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>
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 32a6c44..c530bc5 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -234,11 +234,13 @@
*/
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,
@@ -262,16 +264,30 @@
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;
}