Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756068Ab3GBJyg (ORCPT ); Tue, 2 Jul 2013 05:54:36 -0400 Received: from e28smtp09.in.ibm.com ([122.248.162.9]:55439 "EHLO e28smtp09.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755227Ab3GBJyd (ORCPT ); Tue, 2 Jul 2013 05:54:33 -0400 Message-ID: <51D2A289.7070805@linux.vnet.ibm.com> Date: Tue, 02 Jul 2013 15:21:05 +0530 From: "Srivatsa S. Bhat" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120828 Thunderbird/15.0 MIME-Version: 1.0 To: Michael Wang CC: tglx@linutronix.de, peterz@infradead.org, tj@kernel.org, oleg@redhat.com, paulmck@linux.vnet.ibm.com, rusty@rustcorp.com.au, mingo@kernel.org, akpm@linux-foundation.org, namhyung@kernel.org, walken@google.com, vincent.guittot@linaro.org, laijs@cn.fujitsu.com, David.Laight@aculab.com, rostedt@goodmis.org, xiaoguangrong@linux.vnet.ibm.com, sbw@mit.edu, fweisbec@gmail.com, zhong@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, linux-arch@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Wang YanQing , Shaohua Li , Jan Beulich , liguang Subject: Re: [PATCH v3 10/45] smp: Use get/put_online_cpus_atomic() to prevent CPU offline References: <20130627195136.29830.10445.stgit@srivatsabhat.in.ibm.com> <20130627195418.29830.34958.stgit@srivatsabhat.in.ibm.com> <51D2660A.8000401@linux.vnet.ibm.com> <51D28E69.9060205@linux.vnet.ibm.com> <51D2939C.5090205@linux.vnet.ibm.com> In-Reply-To: <51D2939C.5090205@linux.vnet.ibm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-MML: No X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13070209-2674-0000-0000-000009A54CAF Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4055 Lines: 103 On 07/02/2013 02:17 PM, Michael Wang wrote: > On 07/02/2013 04:25 PM, Srivatsa S. Bhat wrote: >> Hi Michael, >> >> On 07/02/2013 11:02 AM, Michael Wang wrote: >>> Hi, Srivatsa >>> >>> On 06/28/2013 03:54 AM, Srivatsa S. Bhat wrote: >>> [snip] >>>> @@ -625,8 +632,9 @@ EXPORT_SYMBOL(on_each_cpu_mask); >>>> * The function might sleep if the GFP flags indicates a non >>>> * atomic allocation is allowed. >>>> * >>>> - * Preemption is disabled to protect against CPUs going offline but not online. >>>> - * CPUs going online during the call will not be seen or sent an IPI. >>>> + * We use get/put_online_cpus_atomic() to protect against CPUs going >>>> + * offline but not online. CPUs going online during the call will >>>> + * not be seen or sent an IPI. >>> >>> I was a little confused about this comment, if the offline-cpu still >>> have chances to become online, then there is chances that we will pick >>> it from for_each_online_cpu(), isn't it? Did I miss some point? >>> >> >> Whether or not the newly onlined CPU is observed in our for_each_online_cpu() >> loop, is dependent on timing. On top of that, there are 2 paths in the code: >> one which uses a temporary cpumask and the other which doesn't. In the former >> case, if a CPU comes online _after_ we populate the temporary cpumask, then >> we won't send an IPI to that cpu, since the temporary cpumask doesn't contain >> that CPU. Whereas, if we observe the newly onlined CPU in the for_each_online_cpu() >> loop itself (either in the former or the latter case), then yes, we will send >> the IPI to that CPU. > > So it is not 'during the call' but 'during the call of > on_each_cpu_mask()', correct? > Well, as I said, its timing dependent. We might miss the newly onlined CPU in the for_each_online_cpu() loop itself, based on when exactly the CPU was added to the cpu_online_mask. So you can't exactly pin-point the places where you'll miss the CPU and where you won't. Besides, is it _that_ important? It is after all unpredictable.. > The comment position seems like it declaim that during the call of this > func, online-cpu won't be seem and send IPI... > Doesn't matter, AFAICS. The key take-away from that whole comment is: nothing is done to prevent CPUs from coming online while the function is running, whereas the online CPUs are guaranteed to remain online throughout the function. In other words, its a weaker form of get_online_cpus()/put_online_cpus(), providing a one-way synchronization (CPU offline). As long as that idea is conveyed properly, the purpose of that comment is served, IMHO. Regards, Srivatsa S. Bhat >>>> * >>>> * You must not call this function with disabled interrupts or >>>> * from a hardware interrupt handler or from a bottom half handler. >>>> @@ -641,26 +649,26 @@ void on_each_cpu_cond(bool (*cond_func)(int cpu, void *info), >>>> might_sleep_if(gfp_flags & __GFP_WAIT); >>>> >>>> if (likely(zalloc_cpumask_var(&cpus, (gfp_flags|__GFP_NOWARN)))) { >>>> - preempt_disable(); >>>> + get_online_cpus_atomic(); >>>> for_each_online_cpu(cpu) >>>> if (cond_func(cpu, info)) >>>> cpumask_set_cpu(cpu, cpus); >>>> on_each_cpu_mask(cpus, func, info, wait); >>>> - preempt_enable(); >>>> + put_online_cpus_atomic(); >>>> free_cpumask_var(cpus); >>>> } else { >>>> /* >>>> * No free cpumask, bother. No matter, we'll >>>> * just have to IPI them one by one. >>>> */ >>>> - preempt_disable(); >>>> + get_online_cpus_atomic(); >>>> for_each_online_cpu(cpu) >>>> if (cond_func(cpu, info)) { >>>> ret = smp_call_function_single(cpu, func, >>>> info, wait); >>>> WARN_ON_ONCE(!ret); >>>> } >>>> - preempt_enable(); >>>> + put_online_cpus_atomic(); >>>> } >>>> } >>>> EXPORT_SYMBOL(on_each_cpu_cond); >>>> >>> >> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/