From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH v5 45/45] Documentation/cpu-hotplug: Remove references to stop_machine() Date: Fri, 8 Feb 2013 16:16:49 -0800 Message-ID: <20130209001649.GS2666@linux.vnet.ibm.com> References: <20130122073210.13822.50434.stgit@srivatsabhat.in.ibm.com> <20130122074543.13822.44724.stgit@srivatsabhat.in.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20130122074543.13822.44724.stgit@srivatsabhat.in.ibm.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: linux-arm-kernel-bounces@lists.infradead.org Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=m.gmane.org@lists.infradead.org To: "Srivatsa S. Bhat" Cc: linux-doc@vger.kernel.org, peterz@infradead.org, fweisbec@gmail.com, linux-kernel@vger.kernel.org, mingo@kernel.org, linux-arch@vger.kernel.org, linux@arm.linux.org.uk, xiaoguangrong@linux.vnet.ibm.com, wangyun@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, rusty@rustcorp.com.au, rostedt@goodmis.org, rjw@sisk.pl, namhyung@kernel.org, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, netdev@vger.kernel.org, oleg@redhat.com, sbw@mit.edu, tj@kernel.org, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org List-Id: linux-pm@vger.kernel.org On Tue, Jan 22, 2013 at 01:15:48PM +0530, Srivatsa S. Bhat wrote: > Since stop_machine() is no longer used in the CPU offline path, we cannot > disable CPU hotplug using preempt_disable()/local_irq_disable() etc. We > need to use the newly introduced get/put_online_cpus_atomic() APIs. > Reflect this in the documentation. > > Cc: Rob Landley > Cc: linux-doc@vger.kernel.org > Signed-off-by: Srivatsa S. Bhat Reviewed-by: Paul E. McKenney > --- > > Documentation/cpu-hotplug.txt | 17 +++++++++++------ > 1 file changed, 11 insertions(+), 6 deletions(-) > > diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt > index 9f40135..7f907ec 100644 > --- a/Documentation/cpu-hotplug.txt > +++ b/Documentation/cpu-hotplug.txt > @@ -113,13 +113,15 @@ Never use anything other than cpumask_t to represent bitmap of CPUs. > #include > get_online_cpus() and put_online_cpus(): > > -The above calls are used to inhibit cpu hotplug operations. While the > +The above calls are used to inhibit cpu hotplug operations, when invoked from > +non-atomic context (because the above functions can sleep). While the > cpu_hotplug.refcount is non zero, the cpu_online_mask will not change. > -If you merely need to avoid cpus going away, you could also use > -preempt_disable() and preempt_enable() for those sections. > -Just remember the critical section cannot call any > -function that can sleep or schedule this process away. The preempt_disable() > -will work as long as stop_machine_run() is used to take a cpu down. > + > +However, if you are executing in atomic context (ie., you can't afford to > +sleep), and you merely need to avoid cpus going offline, you can use > +get_online_cpus_atomic() and put_online_cpus_atomic() for those sections. > +Just remember the critical section cannot call any function that can sleep or > +schedule this process away. > > CPU Hotplug - Frequently Asked Questions. > > @@ -360,6 +362,9 @@ A: There are two ways. If your code can be run in interrupt context, use > return err; > } > > + If my_func_on_cpu() itself cannot block, use get/put_online_cpus_atomic() > + instead of get/put_online_cpus() to prevent CPUs from going offline. > + > Q: How do we determine how many CPUs are available for hotplug. > A: There is no clear spec defined way from ACPI that can give us that > information today. Based on some input from Natalie of Unisys, >