Re: vmstat: On demand vmstat workers V8

[Christoph Lameter <cl@gentwo.org>]

On Wed, 30 Jul 2014, Lai Jiangshan wrote:

> I think the bug is here, it re-queues the per_cpu(vmstat_work, cpu) which is offline
> (after vmstat_cpuup_callback(CPU_DOWN_PREPARE).  And cpu_stat_off is accessed without
> proper lock.

Ok. I guess we need to make the preemption check output more information
so that it tells us that an operation was performed on a processor that is
down?

> I suggest to use get_cpu_online() or a new cpu_stat_off_mutex to protect it.

If a processor is downed then cpu_stat_off bit should be cleared but also
the worker thread should not run.

> >  	case CPU_DOWN_PREPARE:
> >  	case CPU_DOWN_PREPARE_FROZEN:
> > -		cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
> > -		per_cpu(vmstat_work, cpu).work.func = NULL;
> > +		if (!cpumask_test_and_set_cpu(cpu, cpu_stat_off))
> > +			cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
>
> It is suggest that cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu)) should
> be called unconditionally.  And the cpu should be cleared from cpu_stat_off.
> (you set it, it is BUG according to vmstat_shepherd() and the semantics of the
> cpu_stat_off).

True.

Subject: vmstat ondemand: Fix online/offline races

Do not allow onlining/offlining while the shepherd task is checking
for vmstat threads.

On offlining a processor do the right thing cancelling the vmstat
worker thread if it exista and also exclude it from the shepherd
process checks.

Signed-off-by: Christoph Lameter <cl@linux.com>

Index: linux/mm/vmstat.c
===================================================================
--- linux.orig/mm/vmstat.c	2014-07-30 09:35:54.602662306 -0500
+++ linux/mm/vmstat.c	2014-07-30 09:43:07.109037043 -0500
@@ -1317,6 +1317,7 @@ static void vmstat_shepherd(struct work_
 {
 	int cpu;

+	get_online_cpus();
 	/* Check processors whose vmstat worker threads have been disabled */
 	for_each_cpu(cpu, cpu_stat_off)
 		if (need_update(cpu) &&
@@ -1325,6 +1326,7 @@ static void vmstat_shepherd(struct work_
 			schedule_delayed_work_on(cpu, &per_cpu(vmstat_work, cpu),
 				__round_jiffies_relative(sysctl_stat_interval, cpu));

+	put_online_cpus();

 	schedule_delayed_work(&shepherd,
 		round_jiffies_relative(sysctl_stat_interval));
@@ -1380,8 +1382,8 @@ static int vmstat_cpuup_callback(struct
 		break;
 	case CPU_DOWN_PREPARE:
 	case CPU_DOWN_PREPARE_FROZEN:
-		if (!cpumask_test_and_set_cpu(cpu, cpu_stat_off))
-			cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
+		cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
+		cpumask_clear_cpu(cpu, cpu_stat_off);
 		break;
 	case CPU_DOWN_FAILED:
 	case CPU_DOWN_FAILED_FROZEN:

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