From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751517AbeDXTIE (ORCPT ); Tue, 24 Apr 2018 15:08:04 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:57372 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751003AbeDXTIA (ORCPT ); Tue, 24 Apr 2018 15:08:00 -0400 Date: Tue, 24 Apr 2018 12:09:05 -0700 From: "Paul E. McKenney" To: Joel Fernandes 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 Subject: Re: [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace_ if you can Reply-To: paulmck@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18042419-0008-0000-0000-000002FD6587 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00008914; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000257; SDB=6.01022699; UDB=6.00522020; IPR=6.00801933; MB=3.00020753; MTD=3.00000008; XFM=3.00000015; UTC=2018-04-24 19:07:56 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18042419-0009-0000-0000-000039043707 Message-Id: <20180424190905.GU26088@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-04-24_05:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=2 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1804240181 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Thanx, Paul > also Mathieu, you didn't modify the call_rcu_sched in your prototype > to be changed to use call_srcu, should you be doing that? > > thanks, > > - Joel >