From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933803AbbENORM (ORCPT ); Thu, 14 May 2015 10:17:12 -0400 Received: from e23smtp07.au.ibm.com ([202.81.31.140]:44744 "EHLO e23smtp07.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933284AbbENORJ (ORCPT ); Thu, 14 May 2015 10:17:09 -0400 Message-ID: <5554AE2B.30201@linux.vnet.ibm.com> Date: Thu, 14 May 2015 19:46:11 +0530 From: Shreyas B Prabhu User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.1.0 MIME-Version: 1.0 To: Steven Rostedt CC: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, preeti@linux.vnet.ibm.com, paulmck@linux.vnet.ibm.com Subject: Re: [PATCH] tracing: Add comments explaining cpu online filter for trace events References: <1431533263-5139-1-git-send-email-shreyas@linux.vnet.ibm.com> <20150513122147.4ead9f07@gandalf.local.home> In-Reply-To: <20150513122147.4ead9f07@gandalf.local.home> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15051414-0025-0000-0000-0000017BCC16 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wednesday 13 May 2015 09:51 PM, Steven Rostedt wrote: > On Wed, 13 May 2015 21:37:43 +0530 > "Shreyas B. Prabhu" wrote: > >> trace_mm_page_pcpu_drain, trace_kmem_cache_free, trace_mm_page_free >> and trace_tlb_flush can be potentially called from an offlined cpu. >> Since trace points use RCU and RCU should not be used from offlined >> cpus, we have checks to filter out such calls. Add comments to explain >> this. >> >> Signed-off-by: Shreyas B. Prabhu >> --- >> This applies on top of patches posted here: >> https://lkml.org/lkml/2015/5/8/527 >> >> include/trace/events/kmem.h | 15 +++++++++++++++ >> include/trace/events/tlb.h | 5 +++++ >> 2 files changed, 20 insertions(+) >> >> diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h >> index 6cd975f..9883f2f 100644 >> --- a/include/trace/events/kmem.h >> +++ b/include/trace/events/kmem.h >> @@ -146,6 +146,11 @@ DEFINE_EVENT_CONDITION(kmem_free, kmem_cache_free, >> >> TP_ARGS(call_site, ptr), >> >> + /* >> + * This trace can be potentially called from an offlined cpu. >> + * Since trace points use RCU and RCU should not be used from >> + * offline cpus, filter such calls out. >> + */ >> TP_CONDITION(cpu_online(smp_processor_id())) >> ); >> > > Thanks for the comments, but can't these still be called with > preemption enabled. What happens when CONFIG_DEBUG_PREEMPT is set and > you enable these tracepoints. Wont it trigger a warning about > smp_processor_id() being used in preemptible code? > Yes. It does trigger "using smp_processor_id() in preemptible code" warnings. But as you mentioned in the previous comments, we should be safe even if the trace call happens from a preemptible section. Let me play out the scenarios here again- The task gets migrated after the smp_processor_id() 1. From an online cpu to another online cpu - No impact 2. From an online cpu to an offline cpu - Should never happen 3. From an offline cpu to an online cpu - IIUC, once a cpu has been offlined it returns to cpu_idle_loop, discovers its offline and calls arch_cpu_idle_dead. All this happens with preemption disabled. So this scenario too should never happen. So I don't see any downside to changing smp_processor_id() to raw_smp_processor_id() which will suppress the warnings. If you agree I'll send a patch doing this. Another alternative which is perhaps worth considering is to change __DO_TRACE itself to check for offline cpu, without a trace event specifying the check. This will prevent any currently uncaught and any future tracepoints from using RCU on offline cpus. But I guess it's little extreme considering only a low fraction of tracepoints have potential of being called from offline cpus. Thanks, Shreyas