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=-17.4 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_IN_DEF_DKIM_WL autolearn=ham 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 1270BC4363D for ; Thu, 24 Sep 2020 18:20:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AAD112220C for ; Thu, 24 Sep 2020 18:20:11 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="GdfyMCbq" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728736AbgIXSUK (ORCPT ); Thu, 24 Sep 2020 14:20:10 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37116 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727753AbgIXSUK (ORCPT ); Thu, 24 Sep 2020 14:20:10 -0400 Received: from mail-pj1-x1044.google.com (mail-pj1-x1044.google.com [IPv6:2607:f8b0:4864:20::1044]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 597ACC0613CE for ; Thu, 24 Sep 2020 11:20:10 -0700 (PDT) Received: by mail-pj1-x1044.google.com with SMTP id mm21so157300pjb.4 for ; Thu, 24 Sep 2020 11:20:10 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=eQMwhMR95n+3FqBYI5WeAox8ddPXZxaoyC5f5KTzv1Q=; b=GdfyMCbqMOKTkZTGUricXEzpXRPXMc0br4yflxhC7hdlpcAcJ41FcQce+qdccc3z87 MMHOf+8H5Qr7RybcJieBQ5EQNHlLU5mUBdxeE64mi7MqDPzFTD8FlUGOrNkDsD3zyonP cZvcfl8xL5uDWI7gIgAXwh2E3zr9TiPG7nH676ZY8oREoV9x/hm2xZY621hV4+O5Obyn FzkZiPfL3s8lCz0lJhzkW2b7vZnlvGlJmmvTq3UFIh29qUQ4q7K0h7b0kWCmUVcE3j22 8ctZwQVXWEnL7SHF/Pk+KGEEY4RL75w0Lg0VL8ZNiJbHODxZsUa+28YyqqxNSOWrwW7p 5Wnw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=eQMwhMR95n+3FqBYI5WeAox8ddPXZxaoyC5f5KTzv1Q=; b=ddEI31P+rSYkGEPHPVwGGFPreLvMq3zjunmEJfab2dyzvZpntH224ld3YRx9gO5O9+ N9YB+v8ImbSlB6Wk4RE7adYiDUvjLS7P7PqOkXMkHLCBanyuOJ4fwYOeVbKrJ9A/XMyr nlmB0Hn66iO3jwsVl00jXeT9jFxAgbzwMBIiHLlMUNNMUGV/qBeG+qOqLrEr1D0IneJZ gB5iwyFjl5goP27iTWii5f06NcT+oVmHtzdOZgKkb3ZtcKjwj4iY2/pV6AEIpq2cL82p To9BxOk91hfxL+eP/6uDujsLaR0ybKxTcKcAPJKjARi1OCiNVvN5LJrQ9sOzmCNdt83l WW1Q== X-Gm-Message-State: AOAM530g/qMmhN+JUwrB4ggCawf3SaN4Pg/8Exo65+Aq++7x+qvv+p9c X8xBBI2uYtPRBdfYYrszgTMwg0fYGUGw+0J7v7Ib3Q== X-Google-Smtp-Source: ABdhPJxGK7ykG4+qfUScNswCNWsCaTVZXUZJkcNZiTFJ0sltfxqVFMju3BXl5ivw2bkup5SuTddERmoUfauwxLhCPrg= X-Received: by 2002:a17:90a:c17:: with SMTP id 23mr182252pjs.127.1600971609587; Thu, 24 Sep 2020 11:20:09 -0700 (PDT) MIME-Version: 1.0 References: <20200924170928.466191266@goodmis.org> <20200924171846.993048030@goodmis.org> <2006335081.68212.1600969345189.JavaMail.zimbra@efficios.com> In-Reply-To: <2006335081.68212.1600969345189.JavaMail.zimbra@efficios.com> From: Axel Rasmussen Date: Thu, 24 Sep 2020 11:19:33 -0700 Message-ID: Subject: Re: [PATCH 1/2] tracepoints: Add helper to test if tracepoint is enabled in a header To: Mathieu Desnoyers Cc: rostedt , linux-kernel , Yafang Shao , Andrew Morton , Vlastimil Babka , Michel Lespinasse , Daniel Jordan , Davidlohr Bueso , linux-mm , Ingo Molnar , Joonsoo Kim Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 24, 2020 at 10:42 AM Mathieu Desnoyers wrote: > > ----- On Sep 24, 2020, at 1:09 PM, rostedt rostedt@goodmis.org wrote: > > > From: "Steven Rostedt (VMware)" > > > > As tracepoints are discouraged from being added in a header because it can > > cause side effects if other tracepoints are in headers, the common > > workaround is to add a function call that calls a wrapper function in a > > C file that then calls the tracepoint. But as function calls add overhead, > > this function should only be called when the tracepoint in question is > > enabled. To get around the overhead, a static_branch can be used that only > > gets set when the tracepoint is enabled, and then inside the block of the > > static branch can contain the call to the tracepoint wrapper. > > > > Add a tracepoint_enabled(tp) macro that gets passed the name of the > > tracepoint, and this becomes a static_branch that is enabled when the > > tracepoint is enabled and is a nop when the tracepoint is disabled. > > > > Signed-off-by: Steven Rostedt (VMware) > > --- > > Documentation/trace/tracepoints.rst | 25 ++++++++++++++++++++++ > > include/linux/tracepoint-defs.h | 33 +++++++++++++++++++++++++++++ > > 2 files changed, 58 insertions(+) > > > > diff --git a/Documentation/trace/tracepoints.rst > > b/Documentation/trace/tracepoints.rst > > index 6e3ce3bf3593..833d39ee1c44 100644 > > --- a/Documentation/trace/tracepoints.rst > > +++ b/Documentation/trace/tracepoints.rst > > @@ -146,3 +146,28 @@ with jump labels and avoid conditional branches. > > define tracepoints. Check http://lwn.net/Articles/379903, > > http://lwn.net/Articles/381064 and http://lwn.net/Articles/383362 > > for a series of articles with more details. > > + > > +If you require calling a tracepoint from a header file, it is not > > +recommended to call one directly or to use the trace__enabled() > > +function call, as tracepoints in header files can have side effects if a > > +header is included from a file that has CREATE_TRACE_POINTS set. Instead, > > +include tracepoint-defs.h and use trace_enabled(). > > Tracepoints per-se have no issues being used from header files. The TRACE_EVENT > infrastructure seems to be the cause of this problem. We should fix trace events > rather than require all users to use weird work-arounds thorough the kernel code > base. > > I am not against the idea of a tracepoint_enabled(tp), but I am against the > motivation behind this patch and the new tracepoint user requirements it documents. Perhaps anecdotally, I've found that the situation Steven described occurs not just because of the TRACE_EVENT infrastructure. We also run into this problem when adding tracepoints under any "very core" APIs, i.e. anything that is transiently included from linux/tracepoint.h. For example, I ran into this issue while adding tracepoints under the linux/mmap_lock.h API, because that header is somehow transiently included by linux/tracepoint.h (sorry, I don't have the exact transient include path on hand; I can dig it up if it would be useful). > > > + > > +In a C file:: > > + > > + void do_trace_foo_bar_wrapper(args) > > + { > > + trace_foo_bar(args); > > + } > > + > > +In the header file:: > > + > > + DECLEARE_TRACEPOINT(foo_bar); > > + > > + static inline void some_inline_function() > > + { > > + [..] > > + if (trace_enabled(foo_bar)) > > Is it trace_enabled() or tracepoint_enabled() ? There is a mismatch > between the commit message/code and the documentation. > > Thanks, > > Mathieu > > > + do_trace_foo_bar_wrapper(args); > > + [..] > > + } > > diff --git a/include/linux/tracepoint-defs.h b/include/linux/tracepoint-defs.h > > index b29950a19205..ca2f1f77f6f8 100644 > > --- a/include/linux/tracepoint-defs.h > > +++ b/include/linux/tracepoint-defs.h > > @@ -48,4 +48,37 @@ struct bpf_raw_event_map { > > u32 writable_size; > > } __aligned(32); > > > > +/* > > + * If a tracepoint needs to be called from a header file, it is not > > + * recommended to call it directly, as tracepoints in header files > > + * may cause side-effects. Instead, use trace_enabled() to test > > + * if the tracepoint is enabled, then if it is, call a wrapper > > + * function defined in a C file that will then call the tracepoint. > > + * > > + * For "trace_foo()", you would need to create a wrapper function > > + * in a C file to call trace_foo(): > > + * void trace_bar(args) { trace_foo(args); } > > + * Then in the header file, declare the tracepoint: > > + * DECLARE_TRACEPOINT(foo); > > + * And call your wrapper: > > + * static inline void some_inlined_function() { > > + * [..] > > + * if (tracepoint_enabled(foo)) > > + * trace_bar(args); > > + * [..] > > + * } > > + * > > + * Note: tracepoint_enabled(foo) is equivalent to trace_foo_enabled() > > + * but is safe to have in headers, where trace_foo_enabled() is not. > > + */ > > +#define DECLARE_TRACEPOINT(tp) \ > > + extern struct tracepoint __tracepoint_##tp > > + > > +#ifdef CONFIG_TRACEPOINTS > > +# define tracepoint_enabled(tp) \ > > + static_key_false(&(__tracepoint_##tp).key) > > +#else > > +# define tracepoint_enabled(tracepoint) false > > +#endif > > + > > #endif > > -- > > 2.28.0 > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com