| Message ID | 20130122074543.13822.44724.stgit@srivatsabhat.in.ibm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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 <rob@landley.net> > Cc: linux-doc@vger.kernel.org > Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > --- > > 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 <linux/cpu.h> > 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, >
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 <linux/cpu.h> 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,
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 <rob@landley.net> Cc: linux-doc@vger.kernel.org Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> --- Documentation/cpu-hotplug.txt | 17 +++++++++++------ 1 file changed, 11 insertions(+), 6 deletions(-)