From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace_ if you can Date: Mon, 23 Apr 2018 08:12:22 -0700 Message-ID: <20180423151222.GO26088@linux.vnet.ibm.com> References: <20180417040748.212236-1-joelaf@google.com> <20180418180250.7b6038dddba46b37c94b796c@kernel.org> <20180419054302.GD13370@sejong> <20180423031926.GF26088@linux.vnet.ibm.com> <409016827.14587.1524493888181.JavaMail.zimbra@efficios.com> <20180423105325.7d5d245b@gandalf.local.home> <1045420715.14686.1524495583859.JavaMail.zimbra@efficios.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: rostedt , Joel Fernandes , Namhyung Kim , Masami Hiramatsu , linux-kernel , linux-rt-users , Peter Zijlstra , Ingo Molnar , Tom Zanussi , Thomas Gleixner , Boqun Feng , fweisbec , Randy Dunlap , kbuild test robot , baohong liu , vedang patel , kernel-team To: Mathieu Desnoyers Return-path: Content-Disposition: inline In-Reply-To: <1045420715.14686.1524495583859.JavaMail.zimbra@efficios.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-rt-users.vger.kernel.org On Mon, Apr 23, 2018 at 10:59:43AM -0400, Mathieu Desnoyers wrote: > ----- On Apr 23, 2018, at 10:53 AM, rostedt rostedt@goodmis.org wrote: > > > On Mon, 23 Apr 2018 10:31:28 -0400 (EDT) > > Mathieu Desnoyers wrote: > > > >> I've been wanting to introduce an alternative tracepoint instrumentation > >> "flavor" for e.g. system call entry/exit which rely on SRCU rather than > >> sched-rcu (preempt-off). This would allow taking faults within the > >> instrumentation > >> probe, which makes lots of things easier when fetching data from user-space > >> upon system call entry/exit. This could also be used to cleanly instrument > >> the idle loop. > > > > I'd be OK with such an approach. And I don't think it would be that > > hard to implement. It could be similar to the rcu_idle() tracepoints, > > where each flavor simply passes in what protection it uses for > > DO_TRACE(). We could do linker tricks to tell the tracepoint.c code how > > the tracepoint is protected (add section code, that could be read to > > update flags in the tracepoint). Of course modules that have > > tracepoints could only use the standard preempt ones. > > > > That is, if trace_##event##_srcu(trace_##event##_sp, PARAMS), is used, > > then the trace_##event##_sp would need to be created somewhere. The use > > of trace_##event##_srcu() would create a section entry, and on boot up > > we can see that the use of this tracepoint requires srcu protection > > with a pointer to the trace_##event##_sp srcu_struct. This could be > > used to make sure that trace_#event() call isn't done multiple times > > that uses two different protection flavors. > > > > I'm just brain storming the idea, and I'm sure I screwed up something > > above, but I do believe it is feasible. > > The main open question here is whether we want one SRCU grace period > domain per SRCU tracepoint definition, or just one SRCU domain for all > SRCU tracepoints would be fine. > > I'm not sure what we would gain by having the extra granularity provided > by one SRCU grace period domain per tracepoint, and having a single SRCU > domain for all SRCU tracepoints makes it easy to batch grace period after > bulk tracepoint modifications. I don't see how having multiple SRCU domains would help anything, but perhaps I am missing something basic. thanx, Paul