sched: Remove get_online_cpus() usage

Remove get_online_cpus() usage from the scheduler; there's 4 sites that
use it:

 - sched_init_smp(); where its completely superfluous since we're in
   'early' boot and there simply cannot be any hotplugging.

 - sched_getaffinity(); we already take a raw spinlock to protect the
   task cpus_allowed mask, this disables preemption and therefore
   also stabilizes cpu_online_mask as that's modified using
   stop_machine. However switch to active mask for symmetry with
   sched_setaffinity()/set_cpus_allowed_ptr(). We guarantee active
   mask stability by inserting sync_rcu/sched() into _cpu_down.

 - sched_setaffinity(); we don't appear to need get_online_cpus()
   either, there's two sites where hotplug appears relevant:
    * cpuset_cpus_allowed(); for the !cpuset case we use possible_mask,
      for the cpuset case we hold task_lock, which is a spinlock and
      thus for mainline disables preemption (might cause pain on RT).
    * set_cpus_allowed_ptr(); Holds all scheduler locks and thus has
      preemption properly disabled; also it already deals with hotplug
      races explicitly where it releases them.

 - migrate_swap(); we can make stop_two_cpus() do the heavy lifting for
   us with a little trickery. By adding a sync_sched/rcu() after the
   CPU_DOWN_PREPARE notifier we can provide preempt/rcu guarantees for
   cpu_active_mask. Use these to validate that both our cpus are active
   when queueing the stop work before we queue the stop_machine works
   for take_cpu_down().

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Link: http://lkml.kernel.org/r/20131011123820.GV3081@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2013-10-11 14:38:20 +02:00 committed by Ingo Molnar
parent 746023159c
commit 6acce3ef84
3 changed files with 48 additions and 15 deletions

View file

@ -308,6 +308,23 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
}
smpboot_park_threads(cpu);
/*
* By now we've cleared cpu_active_mask, wait for all preempt-disabled
* and RCU users of this state to go away such that all new such users
* will observe it.
*
* For CONFIG_PREEMPT we have preemptible RCU and its sync_rcu() might
* not imply sync_sched(), so explicitly call both.
*/
#ifdef CONFIG_PREEMPT
synchronize_sched();
#endif
synchronize_rcu();
/*
* So now all preempt/rcu users must observe !cpu_active().
*/
err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
if (err) {
/* CPU didn't die: tell everyone. Can't complain. */

View file

@ -1085,8 +1085,6 @@ int migrate_swap(struct task_struct *cur, struct task_struct *p)
struct migration_swap_arg arg;
int ret = -EINVAL;
get_online_cpus();
arg = (struct migration_swap_arg){
.src_task = cur,
.src_cpu = task_cpu(cur),
@ -1097,6 +1095,10 @@ int migrate_swap(struct task_struct *cur, struct task_struct *p)
if (arg.src_cpu == arg.dst_cpu)
goto out;
/*
* These three tests are all lockless; this is OK since all of them
* will be re-checked with proper locks held further down the line.
*/
if (!cpu_active(arg.src_cpu) || !cpu_active(arg.dst_cpu))
goto out;
@ -1109,7 +1111,6 @@ int migrate_swap(struct task_struct *cur, struct task_struct *p)
ret = stop_two_cpus(arg.dst_cpu, arg.src_cpu, migrate_swap_stop, &arg);
out:
put_online_cpus();
return ret;
}
@ -3710,7 +3711,6 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
struct task_struct *p;
int retval;
get_online_cpus();
rcu_read_lock();
p = find_process_by_pid(pid);
@ -3773,7 +3773,6 @@ out_free_cpus_allowed:
free_cpumask_var(cpus_allowed);
out_put_task:
put_task_struct(p);
put_online_cpus();
return retval;
}
@ -3818,7 +3817,6 @@ long sched_getaffinity(pid_t pid, struct cpumask *mask)
unsigned long flags;
int retval;
get_online_cpus();
rcu_read_lock();
retval = -ESRCH;
@ -3831,12 +3829,11 @@ long sched_getaffinity(pid_t pid, struct cpumask *mask)
goto out_unlock;
raw_spin_lock_irqsave(&p->pi_lock, flags);
cpumask_and(mask, &p->cpus_allowed, cpu_online_mask);
cpumask_and(mask, &p->cpus_allowed, cpu_active_mask);
raw_spin_unlock_irqrestore(&p->pi_lock, flags);
out_unlock:
rcu_read_unlock();
put_online_cpus();
return retval;
}
@ -6494,14 +6491,17 @@ void __init sched_init_smp(void)
sched_init_numa();
get_online_cpus();
/*
* There's no userspace yet to cause hotplug operations; hence all the
* cpu masks are stable and all blatant races in the below code cannot
* happen.
*/
mutex_lock(&sched_domains_mutex);
init_sched_domains(cpu_active_mask);
cpumask_andnot(non_isolated_cpus, cpu_possible_mask, cpu_isolated_map);
if (cpumask_empty(non_isolated_cpus))
cpumask_set_cpu(smp_processor_id(), non_isolated_cpus);
mutex_unlock(&sched_domains_mutex);
put_online_cpus();
hotcpu_notifier(sched_domains_numa_masks_update, CPU_PRI_SCHED_ACTIVE);
hotcpu_notifier(cpuset_cpu_active, CPU_PRI_CPUSET_ACTIVE);

View file

@ -234,11 +234,13 @@ static void irq_cpu_stop_queue_work(void *arg)
*/
int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg)
{
int call_cpu;
struct cpu_stop_done done;
struct cpu_stop_work work1, work2;
struct irq_cpu_stop_queue_work_info call_args;
struct multi_stop_data msdata = {
struct multi_stop_data msdata;
preempt_disable();
msdata = (struct multi_stop_data){
.fn = fn,
.data = arg,
.num_threads = 2,
@ -261,17 +263,31 @@ int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *
cpu_stop_init_done(&done, 2);
set_state(&msdata, MULTI_STOP_PREPARE);
/*
* If we observe both CPUs active we know _cpu_down() cannot yet have
* queued its stop_machine works and therefore ours will get executed
* first. Or its not either one of our CPUs that's getting unplugged,
* in which case we don't care.
*
* This relies on the stopper workqueues to be FIFO.
*/
if (!cpu_active(cpu1) || !cpu_active(cpu2)) {
preempt_enable();
return -ENOENT;
}
/*
* Queuing needs to be done by the lowest numbered CPU, to ensure
* that works are always queued in the same order on every CPU.
* This prevents deadlocks.
*/
call_cpu = min(cpu1, cpu2);
smp_call_function_single(call_cpu, &irq_cpu_stop_queue_work,
smp_call_function_single(min(cpu1, cpu2),
&irq_cpu_stop_queue_work,
&call_args, 0);
preempt_enable();
wait_for_completion(&done.completion);
return done.executed ? done.ret : -ENOENT;
}