From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751277AbaHHR1U (ORCPT ); Fri, 8 Aug 2014 13:27:20 -0400 Received: from cdptpa-outbound-snat.email.rr.com ([107.14.166.228]:25668 "EHLO cdptpa-oedge-vip.email.rr.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751157AbaHHR1T (ORCPT ); Fri, 8 Aug 2014 13:27:19 -0400 Date: Fri, 8 Aug 2014 13:27:16 -0400 From: Steven Rostedt To: Peter Zijlstra Cc: "Paul E. McKenney" , Oleg Nesterov , linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, bobby.prani@gmail.com, masami.hiramatsu.pt@hitachi.com Subject: Re: [PATCH v3 tip/core/rcu 3/9] rcu: Add synchronous grace-period waiting for RCU-tasks Message-ID: <20140808132716.28aa1c14@gandalf.local.home> In-Reply-To: <20140808162714.GG9918@twins.programming.kicks-ass.net> References: <20140807172753.GG3588@twins.programming.kicks-ass.net> <20140807184635.GI3588@twins.programming.kicks-ass.net> <20140807154907.6f59cf6e@gandalf.local.home> <20140807155326.18481e66@gandalf.local.home> <20140807200813.GB3935@laptop> <20140807171823.1a481290@gandalf.local.home> <20140808064020.GZ9918@twins.programming.kicks-ass.net> <20140808101221.21056900@gandalf.local.home> <20140808143413.GB9918@twins.programming.kicks-ass.net> <20140808105858.171da847@gandalf.local.home> <20140808162714.GG9918@twins.programming.kicks-ass.net> X-Mailer: Claws Mail 3.10.1 (GTK+ 2.24.24; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-RR-Connecting-IP: 107.14.168.142:25 X-Cloudmark-Score: 0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 8 Aug 2014 18:27:14 +0200 Peter Zijlstra wrote: > On Fri, Aug 08, 2014 at 10:58:58AM -0400, Steven Rostedt wrote: > > > > > No, they are also used by optimized kprobes. This is why optimized > > > > kprobes depend on !CONFIG_PREEMPT. [ added Masami to the discussion ]. > > > > > > How do those work? Is that one where the INT3 relocates the instruction > > > stream into an alternative 'text' and that JMPs back into the original > > > stream at the end? > > > > No, it's where we replace the 'int3' with a jump to a trampoline that > > simulates an INT3. Speeds things up quite a bit. > > OK, so the trivial 'fix' for that is to patch the probe site like: > > preempt_disable(); INC GS:%__preempt_count > call trampoline; CALL 0xDEADBEEF > preempt_enable(); DEC GS:%__preempt_count > JNZ 1f > CALL ___preempt_schedule > 1f: > > At which point the preempt_disable/enable() are the read side primitives > and call_rcu_sched/synchronize_sched are sufficient to release it. > > With the per-cpu preempt count stuff we have on x86 that is 4 > instructions for the preempt_*() stuff -- they're 'big' instructions > though, since 3 have memops and 2 have a segment prefix. > > Well, this looks like it may make kprobes a bit more complex, and even slow down slightly the optimized probe. Also note that if we add call_rcu_tasks(), then perf function tracing can be called directly instead of being added to the trampoline that disables and enables preemption before calling it. -- Steve