From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context Date: Wed, 19 Dec 2012 01:36:11 +0530 Message-ID: <50D0CCB3.10105@linux.vnet.ibm.com> References: <50C8C4A5.4080104@linux.vnet.ibm.com> <20121212180248.GA24882@redhat.com> <50C8CD52.8040808@linux.vnet.ibm.com> <20121212184849.GA26784@redhat.com> <50C8D739.6030903@linux.vnet.ibm.com> <50C9F38F.3020005@linux.vnet.ibm.com> <20121213161709.GA19125@redhat.com> <50CA0317.90501@linux.vnet.ibm.com> <20121214180345.GA22024@redhat.com> <50D09180.4080703@linux.vnet.ibm.com> <20121218194357.GA27972@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from e28smtp04.in.ibm.com ([122.248.162.4]:36558 "EHLO e28smtp04.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755148Ab2LRUHt (ORCPT ); Tue, 18 Dec 2012 15:07:49 -0500 Received: from /spool/local by e28smtp04.in.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 19 Dec 2012 01:37:13 +0530 In-Reply-To: <20121218194357.GA27972@redhat.com> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: Oleg Nesterov Cc: tglx@linutronix.de, peterz@infradead.org, paulmck@linux.vnet.ibm.com, rusty@rustcorp.com.au, mingo@kernel.org, akpm@linux-foundation.org, namhyung@kernel.org, vincent.guittot@linaro.org, tj@kernel.org, sbw@mit.edu, amit.kucheria@linaro.org, rostedt@goodmis.org, rjw@sisk.pl, wangyun@linux.vnet.ibm.com, xiaoguangrong@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org On 12/19/2012 01:13 AM, Oleg Nesterov wrote: > On 12/18, Srivatsa S. Bhat wrote: >> >> So now that we can't avoid disabling and enabling interrupts, > > Still I think it would be better to not use local_irq_save/restore > directly. Sure, we can use this_cpu_add() itself. I explicitly used local_irq_save/restore here just to explain my question. > And, > >> I was >> wondering if we could exploit this to avoid the smp_mb().. >> >> Maybe this is a stupid question, but I'll shoot it anyway... >> Does local_irq_disable()/enable provide any ordering guarantees by any chance? > > Oh, I do not know. > > But please look at the comment above prepare_to_wait(). It assumes > that even spin_unlock_irqrestore() is not the full barrier. > Semi-permeable barrier.. Hmm.. > In any case. get_online_cpus_atomic() has to use irq_restore, not > irq_enable. And _restore does nothing "special" if irqs were already > disabled, so I think we can't rely on sti. > Right, I forgot about the _restore part. >> I tried thinking about other ways to avoid that smp_mb() in the reader, > > Just in case, I think there is no way to avoid mb() in _get (although > perhaps it can be "implicit"). > Actually, I was trying to avoid mb() in the _fastpath_, when there is no active writer. I missed stating that clearly, sorry. > The writer changes cpu_online_mask and drops the lock. We need to ensure > that the reader sees the change in cpu_online_mask after _get returns. > The write_unlock() will ensure the completion of the update to cpu_online_mask, using the same semi-permeable logic that you pointed above. So readers will see the update as soon as the writer releases the lock, right? >> but was unsuccessful. So if the above assumption is wrong, I guess we'll >> just have to go with the version that uses synchronize_sched() at the >> writer-side. > > In this case we can also convert get_online_cpus() to use percpu_rwsem > and avoid mutex_lock(&cpu_hotplug.lock), but this is minor I guess. > I do not think get_online_cpus() is called too often. > Yes, we could do that as well. I remember you saying that you had some patches for percpu_rwsem to help use it in cpu hotplug code (to make it recursive, IIRC). So, I guess we'll go with the synchronize_sched() approach for percpu rwlocks then. Tejun, it is still worthwhile to expose this as a generic percpu rwlock and then use it inside cpu hotplug code, right? Regards, Srivatsa S. Bhat