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Date: Thu, 9 Oct 2008 18:25:36 +0200
From: Heiko Carstens <heiko.carstens@...ibm.com>
To: Rusty Russell <rusty@...tcorp.com.au>
Cc: jens.axboe@...cle.com, schwidefsky@...ibm.com,
linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH/RFC 0/4] Add stop_machine_get/put_threads to
stop_machine infrastructrue.
On Thu, Oct 09, 2008 at 11:18:27AM +1100, Rusty Russell wrote:
> On Wednesday 08 October 2008 21:14:50 Heiko Carstens wrote:
> > The only thing that needs to be addressed is that workqueue threads aka
> > stop_machine threads are no real time threads now.
> > We would need something like create_workqueue_prio() or
> > create_workqueue_rt(). Would that be acceptable?
>
> Hmm, I was hoping to reuse the kevent threads rather than create YA set
> of threads. But hey, everyone else is doing it.
Reusing the kevent threads wouldn't work. The stopmachine work might be
queued behind a different work which could block on I/O.
Which is what I try to avoid.
> > +static struct workqueue_struct *stop_machine_wq;
> > +static struct work_struct *stop_machine_work;
> > +static struct stop_machine_data active, idle;
> > +static cpumask_t active_cpus;
> Hmm, please make active cpus a const cpumask_t pointer. I'm trying to
> get rid of these kind of decls in another patch series :)
done.
> > /* This is the actual thread which stops the CPU. It exits by itself rather
> > * than waiting for kthread_stop(), because it's easier for hotplug CPU. */
> This comment is no longer valid...
fixed.
> > +static int __init stop_machine_init(void)
> > +{
> > + stop_machine_wq = create_workqueue("kstop");
> > + stop_machine_work = kcalloc(NR_CPUS, sizeof(struct work_struct),
> > + GFP_KERNEL);
> Perhaps make stop_machine_work a per-cpu array of struct work_struct
> instead of initializing it here. Or at least make it a percpu pointer and
> only alloc possible cpus.
For the time being I converted it to alloc_percpu. Might be worth to use
percpu_alloc instead and register a cpu hotplug notifier...
> Does it break cpu hotplug BTW? That's usually the problem.
It works. CPU hotplug is my primary test case, because it usually breaks ;)
So, how about the patch below then? I converted the device_initcall() to an
early_initcall(), so we get stop_machine working before smp works. For that
I had to move the init_workqueues() call a bit.
There are two more things that probably need to be addressed: the priority
of the workqueue should probably be changed to MAX_RT_PRIO-1 like it was
for the kstop threads.
And for num_online_cpus() == 1 we might as well call the simple stop_machine()
version which is defined in include/linux/stop_machine.h for !CONFIG_SMP.
---
init/main.c | 4 -
kernel/stop_machine.c | 109 ++++++++++++++++++--------------------------------
2 files changed, 42 insertions(+), 71 deletions(-)
Index: linux-2.6/init/main.c
===================================================================
--- linux-2.6.orig/init/main.c
+++ linux-2.6/init/main.c
@@ -767,8 +767,6 @@ static void __init do_initcalls(void)
static void __init do_basic_setup(void)
{
rcu_init_sched(); /* needed by module_init stage. */
- /* drivers will send hotplug events */
- init_workqueues();
usermodehelper_init();
driver_init();
init_irq_proc();
@@ -852,6 +850,8 @@ static int __init kernel_init(void * unu
cad_pid = task_pid(current);
+ init_workqueues();
+
smp_prepare_cpus(setup_max_cpus);
do_pre_smp_initcalls();
Index: linux-2.6/kernel/stop_machine.c
===================================================================
--- linux-2.6.orig/kernel/stop_machine.c
+++ linux-2.6/kernel/stop_machine.c
@@ -37,9 +37,13 @@ struct stop_machine_data {
/* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
static unsigned int num_threads;
static atomic_t thread_ack;
-static struct completion finished;
static DEFINE_MUTEX(lock);
+static struct workqueue_struct *stop_machine_wq;
+static struct stop_machine_data active, idle;
+static const cpumask_t *active_cpus;
+static void *stop_machine_work;
+
static void set_state(enum stopmachine_state newstate)
{
/* Reset ack counter. */
@@ -51,21 +55,25 @@ static void set_state(enum stopmachine_s
/* Last one to ack a state moves to the next state. */
static void ack_state(void)
{
- if (atomic_dec_and_test(&thread_ack)) {
- /* If we're the last one to ack the EXIT, we're finished. */
- if (state == STOPMACHINE_EXIT)
- complete(&finished);
- else
- set_state(state + 1);
- }
+ if (atomic_dec_and_test(&thread_ack))
+ set_state(state + 1);
}
-/* This is the actual thread which stops the CPU. It exits by itself rather
- * than waiting for kthread_stop(), because it's easier for hotplug CPU. */
-static int stop_cpu(struct stop_machine_data *smdata)
+/* This is the actual function which stops the CPU. It runs
+ * in the context of a dedicated stopmachine workqueue. */
+static void stop_cpu(struct work_struct *unused)
{
enum stopmachine_state curstate = STOPMACHINE_NONE;
+ struct stop_machine_data *smdata = &idle;
+ int cpu = smp_processor_id();
+ if (!active_cpus) {
+ if (cpu == first_cpu(cpu_online_map))
+ smdata = &active;
+ } else {
+ if (cpu_isset(cpu, *active_cpus))
+ smdata = &active;
+ }
/* Simple state machine */
do {
/* Chill out and ensure we re-read stopmachine_state. */
@@ -90,7 +98,6 @@ static int stop_cpu(struct stop_machine_
} while (curstate != STOPMACHINE_EXIT);
local_irq_enable();
- do_exit(0);
}
/* Callback for CPUs which aren't supposed to do anything. */
@@ -101,78 +108,34 @@ static int chill(void *unused)
int __stop_machine(int (*fn)(void *), void *data, const cpumask_t *cpus)
{
- int i, err;
- struct stop_machine_data active, idle;
- struct task_struct **threads;
+ struct work_struct *sm_work;
+ int i;
+ /* Set up initial state. */
+ mutex_lock(&lock);
+ num_threads = num_online_cpus();
+ active_cpus = cpus;
active.fn = fn;
active.data = data;
active.fnret = 0;
idle.fn = chill;
idle.data = NULL;
- /* This could be too big for stack on large machines. */
- threads = kcalloc(NR_CPUS, sizeof(threads[0]), GFP_KERNEL);
- if (!threads)
- return -ENOMEM;
-
- /* Set up initial state. */
- mutex_lock(&lock);
- init_completion(&finished);
- num_threads = num_online_cpus();
set_state(STOPMACHINE_PREPARE);
- for_each_online_cpu(i) {
- struct stop_machine_data *smdata = &idle;
- struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
-
- if (!cpus) {
- if (i == first_cpu(cpu_online_map))
- smdata = &active;
- } else {
- if (cpu_isset(i, *cpus))
- smdata = &active;
- }
-
- threads[i] = kthread_create((void *)stop_cpu, smdata, "kstop%u",
- i);
- if (IS_ERR(threads[i])) {
- err = PTR_ERR(threads[i]);
- threads[i] = NULL;
- goto kill_threads;
- }
-
- /* Place it onto correct cpu. */
- kthread_bind(threads[i], i);
-
- /* Make it highest prio. */
- if (sched_setscheduler_nocheck(threads[i], SCHED_FIFO, ¶m))
- BUG();
- }
-
- /* We've created all the threads. Wake them all: hold this CPU so one
+ /* Schedule the stop_cpu work on all cpus: hold this CPU so one
* doesn't hit this CPU until we're ready. */
get_cpu();
- for_each_online_cpu(i)
- wake_up_process(threads[i]);
-
+ for_each_online_cpu(i) {
+ sm_work = percpu_ptr(stop_machine_work, i);
+ INIT_WORK(sm_work, stop_cpu);
+ queue_work_on(i, stop_machine_wq, sm_work);
+ }
/* This will release the thread on our CPU. */
put_cpu();
- wait_for_completion(&finished);
+ flush_workqueue(stop_machine_wq);
mutex_unlock(&lock);
-
- kfree(threads);
-
return active.fnret;
-
-kill_threads:
- for_each_online_cpu(i)
- if (threads[i])
- kthread_stop(threads[i]);
- mutex_unlock(&lock);
-
- kfree(threads);
- return err;
}
int stop_machine(int (*fn)(void *), void *data, const cpumask_t *cpus)
@@ -187,3 +150,11 @@ int stop_machine(int (*fn)(void *), void
return ret;
}
EXPORT_SYMBOL_GPL(stop_machine);
+
+static int __init stop_machine_init(void)
+{
+ stop_machine_wq = create_workqueue("kstop");
+ stop_machine_work = alloc_percpu(struct work_struct);
+ return 0;
+}
+early_initcall(stop_machine_init);
--
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