On Tue, May 21, 2013 at 02:17:17PM -0700, Andrew Morton wrote:
> On Fri, 3 May 2013 17:35:44 -0500 Russ Anderson <r...@sgi.com> wrote:
>
> > The routine disable_nonboot_cpus() shuts down cpus sequentially
> > using for_each_online_cpu(cpu) to call cpu_down() one cpu at
> > a time. cpu_down() calls __stop_machine() which stops all
> > the cpus while it disables one. Then it re-enables the remaining
> > cpus, only to do it all over again for the next cpu. The
> > result is that it takes 16 minutes on a 1024 cpu system to
> > disable 1023 cpus.
> >
> > This patch changes disable_nonboot_cpus() to pass a bitmask
> > of cpus to cpu_down() and modifies cpu_down() to only call
> > __stop_machine() once.
> >
> > On a 1024 cpu system this reduces the time it takes to disable
> > all but one cpu from 16 minutes down to 4 minutes.
>
> That's still a helluva long time. What's the kernel *doing* for
> all that time?
Since __raw_notifier_call_chain() isn't thread safe, it is still
called one cpu at a time. See "__cpu_notify(CPU_DOWN_PREPARE" below.
So we still go sequentially for each cpu through the call chain.
These are the two heaviest hitters on the call chain.
sched_init_smp ->
cpuset_cpu_inactive ->
cpuset_update_active_cpus ->
partition_sched_domains(1, NULL, NULL)
schedule_work(&cpuset_hotplug_work);
cpuset_hotplug_workfn
mutex_lock(&cpuset_mutex);
partition_sched_domains()
partition_sched_domains ->
unregister_sched_domain_sysctl() ->
unregister_sysctl_table
register_sched_domain_sysctl
for_each_possible_cpu()
register_sysctl_table
uncore_cpu_nb -> uncore_cpu_notifier
uncore_event_exit_cpu
for_each_online_cpu()
The partition_sched_domains() code gets called for each cpu,
each time destroying and building new domains.
register_sched_domain_sysctl() calls for_each_possible_cpu()
each time.
Likewise uncore_event_exit_cpu() calls for_each_online_cpu()
for each cpu.
Neither of those scale well.
> > --- linux.orig/kernel/cpu.c 2013-05-03 09:56:31.145508321 -0500
> > +++ linux/kernel/cpu.c 2013-05-03 17:01:20.652959400 -0500
> >
> > ...
> >
> > @@ -255,21 +255,21 @@ static int __ref take_cpu_down(void *_pa
> > if (err < 0)
> > return err;
> >
> > - cpu_notify(CPU_DYING | param->mod, param->hcpu);
> > + cpu_notify(CPU_DYING | param->mod, hcpu);
> > /* Park the stopper thread */
> > kthread_park(current);
> > return 0;
> > }
> >
> > /* Requires cpu_add_remove_lock to be held */
> > -static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
> > +static int __ref _cpu_down(const cpumask_t *cpus_to_offline, int
> > tasks_frozen)
>
> _cpu_down() is now misnamed - it downs multiple CPUs.
OK, I'll change it.
> > {
> > - int err, nr_calls = 0;
> > + int err = 0, cpu = 0, nr_calls = 0;
> > void *hcpu = (void *)(long)cpu;
> > + cpumask_var_t cpus_offlined;
> > unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
> > struct take_cpu_down_param tcd_param = {
> > .mod = mod,
> > - .hcpu = hcpu,
> > };
> >
> > if (num_online_cpus() == 1)
> > @@ -278,46 +278,67 @@ static int __ref _cpu_down(unsigned int
> > if (!cpu_online(cpu))
> > return -EINVAL;
> >
> > + if (!alloc_cpumask_var(&cpus_offlined, GFP_KERNEL))
> > + return -ENOMEM;
> > +
> > cpu_hotplug_begin();
> > + cpumask_copy(cpus_offlined, cpus_to_offline);
> >
> > - err = __cpu_notify(CPU_DOWN_PREPARE | mod, hcpu, -1, &nr_calls);
> > - if (err) {
> > - nr_calls--;
> > - __cpu_notify(CPU_DOWN_FAILED | mod, hcpu, nr_calls, NULL);
> > - printk("%s: attempt to take down CPU %u failed\n",
> > + for_each_cpu_mask(cpu, *cpus_to_offline) {
> > + if (!cpu_online(cpu))
> > + continue;
> > + hcpu = (void *)(long)cpu;
> > + err = __cpu_notify(CPU_DOWN_PREPARE | mod, hcpu, -1, &nr_calls);
> > + if (err) {
> > + nr_calls--;
> > + __cpu_notify(CPU_DOWN_FAILED | mod, hcpu, nr_calls,
> > NULL);
> > + pr_err("%s: attempt to take down CPU %u failed\n",
> > __func__, cpu);
> > - goto out_release;
> > + goto out_release;
> > + }
> > + smpboot_park_threads(cpu);
> > }
> > - smpboot_park_threads(cpu);
> >
> > - err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
> > + err = __stop_machine(take_cpu_down, &tcd_param, cpus_to_offline);
> > if (err) {
> > /* CPU didn't die: tell everyone. Can't complain. */
>
> This comment is now inaccurate. "One or more of the CPUs didn't die"?.
>
> I'm not sure what "Can't complain" means. Perhaps expand on this while
> you're there?
OK.
> > - smpboot_unpark_threads(cpu);
> > - cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu);
> > + for_each_cpu_mask(cpu, *cpus_to_offline) {
> > + hcpu = (void *)(long)cpu;
> > + smpboot_unpark_threads(cpu);
> > + cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu);
>
> Is this accurate? What happens if we asked stop_machine() to down 100
> CPUs but it failed at CPU #50? We now tell listeners that we failed to
> down all 100 CPUs? That's not true.
I'll look closer at that.
> > + }
> > goto out_release;
> > }
> > - BUG_ON(cpu_online(cpu));
> >
> > /*
> > * The migration_call() CPU_DYING callback will have removed all
> > * runnable tasks from the cpu, there's only the idle task left now
> > * that the migration thread is done doing the stop_machine thing.
> > - *
> > - * Wait for the stop thread to go away.
> > */
> > - while (!idle_cpu(cpu))
> > - cpu_relax();
> > -
> > - /* This actually kills the CPU. */
> > - __cpu_die(cpu);
> > + for_each_cpu_mask(cpu, *cpus_offlined) {
> > + BUG_ON(cpu_online(cpu));
> >
> > - /* CPU is completely dead: tell everyone. Too late to complain. */
> > - cpu_notify_nofail(CPU_DEAD | mod, hcpu);
> > -
> > - check_for_tasks(cpu);
> > + /*
> > + * Wait for the stop thread to go away.
> > + */
> > + while (!idle_cpu(cpu))
> > + cpu_relax();
> > +
> > + /*
> > + * This actually kills the CPU.
> > + */
> > + __cpu_die(cpu);
> > +
> > + /*
> > + * CPU is completely dead: tell everyone. Too late to complain.
> > + */
> > + hcpu = (void *)(long)cpu;
> > + cpu_notify_nofail(CPU_DEAD | mod, hcpu);
> > + check_for_tasks(cpu);
> > + }
> >
> > out_release:
> > + free_cpumask_var(cpus_offlined);
> > cpu_hotplug_done();
>
> Swap the above two lines and we reduced the locked region by an
> unmeasurable amount!
>
> > if (!err)
> > cpu_notify_nofail(CPU_POST_DEAD | mod, hcpu);
> >
> > ...
> >
--
Russ Anderson, OS RAS/Partitioning Project Lead
SGI - Silicon Graphics Inc r...@sgi.com
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