From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Subject: Re: [PATCH v4 16/27] tracing: Remove regular RCU context for _rcuidle tracepoints (again) Date: Fri, 6 Mar 2020 15:22:46 -0500 (EST) Message-ID: <609624365.20355.1583526166349.JavaMail.zimbra@efficios.com> References: <20200221133416.777099322@infradead.org> <20200221134216.051596115@infradead.org> <20200306104335.GF3348@worktop.programming.kicks-ass.net> <20200306113135.GA8787@worktop.programming.kicks-ass.net> <1896740806.20220.1583510668164.JavaMail.zimbra@efficios.com> <20200306125500.6aa75c0d@gandalf.local.home> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: Received: from mail.efficios.com ([167.114.26.124]:54530 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726300AbgCFUWs (ORCPT ); Fri, 6 Mar 2020 15:22:48 -0500 In-Reply-To: <20200306125500.6aa75c0d@gandalf.local.home> Sender: linux-arch-owner@vger.kernel.org List-ID: To: rostedt Cc: Alexei Starovoitov , Peter Zijlstra , linux-kernel , linux-arch , Ingo Molnar , "Joel Fernandes, Google" , Greg Kroah-Hartman , "Gustavo A. R. Silva" , Thomas Gleixner , paulmck , Josh Triplett , Lai Jiangshan , Andy Lutomirski , Tony Luck , Frederic Weisbecker , dan carpenter , Masami Hiramatsu ----- On Mar 6, 2020, at 12:55 PM, rostedt rostedt@goodmis.org wrote: > On Fri, 6 Mar 2020 11:04:28 -0500 (EST) > Mathieu Desnoyers wrote: > >> If we care about not adding those extra branches on the fast-path, there is >> an alternative way to do things: BPF could provide two distinct probe callbacks, >> one meant for rcuidle tracepoints (which would have the trace_rcu_enter/exit), >> and >> the other for the for 99% of the other callsites which have RCU watching. >> >> I would recommend performing benchmarks justifying the choice of one approach >> over >> the other though. > > I just whipped this up (haven't even tried to compile it), but this should > satisfy everyone. Those that register a callback that needs RCU protection > simply registers with one of the _rcu versions, and all will be done. And > since DO_TRACE is a macro, and rcuidle is a constant, the rcu protection > code will be compiled out for locations that it is not needed. > > With this, perf doesn't even need to do anything extra but register with > the "_rcu" version. > > -- Steve > [...] > diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c > index 73956eaff8a9..1797e20fd471 100644 > --- a/kernel/tracepoint.c > +++ b/kernel/tracepoint.c > @@ -295,6 +295,7 @@ static int tracepoint_remove_func(struct tracepoint *tp, > * @probe: probe handler > * @data: tracepoint data > * @prio: priority of this function over other registered functions > + * @rcu: set to non zero if the callback requires RCU protection > * > * Returns 0 if ok, error value on error. > * Note: if @tp is within a module, the caller is responsible for > @@ -302,8 +303,8 @@ static int tracepoint_remove_func(struct tracepoint *tp, > * performed either with a tracepoint module going notifier, or from > * within module exit functions. > */ > -int tracepoint_probe_register_prio(struct tracepoint *tp, void *probe, > - void *data, int prio) > +int tracepoint_probe_register_prio_rcu(struct tracepoint *tp, void *probe, > + void *data, int prio, int rcu) I agree with the overall approach. Just a bit of nitpicking on the API: I understand that the "prio" argument is a separate argument because it can take many values. However, "rcu" is just a boolean, so I wonder if we should not rather introduce a "int flags" with a bitmask enum, e.g. int tracepoint_probe_register_prio_flags(struct tracepoint *tp, void *probe, void *data, int prio, int flags) where flags would be populated through OR between labels of this enum: enum tracepoint_flags { TRACEPOINT_FLAG_RCU = (1U << 0), }; We can then be future-proof for additional flags without ending up calling e.g. tracepoint_probe_register_featurea_featureb_featurec(tp, probe, data, 0, 1, 0, 1) which seems rather error-prone and less readable than a set of flags. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com