On 10/4/2013 10:10 AM, Sebastian Andrzej Siewior wrote:
* Yang Shi | 2013-10-04 09:36:41 [-0700]:
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2453,8 +2453,11 @@ static void drain_all_stock(struct mem_cgroup *root_memcg, bool sync)
if (!test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
if (cpu == curcpu)
drain_local_stock(&stock->work);
- else
+ else {
+ preempt_enable();
schedule_work_on(cpu, &stock->work);
+ preempt_disable();
+ }
}
What ensures that you don't switch CPUs between preempt_enable() &
preempt_disable() and is curcpu != smp_processor_id() ?
drain_all_stock is called by drain_all_stock_async or
drain_all_stock_sync, and the call in both is protected by mutex:
if (!mutex_trylock(&percpu_charge_mutex))
return;
drain_all_stock(root_memcg, false);
mutex_unlock(&percpu_charge_mutex);
So, I suppose this should be able to protect from migration?
preempt_disable() ensures that the task executing drain_all_stock() is
not moved from cpu1 to cpu5. Lets say we run cpu1, on first invocation
we get we get moved from cpu1 to cpu5 after preempt_enable(). On the
second run we have (1 == 1) and invoke drain_local_stock() the argument
is ignored so we execute drain_local_stock() with data of cpu5. Later we
schedule work for cpu5 again but we never did it for cpu1.
The code here is robust enough that nothing bad happens if
drain_local_stock() is invoked twice on one CPU and the system probably
survives it if one CPU is skipped. However I would prefer not to have
such an example in the queue where it seems that it is okay to just
enable preemption and invoke schedule_work_on() because it breaks the
assumptions which are made by get_cpu().
Ok, I see. Anyway, we can't call schedule_work_on with preempt disabled.
And, I checked the git commit history about the drain_local_stock call,
it sounds it is just an optimization for preventing from deferring local
stock drain to work queue.
So, It sounds safe to remove the get_cpu and the shortcut to make
schedule_work_on called safely as you suggested.
If this sounds fine to you, I'm going to come up with V2.
Thanks,
Yang
Thanks,
Yang
Sebastian
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