From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline Date: Thu, 06 Dec 2012 00:23:33 +0530 Message-ID: <50BF982D.7090704@linux.vnet.ibm.com> References: <20121205184041.3750.64945.stgit@srivatsabhat.in.ibm.com> <20121205184258.3750.31879.stgit@srivatsabhat.in.ibm.com> <50BF96DF.3000500@linux.vnet.ibm.com> <50BF979A.50304@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from e28smtp08.in.ibm.com ([122.248.162.8]:48911 "EHLO e28smtp08.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752399Ab2LESzE (ORCPT ); Wed, 5 Dec 2012 13:55:04 -0500 Received: from /spool/local by e28smtp08.in.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 6 Dec 2012 00:24:48 +0530 In-Reply-To: <50BF979A.50304@linux.vnet.ibm.com> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: "Srivatsa S. Bhat" 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, oleg@redhat.com, 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 Replaying what Tejun wrote: Hello, Oleg. > Replaying what Oleg wrote: > > Hi, > > Sorry I don't understand the context and I can't find this thread > anywhere, so I am not sure I understand... > Weird, lkml cc is missing. Srivatsa? [Now fixed. This thread has lkml CC] >> Replaying what Tejun wrote: >> So, we basically need percpu_rwlock. We already have percpu_rwsem. > > Yes, and with -mm patches it becomes reader-friendly. In particular > see http://marc.info/?l=linux-mm-commits&m=135240650828875 > >> Oleg, it seems >> CPU hotplug needs big-reader rwlock, ideas on how to proceed? >> > > I am going to convert get_online_cpus() to use percpu_down_read(), > this looks simple. > > We already discussed this with Paul, see > > http://marc.info/?l=linux-kernel&m=135248463226031 > > and the whole thread. > > In short, all we need is percpu_down_write_recursive_readers() and > afaics the only complication is lockdep, we need down_read_no_lockdep() > which (like __up_read) doesn't do rwsem_acquire_read(). > So, it's a different thing. There are two mechanism protecting against cpu hotplug - get_online_cpus() and preempt_disable(). The former can be used by ones which can sleep and need to protect against the whole up/down process (DOWN_PREPARE and so on). The latter protects the last step and can be used when the caller can't sleep. Replacing get_online_cpus() w/ percpu_rwsem is great but this thread is about replacing preempt_disable with something finer grained and less heavy on the writer side - IOW, percpu_rwlock as opposed to percpu_rwsem, so, I think the end result would be that CPU hotplug will be protected by percpu_rwsem for the whole part and by percpu_rwlock for the last commit stage. The problem seems that we don't have percpu_rwlock yet. It shouldn't be too difficult to implement, right? Thanks. -- tejun