend logic not take into account the nesting, and this can cause two time extend deltas to happen moving the time stamp much further ahead than the current time. This gets all reset when the ring buffer moves to the next page, but that can cause time to appear to go backwards. This was observed in a trace-cmd recording, and since the data is saved in a file, with trace-cmd report --debug, it was possible to see that this indeed did happen! bash-52501 110d... 81778.908247: sched_switch: bash:52501 [120] S ==> swapper/110:0 [120] [12770284:0x2e8:64] -0 110d... 81778.908757: sched_switch: swapper/110:0 [120] R ==> bash:52501 [120] [509947:0x32c:64] TIME EXTEND: delta:306454770 length:0 bash-52501 110.... 81779.215212: sched_swap_numa: src_pid=52501 src_tgid=52388 src_ngid=52501 src_cpu=110 src_nid=2 dst_pid=52509 dst_tgid=52388 dst_ngid=52501 dst_cpu=49 dst_nid=1 [0:0x378:48] TIME EXTEND: delta:306458165 length:0 bash-52501 110dNh. 81779.521670: sched_wakeup: migration/110:565 [0] success=1 CPU:110 [0:0x3b4:40] and at the next page, caused the time to go backwards: bash-52504 110d... 81779.685411: sched_switch: bash:52504 [120] S ==> swapper/110:0 [120] [8347057:0xfb4:64] CPU:110 [SUBBUFFER START] [81779379165886:0x1320000] -0 110dN.. 81779.379166: sched_wakeup: bash:52504 [120] success=1 CPU:110 [0:0x10:40] -0 110d... 81779.379167: sched_switch: swapper/110:0 [120] R ==> bash:52504 [120] [1168:0x3c:64] Link: https://lkml.kernel.org/r/20200622151815.345d1bf5@oasis.local.home Cc: Ingo Molnar Cc: Andrew Morton Cc: Tom Zanussi Cc: stable@vger.kernel.org Fixes: dc4e2801d400b ("ring-buffer: Redefine the unimplemented RINGBUF_TYPE_TIME_STAMP") Reported-by: Julia Lawall Signed-off-by: Steven Rostedt (VMware) --- kernel/trace/ring_buffer.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index b8e1ca48be50..00867ff82412 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -2427,7 +2427,7 @@ rb_update_event(struct ring_buffer_per_cpu *cpu_buffer, if (unlikely(info->add_timestamp)) { bool abs = ring_buffer_time_stamp_abs(cpu_buffer->buffer); - event = rb_add_time_stamp(event, info->delta, abs); + event = rb_add_time_stamp(event, abs ? info->delta : delta, abs); length -= RB_LEN_TIME_EXTEND; delta = 0; }