From patchwork Tue Aug 20 23:08:59 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Peter Wu X-Patchwork-Id: 1150464 X-Patchwork-Delegate: bpf@iogearbox.net Return-Path: X-Original-To: incoming-bpf@patchwork.ozlabs.org Delivered-To: patchwork-incoming-bpf@bilbo.ozlabs.org Authentication-Results: ozlabs.org; spf=none (mailfrom) smtp.mailfrom=vger.kernel.org (client-ip=209.132.180.67; helo=vger.kernel.org; envelope-from=bpf-owner@vger.kernel.org; receiver=) Authentication-Results: ozlabs.org; dmarc=none (p=none dis=none) header.from=lekensteyn.nl Authentication-Results: ozlabs.org; dkim=pass (2048-bit key; secure) header.d=lekensteyn.nl header.i=@lekensteyn.nl header.b="Ncim4CHF"; dkim-atps=neutral Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by ozlabs.org (Postfix) with ESMTP id 46Cmh86DCBz9sNf for ; Wed, 21 Aug 2019 09:09:12 +1000 (AEST) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726362AbfHTXJJ (ORCPT ); Tue, 20 Aug 2019 19:09:09 -0400 Received: from lekensteyn.nl ([178.21.112.251]:41075 "EHLO lekensteyn.nl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726354AbfHTXJI (ORCPT ); Tue, 20 Aug 2019 19:09:08 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lekensteyn.nl; s=s2048-2015-q1; h=Content-Transfer-Encoding:MIME-Version:References:In-Reply-To:Message-Id:Date:Subject:Cc:To:From; bh=Q+UDWMYEYIrMXXf99yl5++TT8RhBbJ8ffMGbmG6M8fY=; b=Ncim4CHFR1Q5+pd5BNiNHzCvws2Ijii1/CviLZYp5ZSWR4Knp+bFWtUXA4inj+VXxP/LbmslA8R0lI+PQszg7deLDswysKr8qIESiNHsUEg3fpY3T2WheqhouGW6ViDxZKJqPo0FYcuaHnT7A35lEKVss95cKOjpB23BLJ/RIPUimBmtZoZawJtSzRQmdzanEaEHM2WAxLzQof1XsglWFMlQZvZqrMkrWmTvWdHvAtDtrXyPU9HNWjnlIaN0e2oTtV8p4EcWyUUyvn8bMyRKk80RLyX8POgnhIX2G3hIJlaEKIdtmivX62gLQIip1fFKrkMGABkjKA+zKqSVO+AtfQ==; Received: by lekensteyn.nl with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.84_2) (envelope-from ) id 1i0DFR-00055S-D6; Wed, 21 Aug 2019 01:09:05 +0200 From: Peter Wu To: Alexei Starovoitov , Daniel Borkmann Cc: netdev@vger.kernel.org, bpf@vger.kernel.org Subject: [PATCH v2 3/4] bpf: clarify when bpf_trace_printk discards lines Date: Wed, 21 Aug 2019 00:08:59 +0100 Message-Id: <20190820230900.23445-4-peter@lekensteyn.nl> X-Mailer: git-send-email 2.22.0 In-Reply-To: <20190820230900.23445-1-peter@lekensteyn.nl> References: <20190820230900.23445-1-peter@lekensteyn.nl> MIME-Version: 1.0 X-Spam-Score: -0.0 (/) X-Spam-Status: No, hits=-0.0 required=5.0 tests=NO_RELAYS=-0.001 autolearn=unavailable autolearn_force=no Sender: bpf-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org I opened /sys/kernel/tracing/trace once and kept reading from it. bpf_trace_printk somehow did not seem to work, no entries were appended to that trace file. It turns out that tracing is disabled when that file is open. Save the next person some time and document this. The trace file is described in Documentation/trace/ftrace.rst, however the implication "tracing is disabled" did not immediate translate to "bpf_trace_printk silently discards entries". Signed-off-by: Peter Wu --- include/uapi/linux/bpf.h | 2 ++ 1 file changed, 2 insertions(+) diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h index 9ca333c3ce91..e4236e357ed9 100644 --- a/include/uapi/linux/bpf.h +++ b/include/uapi/linux/bpf.h @@ -575,6 +575,8 @@ union bpf_attr { * limited to five). * * Each time the helper is called, it appends a line to the trace. + * Lines are discarded while *\/sys/kernel/debug/tracing/trace* is + * open, use *\/sys/kernel/debug/tracing/trace_pipe* to avoid this. * The format of the trace is customizable, and the exact output * one will get depends on the options set in * *\/sys/kernel/debug/tracing/trace_options* (see also the