From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: Re: [RFC PATCH v3 7/9] yield_to(), cpu-hotplug: Prevent offlining of other CPUs properly Date: Mon, 10 Dec 2012 01:27:58 +0530 Message-ID: <50C4ED46.3070100@linux.vnet.ibm.com> References: <20121207173702.27305.1486.stgit@srivatsabhat.in.ibm.com> <20121207173950.27305.39499.stgit@srivatsabhat.in.ibm.com> <20121209194826.GB2816@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from e28smtp09.in.ibm.com ([122.248.162.9]:54701 "EHLO e28smtp09.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758902Ab2LIT71 (ORCPT ); Sun, 9 Dec 2012 14:59:27 -0500 Received: from /spool/local by e28smtp09.in.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 10 Dec 2012 01:29:12 +0530 In-Reply-To: <20121209194826.GB2816@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 01:18 AM, Oleg Nesterov wrote: > On 12/07, Srivatsa S. Bhat wrote: >> >> Once stop_machine() is gone from the CPU offline path, we won't be able to >> depend on local_irq_save() to prevent CPUs from going offline from under us. > > OK, I guess we need to avoid resched_task()->smp_send_reschedule() > after __cpu_disable() and before migrate_tasks(). > Yes. > But, whatever problem we have, > >> Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going offline, >> while invoking from atomic context. > > it should be solved, so... > >> - if (preempt && rq != p_rq) >> + if (preempt && rq != p_rq && cpu_online(task_cpu(p))) > > Why do we need this change? > > Afaics, you could add BUG_ON(!cpu_online(...)) instead? > > I am just curious. > Oh, I think that's a remnant of v1 (which needed readers to use cpu_online_stable()). You're right, we don't need it. Or we could put a BUG_ON instead, like you suggested. Regards, Srivatsa S. Bhat