From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context Date: Tue, 11 Dec 2012 18:35:22 +0530 Message-ID: <50C72F92.9030704@linux.vnet.ibm.com> References: <20121207173702.27305.1486.stgit@srivatsabhat.in.ibm.com> <20121207173759.27305.84316.stgit@srivatsabhat.in.ibm.com> <20121209191437.GA2816@redhat.com> <50C4EB79.5050203@linux.vnet.ibm.com> <20121209211338.GA8090@redhat.com> <50C56CC0.4000200@linux.vnet.ibm.com> <20121210172859.GB28479@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from e28smtp07.in.ibm.com ([122.248.162.7]:35009 "EHLO e28smtp07.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753292Ab2LKNG5 (ORCPT ); Tue, 11 Dec 2012 08:06:57 -0500 Received: from /spool/local by e28smtp07.in.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 11 Dec 2012 18:36:25 +0530 In-Reply-To: <20121210172859.GB28479@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 12/10/2012 10:58 PM, Oleg Nesterov wrote: > On 12/10, Srivatsa S. Bhat wrote: >> >> On 12/10/2012 02:43 AM, Oleg Nesterov wrote: >>> Damn, sorry for noise. I missed this part... >>> >>> On 12/10, Srivatsa S. Bhat wrote: >>>> >>>> On 12/10/2012 12:44 AM, Oleg Nesterov wrote: >>>>> the latency. And I guess something like kick_all_cpus_sync() is "too heavy". >>>> >>>> I hadn't considered that. Thinking of it, I don't think it would help us.. >>>> It won't get rid of the currently running preempt_disable() sections no? >>> >>> Sure. But (again, this is only my feeling so far) given that get_online_cpus_atomic() >>> does cli/sti, >> >> Ah, that one! Actually, the only reason I do that cli/sti is because, potentially >> interrupt handlers can be hotplug readers too. So we need to protect the portion >> of the code of get_online_cpus_atomic() which is not re-entrant. > > Yes, I understand. > >>> this can help to implement ensure-the-readers-must-see-the-pending-writer. >>> IOW this might help to implement sync-with-readers. >>> >> >> 2 problems: >> >> 1. It won't help with cases like this: >> >> preempt_disable() >> ... >> preempt_disable() >> ... >> <------- Here >> ... >> preempt_enable() >> ... >> preempt_enable() > > No, I meant that kick_all_cpus_sync() can be used to synchronize with > cli/sti in get_online_cpus_atomic(), just like synchronize_sched() does > in the code I posted a minute ago. > Ah, OK. >> 2. Part of the reason we want to get rid of stop_machine() is to avoid the >> latency it induces on _all_ CPUs just to take *one* CPU offline. If we use >> kick_all_cpus_sync(), we get into that territory again : we unfairly interrupt >> every CPU, _even when_ that CPU's existing preempt_disabled() sections might >> not actually be hotplug readers! (ie., not bothered about CPU Hotplug). > > I agree, that is why I said it is "too heavy". > Got it :) Regards, Srivatsa S. Bhat