From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751384AbeDXTQl (ORCPT ); Tue, 24 Apr 2018 15:16:41 -0400 Received: from mail-io0-f176.google.com ([209.85.223.176]:41691 "EHLO mail-io0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750766AbeDXTQj (ORCPT ); Tue, 24 Apr 2018 15:16:39 -0400 X-Google-Smtp-Source: AB8JxZr0zITOf89O6GvTbvku+N2MbylNimAoqvDTh8hIeCLBkJYWzHBHtuyXy91PBxJ3lYRqN7RkM8pXOihntyAV3eU= MIME-Version: 1.0 In-Reply-To: <20180424190905.GU26088@linux.vnet.ibm.com> References: <1045420715.14686.1524495583859.JavaMail.zimbra@efficios.com> <20180423121800.47b173af@gandalf.local.home> <1212130312.14753.1524503541789.JavaMail.zimbra@efficios.com> <20180423172244.694dbc9d@gandalf.local.home> <20180424155655.GA820@linux.vnet.ibm.com> <20180424172658.GT26088@linux.vnet.ibm.com> <20180424182302.GA404@linux.vnet.ibm.com> <20180424182623.GA1357@linux.vnet.ibm.com> <20180424190905.GU26088@linux.vnet.ibm.com> From: Joel Fernandes Date: Tue, 24 Apr 2018 12:16:37 -0700 Message-ID: Subject: Re: [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace_ if you can To: Paul McKenney Cc: Steven Rostedt , Mathieu Desnoyers , 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 Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 24, 2018 at 12:09 PM, Paul E. McKenney wrote: > On Tue, Apr 24, 2018 at 11:59:32AM -0700, Joel Fernandes wrote: >> Hi Paul, >> >> On Tue, Apr 24, 2018 at 11:26 AM, Paul E. McKenney >> wrote: >> > On Tue, Apr 24, 2018 at 11:23:02AM -0700, Paul E. McKenney wrote: >> >> On Tue, Apr 24, 2018 at 10:26:58AM -0700, Paul E. McKenney wrote: >> >> > On Tue, Apr 24, 2018 at 09:01:34AM -0700, Joel Fernandes wrote: >> >> > > On Tue, Apr 24, 2018 at 8:56 AM, Paul E. McKenney >> >> > > wrote: >> >> > > > On Mon, Apr 23, 2018 at 05:22:44PM -0400, Steven Rostedt wrote: >> >> > > >> On Mon, 23 Apr 2018 13:12:21 -0400 (EDT) >> >> > > >> Mathieu Desnoyers wrote: >> >> > > >> >> >> > > >> >> >> > > >> > I'm inclined to explicitly declare the tracepoints with their given >> >> > > >> > synchronization method. Tracepoint probe callback functions for currently >> >> > > >> > existing tracepoints expect to have preemption disabled when invoked. >> >> > > >> > This assumption will not be true anymore for srcu-tracepoints. >> >> > > >> >> >> > > >> Actually, why not have a flag attached to the tracepoint_func that >> >> > > >> states if it expects preemption to be enabled or not? If a >> >> > > >> trace_##event##_srcu() is called, then simply disable preemption before >> >> > > >> calling the callbacks for it. That way if a callback is fine for use >> >> > > >> with srcu, then it would require calling >> >> > > >> >> >> > > >> register_trace_##event##_may_sleep(); >> >> > > >> >> >> > > >> Then if someone uses this on a tracepoint where preemption is disabled, >> >> > > >> we simply do not call it. >> >> > > > >> >> > > > One more stupid question... If we are having to trace so much stuff >> >> > > > in the idle loop, are we perhaps grossly overstating the extent of that >> >> > > > "idle" loop? For being called "idle", this code seems quite busy! >> >> > > >> >> > > ;-) >> >> > > The performance hit I am observing is when running a heavy workload, >> >> > > like hackbench or something like that. That's what I am trying to >> >> > > correct. >> >> > > By the way is there any limitation on using SRCU too early during >> >> > > boot? I backported Mathieu's srcu tracepoint patches but the kernel >> >> > > hangs pretty early in the boot. I register lockdep probes in >> >> > > start_kernel. I am hoping that's not why. >> >> > > >> >> > > I could also have just screwed up the backporting... may be for my >> >> > > testing, I will just replace the rcu API with the srcu instead of all >> >> > > of Mathieu's new TRACE_EVENT macros for SRCU, since all I am trying to >> >> > > do right now is measure the performance of my patches with SRCU. >> >> > >> >> > Gah, yes, there is an entry on my capacious todo list on making SRCU >> >> > grace periods work during early boot and mid-boot. Let me see what >> >> > I can do... >> >> >> >> OK, just need to verify that you are OK with call_srcu()'s callbacks >> >> not being invoked until sometime during core_initcall() time. (If you >> >> really do need them to be invoked before that, in theory it is possible, >> >> but in practice it is weird, even for RCU.) >> > >> > Oh, and that early at boot, you will need to use DEFINE_SRCU() or >> > DEFINE_STATIC_SRCU() rather than dynamic allocation and initialization. >> >> Oh ok. >> >> About call_rcu, calling it later may be an issue since we register the >> probes in start_kernel, for the first probe call_rcu will be sched, >> but for the second one I think it'll try to call_rcu to get rid of the >> first one. >> >> This is the relevant code that gets called when probes are added: >> >> static inline void release_probes(struct tracepoint_func *old) >> { >> if (old) { >> struct tp_probes *tp_probes = container_of(old, >> struct tp_probes, probes[0]); >> call_rcu_sched(&tp_probes->rcu, rcu_free_old_probes); >> } >> } >> >> Maybe we can somehow defer the call_srcu until later? Would that be possible? > > You will be able to invoke call_srcu() early if you wish, it is just that > the specified SRCU callback won't be invoked until core_initcall() time. > Yes, that should be fine then. Also I think I see no issue with static initialization and allocation as you were suggesting in your earlier email so that's also Ok. Let me know when you have anything I can test, thanks a lot. I'll pause my testing of srcu for now then and focus on the other parts of my patchset. thanks, - Joel